Deep Space Network Operations Center at JPL, Pasadena (California) in 1993.
DSN currently consists of three deep-space communications facilities
located such that a distant spacecraft is always in view of at least one
station. They are:
Each facility is situated in semi-mountainous, bowl-shaped terrain to help shield against radio frequency interference.
The strategic placement of the stations permits constant observation of
spacecraft as the Earth rotates, which helps to make the DSN the
largest and most sensitive scientific telecommunications system in the
world.
The DSN supports NASA's contribution to the scientific investigation of the Solar System: It provides a two-way communications link that guides and controls various NASA uncrewed interplanetary space probes, and brings back the images and new scientific information these probes collect. All DSN antennas are steerable, high-gain, parabolic reflector antennas.
The antennas and data delivery systems make it possible to:
The antennas at all three DSN Complexes communicate directly with the
Deep Space Operations Center (also known as Deep Space Network
operations control center) located at the JPL facilities in Pasadena, California.
In the early years, the operations control center did not have a
permanent facility. It was a provisional setup with numerous desks and
phones installed in a large room near the computers used to calculate
orbits. In July 1961, NASA started the construction of the permanent
facility, Space Flight Operations Facility (SFOF). The facility was
completed in October 1963 and dedicated on May 14, 1964. In the initial
setup of the SFOF, there were 31 consoles, 100 closed-circuit television
cameras, and more than 200 television displays to support Ranger 6 to Ranger 9 and Mariner 4.
Currently, the operations center personnel at SFOF monitor and
direct operations, and oversee the quality of spacecraft telemetry and
navigation data delivered to network users. In addition to the DSN
complexes and the operations center, a ground communications facility
provides communications that link the three complexes to the operations
center at JPL, to space flight control centers in the United States and
overseas, and to scientists around the world.
Deep space
View
from the Earth's north pole, showing the field of view of the main DSN
antenna locations. Once a mission gets more than 30,000 km (19,000 mi)
from Earth, it is always in view of at least one of the stations.
Tracking vehicles in deep space is quite different from tracking missions in low Earth orbit
(LEO). Deep space missions are visible for long periods of time from a
large portion of the Earth's surface, and so require few stations (the
DSN has only three main sites). These few stations, however, require
huge antennas, ultra-sensitive receivers, and powerful transmitters in
order to transmit and receive over the vast distances involved.
Deep space is defined in several different ways. According to a
1975 NASA report, the DSN was designed to communicate with "spacecraft
traveling approximately 16,000 km (10,000 miles) from Earth to the
farthest planets of the solar system." JPL diagrams
state that at an altitude of 30,000 km (19,000 mi), a spacecraft is
always in the field of view of one of the tracking stations.
The International Telecommunication Union,
which sets aside various frequency bands for deep space and near Earth
use, defines "deep space" to start at a distance of 2 million km
(1.2 million mi) from the Earth's surface.
Because the Moon, the Earth-moon Lagrange points, and the Earth–Sun Lagrangian points L1 and L2, are all closer than 2 million km from Earth (distances are here), they are considered near space and cannot use the ITU's deep space bands.
The forerunner of the DSN was established in January 1958, when JPL, then under contract to the U.S. Army, deployed portable radio tracking stations in Nigeria, Singapore, and California to receive telemetry and plot the orbit of the Army-launched Explorer 1, the first successful U.S. satellite. NASA
was officially established on October 1, 1958, to consolidate the
separately developing space-exploration programs of the US Army, US Navy, and US Air Force into one civilian organization.
On December 3, 1958, JPL was transferred from the US Army to NASA
and given responsibility for the design and execution of lunar and
planetary exploration programs using remotely controlled spacecraft.
Shortly after the transfer, NASA established the concept of the Deep
Space Network as a separately managed and operated communications system
that would accommodate all deep space
missions, thereby avoiding the need for each flight project to acquire
and operate its own specialized space communications network. The DSN
was given responsibility for its own research, development, and
operation in support of all of its users. Under this concept, it has
become a world leader in the development of low-noise receivers; large
parabolic-dish antennas; tracking, telemetry, and command systems;
digital signal processing; and deep space navigation. The Deep Space
Network formally announced its intention to send missions into deep
space on Christmas Eve 1963; it has remained in continuous operation in
one capacity or another ever since.
The largest antennas of the DSN are often called on during
spacecraft emergencies. Almost all spacecraft are designed so normal
operation can be conducted on the smaller (and more economical) antennas
of the DSN, but during an emergency the use of the largest antennas is
crucial. This is because a troubled spacecraft may be forced to use less
than its normal transmitter power, attitude control problems may preclude the use of high-gain antennas,
and recovering every bit of telemetry is critical to assessing the
health of the spacecraft and planning the recovery. The most famous
example is the Apollo 13
mission, where limited battery power and inability to use the
spacecraft's high-gain antennas reduced signal levels below the
capability of the Manned Space Flight Network, and the use of the biggest DSN antennas (and the Australian Parkes Observatoryradio telescope)
was critical to saving the lives of the astronauts. While Apollo was
also a US mission, DSN provides this emergency service to other space
agencies as well, in a spirit of inter-agency and international
cooperation. For example, the recovery of the Solar and Heliospheric Observatory (SOHO) mission of the European Space Agency (ESA) would not have been possible without the use of the largest DSN facilities.
DSN and the Apollo program
Although
normally tasked with tracking uncrewed spacecraft, the Deep Space
Network (DSN) also contributed to the communication and tracking of Apollo missions to the Moon, although primary responsibility was held by the Manned Space Flight Network
(MSFN). The DSN designed the MSFN stations for lunar communication and
provided a second antenna at each MSFN site (the MSFN sites were near
the DSN sites for just this reason). Two antennas at each site were
needed both for redundancy and because the beam widths of the large
antennas needed were too small to encompass both the lunar orbiter and
the lander at the same time. DSN also supplied some larger antennas as
needed, in particular for television broadcasts from the Moon, and
emergency communications such as Apollo 13.
Excerpt from a NASA report describing how the DSN and MSFN cooperated for Apollo:
Another critical step in the
evolution of the Apollo Network came in 1965 with the advent of the DSN
Wing concept. Originally, the participation of DSN 26-m antennas during
an Apollo Mission was to be limited to a backup role. This was one
reason why the MSFN 26-m sites were collocated with the DSN sites at
Goldstone, Madrid, and Canberra. However, the presence of two,
well-separated spacecraft during lunar operations stimulated the
rethinking of the tracking and communication problem. One thought was to
add a dual S-band RF system to each of the three 26-m MSFN antennas,
leaving the nearby DSN 26-m antennas still in a backup role.
Calculations showed, though, that a 26-m antenna pattern centered on the
landed Lunar Module would suffer a 9-to-12 db loss at the lunar
horizon, making tracking and data acquisition of the orbiting Command
Service Module difficult, perhaps impossible. It made sense to use both
the MSFN and DSN antennas simultaneously during the all-important lunar
operations. JPL was naturally reluctant to compromise the objectives of
its many uncrewed spacecraft by turning three of its DSN stations over
to the MSFN for long periods. How could the goals of both Apollo and
deep space exploration be achieved without building a third 26-m antenna
at each of the three sites or undercutting planetary science missions?
The solution came in early 1965 at a
meeting at NASA Headquarters, when Eberhardt Rechtin suggested what is
now known as the "wing concept". The wing approach involves constructing
a new section or "wing" to the main building at each of the three
involved DSN sites. The wing would include a MSFN control room and the
necessary interface equipment to accomplish the following:
Permit tracking and two-way data transfer with either spacecraft during lunar operations.
Permit tracking and two-way data transfer with the combined spacecraft during the flight to the Moon.
Provide backup for the collocated MSFN site passive track
(spacecraft to ground RF links) of the Apollo spacecraft during
trans-lunar and trans-earth phases.
With this arrangement, the DSN station could be quickly switched from
a deep-space mission to Apollo and back again. GSFC personnel would
operate the MSFN equipment completely independently of DSN personnel.
Deep space missions would not be compromised nearly as much as if the
entire station's equipment and personnel were turned over to Apollo for
several weeks.
The details of this cooperation and operation are available in a two-volume technical report from JPL.
Management
The network is a NASA facility and is managed and operated for NASA by JPL, which is part of the California Institute of Technology
(Caltech). The Interplanetary Network Directorate (IND) manages the
program within JPL and is charged with the development and operation of
it. The IND is considered to be JPL's focal point for all matters
relating to telecommunications, interplanetary navigation, information
systems, information technology, computing, software engineering, and
other relevant technologies. While the IND is best known for its duties
relating to the Deep Space Network, the organization also maintains the
JPL Advanced Multi-Mission Operations System (AMMOS) and JPL's Institutional Computing and Information Services (ICIS).
Peraton (formerly Harris Corporation)
is under contract to JPL for the DSN's operations and maintenance.
Peraton has responsibility for managing the Goldstone complex, operating
the DSOC, and for DSN operations, mission planning, operations
engineering, and logistics.
Each complex consists of at least four deep space terminals equipped
with ultra-sensitive receiving systems and large parabolic-dish
antennas. There are:
Five of the 34-meter (112 ft) beam waveguide antennas were added to
the system in the late 1990s. Three were located at Goldstone, and one
each at Canberra and Madrid. A second 34-meter (112 ft) beam waveguide
antenna (the network's sixth) was completed at the Madrid complex in
2004.
In order to meet the current and future needs of deep space
communication services, a number of new Deep Space Station antennas had
to be built at the existing Deep Space Network sites. At the Canberra
Deep Space Communication Complex the first of these was completed in
October 2014 (DSS35), with a second becoming operational in October 2016
(DSS36). A new 34 meter dish (DSS53) became operational at the Madrid complex in February 2022.
By 2025, the 70-meter antennas at all three locations will be
decommissioned and replaced with 34-meter BWG antennas that will be
arrayed. All systems will be upgraded to have X-band uplink capabilities
and both X and Ka-band downlink capabilities.
The general capabilities of the DSN have not substantially changed since the beginning of the Voyager
Interstellar Mission in the early 1990s. However, many advancements in
digital signal processing, arraying and error correction have been
adopted by the DSN.
The ability to array several antennas was incorporated to improve the data returned from the Voyager 2Neptune encounter, and extensively used for the Galileo mission, when the spacecraft's high-gain antenna failed to deploy and as a result Galileo was forced to resort to operating solely off its low-gain antennas.
The DSN array currently available since the Galileo
mission can link the 70-meter (230 ft) dish antenna at the Deep Space
Network complex in Goldstone, California, with an identical antenna
located in Australia, in addition to two 34-meter (112 ft) antennas at
the Canberra complex. The California and Australia sites were used
concurrently to pick up communications with Galileo.
Arraying of antennas within the three DSN locations is also used.
For example, a 70-meter (230 ft) dish antenna can be arrayed with a
34-meter dish. For especially vital missions, like Voyager 2, non-DSN facilities normally used for radio astronomy can be added to the array. In particular, the Canberra 70-meter (230 ft) dish can be arrayed with the Parkes Radio Telescope in Australia; and the Goldstone 70-meter dish can be arrayed with the Very Large Array of antennas in New Mexico. Also, two or more 34-meter (112 ft) dishes at one DSN location are commonly arrayed together.
All the stations are remotely operated from a centralized Signal
Processing Center at each complex. These Centers house the electronic
subsystems that point and control the antennas, receive and process the
telemetry data, transmit commands, and generate the spacecraft
navigation data. Once the data are processed at the complexes, they are
transmitted to JPL for further processing and for distribution to
science teams over a modern communications network.
Especially at Mars, there are often many spacecraft within the
beam width of an antenna. For operational efficiency, a single antenna
can receive signals from multiple spacecraft at the same time. This
capability is called Multiple Spacecraft Per Aperture, or MSPA.
Currently, the DSN can receive up to 4 spacecraft signals at the same
time, or MSPA-4. However, apertures cannot currently be shared for
uplink. When two or more high-power carriers are used simultaneously,
very high order intermodulation products fall in the receiver bands,
causing interference to the much (25 orders of magnitude) weaker
received signals. Therefore, only one spacecraft at a time can get an uplink, though up to 4 can be received.
There are a number of limitations to the current DSN, and a number of challenges going forward.
The Deep Space Network nodes are all on Earth. Therefore, data
transmission rates from/to spacecraft and space probes are severely
constrained due to the distances from Earth. For now it can connect with
the Mars orbiters in the Mars Relay Network for faster and more flexible communications with spacecraft and landers on Mars. Adding dedicated communication satellites elsewhere in space, to handle multiparty, multi-mission use, such as the canceled Mars Telecommunications Orbiter, would increase flexibility towards some sort of Interplanetary Internet.
The need to support "legacy" missions that have remained operational
beyond their original lifetimes but are still returning scientific
data. Programs such as Voyager have been operating long past their original mission termination date. They also need some of the largest antennas.
Replacing major components can cause problems as it can leave an antenna out of service for months at a time.
The older 70 m antennas are reaching the end of their lives. At some
point these will need to be replaced. The leading candidate for 70 m
replacement had been an array of smaller dishes,
but more recently the decision was taken to expand the provision of
34-meter (112 ft) BWG antennas at each complex to a total of 4. All the 34-meter HEF antennas have been replaced.
New spacecraft intended for missions beyond geocentric orbits are being equipped to use the beacon mode service, which allows such missions to operate without the DSN most of the time.
DSN and radio science
Illustration of Juno and Jupiter. Juno
is in a polar orbit that takes it close to Jupiter as it passes from
north to south, getting a view of both poles. During the GS experiment
it must point its antenna at the Deep Space Network on Earth to pick up a
special signal sent from DSN.
The DSN forms one portion of the radio sciences experiment included
on most deep space missions, where radio links between spacecraft and
Earth are used to investigate planetary science, space physics and
fundamental physics. The experiments include radio occultations,
gravity field determination and celestial mechanics, bistatic
scattering, doppler wind experiments, solar corona characterization, and
tests of fundamental physics.
For example, the Deep Space Network forms one component of the gravity science experiment on Juno. This includes special communication hardware on Juno and uses its communication system. The DSN radiates a Ka-band uplink, which is picked up by Juno's
Ka-Band communication system and then processed by a special
communication box called KaTS, and then this new signal is sent back the
DSN.
This allows the velocity of the spacecraft over time to be determined
with a level of precision that allows a more accurate determination of
the gravity field at planet Jupiter.
Another radio science experiment is REX on the New Horizons
spacecraft to Pluto-Charon. REX received a signal from Earth as it was
occulted by Pluto, to take various measurements of that system of
bodies.
In the Americas and many other parts of the world, VHF Band I was used for the transmission of analog television. As part of the worldwide transition to digital terrestrial television most countries require broadcasters to air television in the VHF range using digital, rather than analog encoding.
Propagation characteristics
Radio waves in the VHF band propagate mainly by line-of-sight and ground-bounce paths; unlike in the HF band there is only some reflection at lower frequencies from the ionosphere (skywave propagation). They do not follow the contour of the Earth as ground waves
and so are blocked by hills and mountains, although because they are
weakly refracted (bent) by the atmosphere they can travel somewhat
beyond the visual horizon
out to about 160 km (100 miles). They can penetrate building walls and
be received indoors, although in urban areas reflections from buildings
cause multipath propagation, which can interfere with television reception. Atmospheric radio noise and interference (RFI)
from electrical equipment is less of a problem in this and higher
frequency bands than at lower frequencies. The VHF band is the first
band at which efficient transmitting antennas are small enough that they
can be mounted on vehicles and portable devices, so the band is used
for two-wayland mobile radio systems, such as walkie-talkies, and two way radio communication with aircraft (Airband) and ships (marine radio). Occasionally, when conditions are right, VHF waves can travel long distances by tropospheric ducting due to refraction by temperature gradients in the atmosphere.
Line-of-sight calculation
"Rabbit-ears" VHF television antenna (the small loop is a separate UHF antenna)
VHF transmission range is a function of transmitter power, receiver
sensitivity, and distance to the horizon, since VHF signals propagate
under normal conditions as a near line-of-sight phenomenon. The distance to the radio horizon
is slightly extended over the geometric line of sight to the horizon,
as radio waves are weakly bent back toward the Earth by the atmosphere.
An approximation to calculate the line-of-sight horizon distance (on Earth) is:
distance in nautical miles = where is the height of the antenna in feet
distance in kilometers = where is the height of the antenna in meters.
These approximations are only valid for antennas at heights that are
small compared to the radius of the Earth. They may not necessarily be
accurate in mountainous areas, since the landscape may not be
transparent enough for radio waves.
In engineered communications systems, more complex calculations
are required to assess the probable coverage area of a proposed
transmitter station.
A VHF television broadcasting antenna. This is a common type called a super turnstile or batwing antenna.
VHF is the first band at which wavelengths are small enough that
efficient transmitting antennas are short enough to mount on vehicles
and handheld devices, a quarter wave whip antenna at VHF frequencies is 25 cm to 2.5 meter (10 inches to 8 feet) long. So the VHF and UHF wavelengths are used for two-way radios in vehicles, aircraft, and handheld transceivers and walkie-talkies. Portable radios usually use whips or rubber ducky antennas, while base stations usually use larger fiberglass whips or collinear arrays of vertical dipoles.
The VHF TV band in Australia was originally allocated channels 1 to
10-with channels 2, 7 and 9 assigned for the initial services in Sydney
and Melbourne, and later the same channels were assigned in Brisbane,
Adelaide and Perth. Other capital cities and regional areas used a
combination of these and other frequencies as available. The initial
commercial services in Hobart and Darwin were respectively allocated
channels 6 and 8 rather than 7 or 9.
By the early 1960s it became apparent that the 10 VHF channels
were insufficient to support the growth of television services. This was
rectified by the addition of three additional frequencies-channels 0,
5A and 11. Older television sets using rotary dial tuners required
adjustment to receive these new channels. Most TVs of that era were not
equipped to receive these broadcasts, and so were modified at the
owners' expense to be able to tune into these bands; otherwise the owner
had to buy a new TV.
Several TV stations were allocated to VHF channels 3, 4 and 5,
which were within the FM radio bands although not yet used for that
purpose. A couple of notable examples were NBN-3Newcastle, WIN-4Wollongong and ABCNewcastle
on channel 5. While some Channel 5 stations were moved to 5A in the
1970s and 80s, beginning in the 1990s, the Australian Broadcasting
Authority began a process to move these stations to UHF
bands to free up valuable VHF spectrum for its original purpose of FM
radio. In addition, by 1985 the federal government decided new TV
stations are to be broadcast on the UHF band.
Two new VHF channels, 9A and 12, have since been made available and are being used primarily for digital services (e.g. ABC
in capital cities) but also for some new analogue services in regional
areas. Because channel 9A is not used for television services in or near
Sydney, Melbourne, Brisbane, Adelaide or Perth, digital radio in those cities are broadcast on DAB frequencies blocks 9A, 9B and 9C.
VHF radio is also used for marine Radio as per its long-distance reachability comparing UHF frequencies.
Until 2013, the four main free-to-air TV stations in New Zealand used the VHF television bands (Band I and Band III)
to transmit to New Zealand households. Other stations, including a
variety of pay and regional free-to-air stations, were forced to
broadcast in the UHF
band, since the VHF band had been very overloaded with four stations
sharing a very small frequency band, which was so overcrowded that one
or more channels would not be available in some smaller towns.
However, at the end of 2013,
all television channels stopped broadcasting on the VHF bands, as New
Zealand moved to digital television broadcasting, requiring all stations
to either broadcast on UHF or satellite (where UHF was unavailable)
utilising the Freeview service.
British television originally used VHF band I and band III. Television on VHF was in black and white with 405-line format (although there were experiments with all three colour systems-NTSC, PAL, and SECAM-adapted for the 405-line system in the late 1950s and early 1960s).
British colour television was broadcast on UHF
(channels 21–69), beginning in the late 1960s. From then on, TV was
broadcast on both VHF and UHF (VHF being a monochromatic downconversion
from the 625-line colour signal), with the exception of BBC2 (which had always broadcast solely on UHF). The last British VHF TV transmitters closed down on January 3, 1985. VHF band III is now used in the UK for digital audio broadcasting, and VHF band II is used for FM radio, as it is in most of the world.
Frequency
assignments between US and Canadian users are closely coordinated since
much of the Canadian population is within VHF radio range of the US
border. Certain discrete frequencies are reserved for radio astronomy.
The general services in the VHF band are:
30–49.6 MHz: Licensed 2-way land mobile communication, with various sub-bands.
43–50 MHz: Cordless telephones,
49 MHz FM walkie-talkies and radio controlled toys, and mixed 2-way
mobile communication. The FM broadcast band originally operated here
(42–50 MHz) before it was moved to 88–108 MHz.
72–76 MHz: Radio controlled models, industrial remote control, and other devices. Model aircraft operate on 72 MHz while surface models operate on 75 MHz in the US and Canada, air navigation beacons 74.8–75.2 MHz.
76–88 MHz TV channels 5–6 (VHF-Lo)
87.5–108 MHz: FM radio broadcasting (87.9–91.9 non-commercial, 92–108 commercial in the United States) (known as "Band II" internationally)
174–240 MHz, known as "Band III"
internationally. A number of DTV channels have begun broadcasting here,
especially many of the stations which were assigned to these channels
for previous analog operation.
174–216 MHz television channels 7–13 (VHF-Hi)
174–216 MHz: professional wireless microphones (low power, certain exact frequencies only)
225 MHz and above (UHF): Military aircraft radio, 243 MHz is an emergency frequency (225–400 MHz) AM, including HAVE QUICK, dGPS RTCM-104
Cable television, though not transmitted aerially, uses a spectrum of frequencies overlapping VHF.
VHF television
The U.S. FCC
allocated television broadcasting to a channelized roster as early as
1938 with 19 channels. That changed three more times: in 1940 when
Channel 19 was deleted and several channels changed frequencies, then in
1946 with television going from 18 channels to 13 channels, again with
different frequencies, and finally in 1948 with the removal of Channel 1 (analog channels 2–13 remain as they were, even on cable television). Channels 14–19 later appeared on the UHF band, while channel 1 remains unused.
87.5–87.9 MHz
87.5–87.9 MHz is a radio frequency which, in most of the world, is used for FM broadcasting. In North America,
however, this bandwidth is allocated to VHF television channel 6
(82–88 MHz). The analog audio for TV channel 6 is broadcast at 87.75 MHz
(adjustable down to 87.74). Several stations, known as Frankenstations, most notably those joining the Pulse 87
franchise, have operated on this frequency as radio stations, though
they use television licenses. As a result, FM radio receivers such as
those found in automobiles which are designed to tune into this
frequency range could receive the audio for analog-mode programming on
the local TV channel 6 while in North America. The practice largely
ended with the DTV transition in 2009, although some still exist.
The FM broadcast channel at 87.9 MHz is normally off-limits for
FM audio broadcasting; it is reserved for displaced class D stations
which have no other frequencies in the normal 88.1–107.9 MHz subband to
move to. So far, only two stations have qualified to operate on
87.9 MHz: 10–watt KSFH in Mountain View, California and 34–watt translator K200AA in Sun Valley, Nevada.
Unlicensed operation
In
some countries, particularly the United States and Canada, limited
low-power license-free operation is available in the FM broadcast band
for purposes such as micro-broadcasting and sending output from CD
or digital media players to radios without auxiliary-in jacks, though
this is illegal in some other countries. This practice was legalised in
the United Kingdom on 8 December 2006.
The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radio navigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite systems (GNSS) that provides geolocation and time information to a GPS receiver anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites.
It does not require the user to transmit any data, and operates
independently of any telephonic or Internet reception, though these
technologies can enhance the usefulness of the GPS positioning
information. It provides critical positioning capabilities to military,
civil, and commercial users around the world. Although the United States
government created, controls and maintains the GPS system, it is freely
accessible to anyone with a GPS receiver.
The GPS project was started by the U.S. Department of Defense
in 1973. The first prototype spacecraft was launched in 1978 and the
full constellation of 24 satellites became operational in 1993.
Originally limited to use by the United States military, civilian use
was allowed from the 1980s following an executive order from President Ronald Reagan after the Korean Air Lines Flight 007 disaster.
Advances in technology and new demands on the existing system have now
led to efforts to modernize the GPS and implement the next generation of
GPS Block IIIA satellites and Next Generation Operational Control System (OCX). which was authorized by the U.S. Congress in 2000.
From the early 1990s, GPS positional accuracy was degraded by the United States government by a program called Selective Availability, which could selectively degrade or deny access to the system at any time, as happened to the Indian military in 1999 during the Kargil War. However, this practice was discontinued on May 1, 2000, in accordance with a bill signed into law by President Bill Clinton. As a result, several countries — including Russia, China, India, Japan, and the European Union — have developed or are in the process of setting up other global or regional satellite navigation systems.
When selective availability was lifted in 2000, GPS had about a
five-meter (16 ft) accuracy. GPS receivers that use the L5 band have
much higher accuracy, pinpointing to within 30 centimeters (12 in),
while high-end users (typically engineering and land surveying
applications) are able to have accuracy on several of the bandwidth
signals to within two centimeters, and even sub-millimeter accuracy for
long-term measurements. Consumer devices, like smartphones, can be as accurate as to within 4.9 m (or better with assistive services like Wi-Fi positioning also enabled). As of May 2021,
16 GPS satellites are broadcasting L5 signals, and the signals are
considered pre-operational, scheduled to reach 24 satellites by
approximately 2027.
History
GPS constellation system animation
The GPS project was launched in the United States in 1973 to overcome the limitations of previous navigation systems, combining ideas from several predecessors, including classified engineering design studies from the 1960s. The U.S. Department of Defense
developed the system, which originally used 24 satellites, for use by
the United States military, and became fully operational in 1995.
Civilian use was allowed from the 1980s. Roger L. Easton of the Naval Research Laboratory, Ivan A. Getting of The Aerospace Corporation, and Bradford Parkinson of the Applied Physics Laboratory are credited with inventing it. The work of Gladys West
is credited as instrumental in the development of computational
techniques for detecting satellite positions with the precision needed
for GPS.
The design of GPS is based partly on similar ground-based radio-navigation systems, such as LORAN and the Decca Navigator, developed in the early 1940s.
In 1955, Friedwardt Winterberg proposed a test of general relativity—detecting
time slowing in a strong gravitational field using accurate atomic
clocks placed in orbit inside artificial satellites. Special and general
relativity predicted that the clocks on GPS satellites, as observed by
those on Earth, run 38 microseconds faster per day than those on the
Earth. The design of GPS corrects for this difference; because without
doing so, GPS calculated positions would accumulate errors of up to 10
kilometers per day (6 mi/d).
Predecessors
When the Soviet Union launched its first artificial satellite (Sputnik 1) in 1957, two American physicists, William Guier and George Weiffenbach, at Johns Hopkins University's Applied Physics Laboratory (APL) decided to monitor its radio transmissions. Within hours they realized that, because of the Doppler effect, they could pinpoint where the satellite was along its orbit. The Director of the APL gave them access to their UNIVAC to do the heavy calculations required.
Early the next year, Frank McClure, the deputy director of the
APL, asked Guier and Weiffenbach to investigate the inverse problem:
pinpointing the user's location, given the satellite's. (At the time,
the Navy was developing the submarine-launched Polaris missile, which required them to know the submarine's location.) This led them and APL to develop the TRANSIT system. In 1959, ARPA (renamed DARPA in 1972) also played a role in TRANSIT.
TRANSIT was first successfully tested in 1960. It used a constellation of five satellites and could provide a navigational fix approximately once per hour.
In 1967, the U.S. Navy developed the Timation satellite, which proved the feasibility of placing accurate clocks in space, a technology required for GPS.
In the 1970s, the ground-based OMEGA navigation system, based on phase comparison of signal transmission from pairs of stations,
became the first worldwide radio navigation system. Limitations of
these systems drove the need for a more universal navigation solution
with greater accuracy.
Although there were wide needs for accurate navigation in
military and civilian sectors, almost none of those was seen as
justification for the billions of dollars it would cost in research,
development, deployment, and operation of a constellation of navigation
satellites. During the Cold Wararms race,
the nuclear threat to the existence of the United States was the one
need that did justify this cost in the view of the United States
Congress. This deterrent effect is why GPS was funded. It is also the
reason for the ultra-secrecy at that time. The nuclear triad consisted of the United States Navy's submarine-launched ballistic missiles (SLBMs) along with United States Air Force (USAF) strategic bombers and intercontinental ballistic missiles (ICBMs). Considered vital to the nuclear deterrence posture, accurate determination of the SLBM launch position was a force multiplier.
Precise navigation would enable United States ballistic missile submarines to get an accurate fix of their positions before they launched their SLBMs.
The USAF, with two thirds of the nuclear triad, also had requirements
for a more accurate and reliable navigation system. The U.S. Navy and
U.S. Air Force were developing their own technologies in parallel to
solve what was essentially the same problem.
To increase the survivability of ICBMs, there was a proposal to use mobile launch platforms (comparable to the Soviet SS-24 and SS-25) and so the need to fix the launch position had similarity to the SLBM situation.
In 1960, the Air Force proposed a radio-navigation system called
MOSAIC (MObile System for Accurate ICBM Control) that was essentially a
3-D LORAN. A follow-on study, Project 57, was performed in 1963 and it
was "in this study that the GPS concept was born". That same year, the
concept was pursued as Project 621B, which had "many of the attributes
that you now see in GPS" and promised increased accuracy for Air Force bombers as well as ICBMs.
Updates from the Navy TRANSIT system were too slow for the high speeds of Air Force operation. The Naval Research Laboratory (NRL) continued making advances with their Timation (Time Navigation) satellites, first launched in 1967, second launched in 1969, with the third in 1974 carrying the first atomic clock into orbit and the fourth launched in 1977.
Another important predecessor to GPS came from a different branch of the United States military. In 1964, the United States Army orbited its first Sequential Collation of Range (SECOR) satellite used for geodetic surveying.
The SECOR system included three ground-based transmitters at known
locations that would send signals to the satellite transponder in orbit.
A fourth ground-based station, at an undetermined position, could then
use those signals to fix its location precisely. The last SECOR
satellite was launched in 1969.
Development
With
these parallel developments in the 1960s, it was realized that a
superior system could be developed by synthesizing the best technologies
from 621B, Transit, Timation, and SECOR in a multi-service program.
Satellite orbital position errors, induced by variations in the gravity
field and radar refraction among others, had to be resolved. A team led
by Harold L Jury of Pan Am Aerospace Division in Florida from 1970 to
1973, used real-time data assimilation and recursive estimation to do
so, reducing systematic and residual errors to a manageable level to
permit accurate navigation.
During Labor Day weekend in 1973, a meeting of about twelve military officers at the Pentagon discussed the creation of a Defense Navigation Satellite System (DNSS). It was at this meeting that the real synthesis that became GPS was created. Later that year, the DNSS program was named Navstar.
Navstar is often erroneously considered an acronym for "NAVigation
System Using Timing and Ranging" but was never considered as such by the
GPS Joint Program Office (TRW may have once advocated for a different
navigational system that used that acronym).
With the individual satellites being associated with the name Navstar
(as with the predecessors Transit and Timation), a more fully
encompassing name was used to identify the constellation of Navstar
satellites, Navstar-GPS. Ten "Block I" prototype satellites were launched between 1978 and 1985 (an additional unit was destroyed in a launch failure).
The effect of the ionosphere on radio transmission was investigated in a geophysics laboratory of Air Force Cambridge Research Laboratory, renamed to Air Force Geophysical Research Lab (AFGRL) in 1974. AFGRL developed the Klobuchar model for computing ionospheric corrections to GPS location. Of note is work done by Australian space scientist Elizabeth Essex-Cohen at AFGRL in 1974. She was concerned with the curving of the paths of radio waves (atmospheric refraction) traversing the ionosphere from NavSTAR satellites.
After Korean Air Lines Flight 007, a Boeing 747 carrying 269 people, was shot down by a Soviet interceptor aircraft after straying in prohibited airspace because of navigational errors, in the vicinity of Sakhalin and Moneron Islands, President Ronald Reagan issued a directive making GPS freely available for civilian use, once it was sufficiently developed, as a common good. The first Block II satellite was launched on February 14, 1989,
and the 24th satellite was launched in 1994. The GPS program cost at
this point, not including the cost of the user equipment but including
the costs of the satellite launches, has been estimated at US$5 billion
(equivalent to $10 billion in 2022).
Initially, the highest-quality signal was reserved for military
use, and the signal available for civilian use was intentionally
degraded, in a policy known as Selective Availability. This changed on May 1, 2000, with President Bill Clinton
signing a policy directive to turn off Selective Availability to
provide the same accuracy to civilians that was afforded to the
military. The directive was proposed by the U.S. Secretary of Defense, William Perry, in view of the widespread growth of differential GPS
services by private industry to improve civilian accuracy. Moreover,
the U.S. military was developing technologies to deny GPS service to
potential adversaries on a regional basis. Selective Availability was removed from the GPS architecture beginning with GPS-III.
Since its deployment, the U.S. has implemented several
improvements to the GPS service, including new signals for civil use and
increased accuracy and integrity for all users, all the while
maintaining compatibility with existing GPS equipment. Modernization of
the satellite system has been an ongoing initiative by the U.S.
Department of Defense through a series of satellite acquisitions to meet the growing needs of the military, civilians, and the commercial market.
As of early 2015, high-quality Standard Positioning Service (SPS)
GPS receivers provided horizontal accuracy of better than 3.5 meters
(11 ft), although many factors such as receiver and antenna quality and atmospheric issues can affect this accuracy.
GPS is owned and operated by the United States government as a
national resource. The Department of Defense is the steward of GPS. The Interagency GPS Executive Board (IGEB)
oversaw GPS policy matters from 1996 to 2004. After that, the National
Space-Based Positioning, Navigation and Timing Executive Committee was
established by presidential directive in 2004 to advise and coordinate
federal departments and agencies on matters concerning the GPS and
related systems.
The executive committee is chaired jointly by the Deputy Secretaries of
Defense and Transportation. Its membership includes equivalent-level
officials from the Departments of State, Commerce, and Homeland
Security, the Joint Chiefs of Staff and NASA.
Components of the executive office of the president participate as
observers to the executive committee, and the FCC chairman participates
as a liaison.
The U.S. Department of Defense is required by law to "maintain a
Standard Positioning Service (as defined in the federal radio navigation
plan and the standard positioning service signal specification) that
will be available on a continuous, worldwide basis" and "develop
measures to prevent hostile use of GPS and its augmentations without
unduly disrupting or degrading civilian uses".
In 1972, the USAF Central Inertial Guidance Test Facility
(Holloman AFB) conducted developmental flight tests of four prototype
GPS receivers in a Y configuration over White Sands Missile Range, using ground-based pseudo-satellites.
In 1978, the first experimental Block-I GPS satellite was launched.
In 1983, after Soviet interceptor aircraft shot down the civilian airliner KAL 007 that strayed into prohibited airspace because of navigational errors, killing all 269 people on board, U.S. President Ronald Reagan announced that GPS would be made available for civilian uses once it was completed,
although it had been publicly known as early as 1979, that the CA code
(Coarse/Acquisition code) would be available to civilian users.
By 1985, ten more experimental Block-I satellites had been launched to validate the concept.
On February 14, 1989, the first modern Block-II satellite was launched.
The Gulf War from 1990 to 1991 was the first conflict in which the military widely used GPS.
In 1991, a project to create a miniature GPS receiver successfully
ended, replacing the previous 16 kg (35 lb) military receivers with a
1.25 kg (2.8 lb) handheld receiver.
In 1991, TomTom, a Dutch sat-nav manufacturer was founded.
By December 1993, GPS achieved initial operational capability (IOC), with a full constellation (24 satellites) available and providing the Standard Positioning Service (SPS).
Full Operational Capability (FOC) was declared by Air Force Space Command (AFSPC) in April 1995, signifying full availability of the military's secure Precise Positioning Service (PPS).
In 1996, recognizing the importance of GPS to civilian users as well as military users, U.S. President Bill Clinton issued a policy directive declaring GPS a dual-use system and establishing an Interagency GPS Executive Board to manage it as a national asset.
In 1998, United States Vice President Al Gore
announced plans to upgrade GPS with two new civilian signals for
enhanced user accuracy and reliability, particularly with respect to
aviation safety, and in 2000 the United States Congress authorized the effort, referring to it as GPS III.
On May 2, 2000 "Selective Availability" was discontinued as a result
of the 1996 executive order, allowing civilian users to receive a
non-degraded signal globally.
In 2004, the United States government signed an agreement with the
European Community establishing cooperation related to GPS and Europe's Galileo system.
In 2004, United States President George W. Bush
updated the national policy and replaced the executive board with the
National Executive Committee for Space-Based Positioning, Navigation,
and Timing.
In 2005, the first modernized GPS satellite was launched and began
transmitting a second civilian signal (L2C) for enhanced user
performance.
On September 14, 2007, the aging mainframe-based Ground Segment Control System was transferred to the new Architecture Evolution Plan.
On May 19, 2009, the United States Government Accountability Office issued a report warning that some GPS satellites could fail as soon as 2010.
On May 21, 2009, the Air Force Space Command allayed fears of GPS failure, saying: "There's only a small risk we will not continue to exceed our performance standard."
On January 11, 2010, an update of ground control systems caused a
software incompatibility with 8,000 to 10,000 military receivers
manufactured by a division of Trimble Navigation Limited of Sunnyvale,
Calif.
On February 25, 2010, the U.S. Air Force awarded the contract to Raytheon Company
to develop the GPS Next Generation Operational Control System (OCX) to
improve accuracy and availability of GPS navigation signals, and serve
as a critical part of GPS modernization.
Awards
AFSPC
Vice Commander Lt. Gen. DT Thompson presents Gladys West with an award
as she is inducted into the Air Force Space and Missile Pioneers Hall of
Fame.
On February 10, 1993, the National Aeronautic Association selected the GPS Team as winners of the 1992 Robert J. Collier Trophy, the US's most prestigious aviation award. This team combines researchers from the Naval Research Laboratory, the USAF, the Aerospace Corporation, Rockwell International Corporation, and IBM
Federal Systems Company. The citation honors them "for the most
significant development for safe and efficient navigation and
surveillance of air and spacecraft since the introduction of radio
navigation 50 years ago".
Ivan Getting, emeritus president of The Aerospace Corporation and an engineer at MIT, established the basis for GPS, improving on the World War II land-based radio system called LORAN (Long-range Radio Aid to Navigation).
Bradford Parkinson, professor of aeronautics and astronautics at Stanford University,
conceived the present satellite-based system in the early 1960s and
developed it in conjunction with the U.S. Air Force. Parkinson served
twenty-one years in the Air Force, from 1957 to 1978, and retired with
the rank of colonel.
Francis X. Kane
(Col. USAF, ret.) was inducted into the U.S. Air Force Space and
Missile Pioneers Hall of Fame at Lackland A.F.B., San Antonio, Texas,
March 2, 2010, for his role in space technology development and the
engineering design concept of GPS conducted as part of Project 621B.
On October 4, 2011, the International Astronautical Federation
(IAF) awarded the Global Positioning System (GPS) its 60th Anniversary
Award, nominated by IAF member, the American Institute for Aeronautics
and Astronautics (AIAA). The IAF Honors and Awards Committee recognized
the uniqueness of the GPS program and the exemplary role it has played
in building international collaboration for the benefit of humanity.
On December 6, 2018, Gladys West was inducted into the Air Force
Space and Missile Pioneers Hall of Fame in recognition of her work on an
extremely accurate geodetic Earth model, which was ultimately used to
determine the orbit of the GPS constellation.
On February 12, 2019, four founding members of the project were
awarded the Queen Elizabeth Prize for Engineering with the chair of the
awarding board stating: "Engineering is the foundation of civilisation;
there is no other foundation; it makes things happen. And that's exactly
what today's Laureates have done – they've made things happen. They've
re-written, in a major way, the infrastructure of our world."
Principles
The GPS satellites carry very stable atomic clocks
that are synchronized with one another and with the reference atomic
clocks at the ground control stations; any drift of the clocks aboard
the satellites from the reference time maintained on the ground stations
is corrected regularly. Since the speed of radio waves (speed of light)
is constant and independent of the satellite speed, the time delay
between when the satellite transmits a signal and the ground station
receives it is proportional to the distance from the satellite to the
ground station. With the distance information collected from multiple
ground stations, the location coordinates of any satellite at any time
can be calculated with great precision.
Each GPS satellite carries an accurate record of its own position
and time, and broadcasts that data continuously. Based on data received
from multiple GPS satellites, an end user's GPS receiver can calculate its own four-dimensional position in spacetime;
However, at a minimum, four satellites must be in view of the receiver
for it to compute four unknown quantities (three position coordinates
and the deviation of its own clock from satellite time).
More detailed description
Each GPS satellite continually broadcasts a signal (carrier wave with modulation) that includes:
A pseudorandom
code (sequence of ones and zeros) that is known to the receiver. By
time-aligning a receiver-generated version and the receiver-measured
version of the code, the time of arrival (TOA) of a defined point in the
code sequence, called an epoch, can be found in the receiver clock time
scale
A message that includes the time of transmission (TOT) of the code
epoch (in GPS time scale) and the satellite position at that time
Conceptually, the receiver measures the TOAs (according to its own
clock) of four satellite signals. From the TOAs and the TOTs, the
receiver forms four time of flight
(TOF) values, which are (given the speed of light) approximately
equivalent to receiver-satellite ranges plus time difference between the
receiver and GPS satellites multiplied by speed of light, which are
called pseudo-ranges. The receiver then computes its three-dimensional
position and clock deviation from the four TOFs.
In practice the receiver position (in three dimensional Cartesian coordinates
with origin at the Earth's center) and the offset of the receiver clock
relative to the GPS time are computed simultaneously, using the navigation equations to process the TOFs.
The receiver's Earth-centered solution location is usually converted to latitude, longitude and height relative to an ellipsoidal Earth model. The height may then be further converted to height relative to the geoid, which is essentially mean sea level. These coordinates may be displayed, such as on a moving map display, or recorded or used by some other system, such as a vehicle guidance system.
Although usually not formed explicitly in the receiver processing,
the conceptual time differences of arrival (TDOAs) define the
measurement geometry. Each TDOA corresponds to a hyperboloid of revolution (see Multilateration).
The line connecting the two satellites involved (and its extensions)
forms the axis of the hyperboloid. The receiver is located at the point
where three hyperboloids intersect.
It is sometimes incorrectly said that the user location is at the
intersection of three spheres. While simpler to visualize, this is the
case only if the receiver has a clock synchronized with the satellite
clocks (i.e., the receiver measures true ranges to the satellites rather
than range differences). There are marked performance benefits to the
user carrying a clock synchronized with the satellites. Foremost is that
only three satellites are needed to compute a position solution. If it
were an essential part of the GPS concept that all users needed to carry
a synchronized clock, a smaller number of satellites could be deployed,
but the cost and complexity of the user equipment would increase.
Receiver in continuous operation
The description above is representative of a receiver start-up situation. Most receivers have a track algorithm, sometimes called a tracker,
that combines sets of satellite measurements collected at different
times—in effect, taking advantage of the fact that successive receiver
positions are usually close to each other. After a set of measurements
are processed, the tracker predicts the receiver location corresponding
to the next set of satellite measurements. When the new measurements are
collected, the receiver uses a weighting scheme to combine the new
measurements with the tracker prediction. In general, a tracker can (a)
improve receiver position and time accuracy, (b) reject bad
measurements, and (c) estimate receiver speed and direction.
The disadvantage of a tracker is that changes in speed or
direction can be computed only with a delay, and that derived direction
becomes inaccurate when the distance traveled between two position
measurements drops below or near the random error of position measurement. GPS units can use measurements of the Doppler shift of the signals received to compute velocity accurately. More advanced navigation systems use additional sensors like a compass or an inertial navigation system to complement GPS.
GPS requires four or more satellites to be visible for accurate navigation. The solution of the navigation equations
gives the position of the receiver along with the difference between
the time kept by the receiver's on-board clock and the true time-of-day,
thereby eliminating the need for a more precise and possibly
impractical receiver based clock. Applications for GPS such as time transfer, traffic signal timing, and synchronization of cell phone base stations, make use of
this cheap and highly accurate timing. Some GPS applications use this
time for display, or, other than for the basic position calculations, do
not use it at all.
Although four satellites are required for normal operation, fewer
apply in special cases. If one variable is already known, a receiver
can determine its position using only three satellites. For example, a
ship on the open ocean usually has a known elevation close to 0m, and the elevation of an aircraft may be known. Some GPS receivers may use additional clues or assumptions such as reusing the last known altitude, dead reckoning, inertial navigation,
or including information from the vehicle computer, to give a (possibly
degraded) position when fewer than four satellites are visible.
Structure
The current GPS consists of three major segments. These are the space segment, a control segment, and a user segment. The U.S. Space Force develops, maintains, and operates the space and control segments. GPS satellites broadcast signals
from space, and each GPS receiver uses these signals to calculate its
three-dimensional location (latitude, longitude, and altitude) and the
current time.
Unlaunched GPS block II-A satellite on display at the San Diego Air & Space MuseumA visual example of a 24-satellite GPS constellation in motion with the Earth rotating. Notice how the number of satellites in view from a given point on the Earth's surface changes with time. The point in this example is in Golden, Colorado, USA (39.7469°N 105.2108°W).
The space segment (SS) is composed of 24 to 32 satellites, or Space Vehicles (SV), in medium Earth orbit,
and also includes the payload adapters to the boosters required to
launch them into orbit. The GPS design originally called for 24 SVs,
eight each in three approximately circular orbits, but this was modified to six orbital planes with four satellites each. The six orbit planes have approximately 55° inclination (tilt relative to the Earth's equator) and are separated by 60° right ascension of the ascending node (angle along the equator from a reference point to the orbit's intersection). The orbital period is one-half of a sidereal day, i.e., 11 hours and 58 minutes, so that the satellites pass over the same locations or almost the same locations every day. The orbits are arranged so that at least six satellites are always within line of sight from everywhere on the Earth's surface (see animation at right).
The result of this objective is that the four satellites are not evenly
spaced (90°) apart within each orbit. In general terms, the angular
difference between satellites in each orbit is 30°, 105°, 120°, and 105°
apart, which sum to 360°.
Orbiting at an altitude of approximately 20,200 km (12,600 mi); orbital radius of approximately 26,600 km (16,500 mi), each SV makes two complete orbits each sidereal day, repeating the same ground track each day.
This was very helpful during development because even with only four
satellites, correct alignment means all four are visible from one spot
for a few hours each day. For military operations, the ground track
repeat can be used to ensure good coverage in combat zones.
As of February 2019, there are 31 satellites in the GPS constellation,
27 of which are in use at a given time with the rest allocated as
stand-bys. A 32nd was launched in 2018, but as of July 2019 is still in
evaluation. More decommissioned satellites are in orbit and available as
spares. The additional satellites improve the precision of GPS receiver
calculations by providing redundant measurements. With the increased
number of satellites, the constellation was changed to a nonuniform
arrangement. Such an arrangement was shown to improve accuracy but also
improves reliability and availability of the system, relative to a
uniform system, when multiple satellites fail.
With the expanded constellation, nine satellites are usually visible at
any time from any point on the Earth with a clear horizon, ensuring
considerable redundancy over the minimum four satellites needed for a
position.
The MCS can also access Satellite Control Network (SCN) ground antennas (for additional command and control capability) and NGA (National Geospatial-Intelligence Agency) monitor stations. The flight paths of the satellites are tracked by dedicated U.S. Space Force monitoring stations in Hawaii, Kwajalein Atoll, Ascension Island, Diego Garcia, Colorado Springs, Colorado and Cape Canaveral, along with shared NGA monitor stations operated in England, Argentina, Ecuador, Bahrain, Australia and Washington DC. The tracking information is sent to the MCS at Schriever Space Force Base 25 km (16 mi) ESE of Colorado Springs, which is operated by the 2nd Space Operations Squadron
(2 SOPS) of the U.S. Space Force. Then 2 SOPS contacts each GPS
satellite regularly with a navigational update using dedicated or shared
(AFSCN) ground antennas (GPS dedicated ground antennas are located at Kwajalein, Ascension Island, Diego Garcia, and Cape Canaveral). These updates synchronize the atomic clocks on board the satellites to within a few nanoseconds of each other, and adjust the ephemeris of each satellite's internal orbital model. The updates are created by a Kalman filter that uses inputs from the ground monitoring stations, space weather information, and various other inputs.
When a satellite's orbit is being adjusted, the satellite is marked unhealthy,
so receivers do not use it. After the maneuver, engineers track the new
orbit from the ground, upload the new ephemeris, and mark the satellite
healthy again.
The operation control segment (OCS) currently serves as the
control segment of record. It provides the operational capability that
supports GPS users and keeps the GPS operational and performing within
specification.
OCS successfully replaced the legacy 1970s-era mainframe computer
at Schriever Air Force Base in September 2007. After installation, the
system helped enable upgrades and provide a foundation for a new
security architecture that supported U.S. armed forces.
OCS will continue to be the ground control system of record until the new segment, Next Generation GPS Operation Control System
(OCX), is fully developed and functional. The new capabilities provided
by OCX will be the cornerstone for revolutionizing GPS's mission
capabilities, enabling theU.S. Space Force to greatly enhance GPS operational services to U.S.
combat forces, civil partners and myriad domestic and international
users. The GPS OCX program also will reduce cost, schedule and technical
risk. It is designed to provide 50%
sustainment cost savings through efficient software architecture and
Performance-Based Logistics. In addition, GPS OCX is expected to cost
millions less than the cost to upgrade OCS while providing four times
the capability.
The GPS OCX program represents a critical part of GPS
modernization and provides significant information assurance
improvements over the current GPS OCS program.
OCX will have the ability to control and manage GPS legacy
satellites as well as the next generation of GPS III satellites, while
enabling the full array of military signals.
Built on a flexible architecture that can rapidly adapt to the
changing needs of today's and future GPS users allowing immediate access
to GPS data and constellation status through secure, accurate and
reliable information.
Provides the warfighter with more secure, actionable and predictive information to enhance situational awareness.
Enables new modernized signals (L1C, L2C, and L5) and has M-code capability, which the legacy system is unable to do.
Provides significant information assurance improvements over the
current program including detecting and preventing cyber attacks, while
isolating, containing and operating during such attacks.
Supports higher volume near real-time command and control capabilities and abilities.
On September 14, 2011,
the U.S. Air Force announced the completion of GPS OCX Preliminary
Design Review and confirmed that the OCX program is ready for the next
phase of development.
The GPS OCX program has missed major milestones and pushed its
launch into 2021, 5 years past the original deadline. According to the
Government Accounting Office in 2019, the 2021 deadline looked shaky.
GPS
receivers come in a variety of formats, from devices integrated into
cars, phones, and watches, to dedicated devices such as these.The first portable GPS unit, a Leica WM 101, displayed at the Irish National Science Museum at Maynooth
The user segment (US) is composed of hundreds of thousands of U.S.
and allied military users of the secure GPS Precise Positioning Service,
and tens of millions of civil, commercial and scientific users of the
Standard Positioning Service. In general, GPS receivers are composed of
an antenna, tuned to the frequencies transmitted by the satellites,
receiver-processors, and a highly stable clock (often a crystal oscillator). They may also include a display for providing location and speed information to the user.
GPS receivers may include an input for differential corrections, using the RTCM SC-104 format. This is typically in the form of an RS-232 port at 4,800 bit/s speed. Data is actually sent at a much lower rate, which limits the accuracy of the signal sent using RTCM. Receivers with internal DGPS receivers can outperform those using external RTCM data. As of 2006, even low-cost units commonly include Wide Area Augmentation System (WAAS) receivers.
A typical GPS receiver with integrated antenna
Many GPS receivers can relay position data to a PC or other device using the NMEA 0183 protocol. Although this protocol is officially defined by the National Marine Electronics Association (NMEA), references to this protocol have been compiled from public records, allowing open source tools like gpsd to read the protocol without violating intellectual property laws. Other proprietary protocols exist as well, such as the SiRF and MTK protocols. Receivers can interface with other devices using methods including a serial connection, USB, or Bluetooth.
While originally a military project, GPS is considered a dual-use technology, meaning it has significant civilian applications as well.
GPS has become a widely deployed and useful tool for commerce,
scientific uses, tracking, and surveillance. GPS's accurate time
facilitates everyday activities such as banking, mobile phone
operations, and even the control of power grids by allowing well
synchronized hand-off switching.
Civilian
This antenna is mounted on the roof of a hut containing a scientific experiment needing precise timing.
Many civilian applications use one or more of GPS's three basic
components: absolute location, relative movement, and time transfer.
Amateur radio: clock synchronization required for several digital modes such as FT8, FT4 and JS8; also used with APRS for position reporting; is often critical during emergency and disaster communications support.
Atmosphere: studying the troposphere delays (recovery of the water vapor content) and ionosphere delays (recovery of the number of free electrons). Recovery of Earth surface displacements due to the atmospheric pressure loading.
Automated vehicle: applying location and routes for cars and trucks to function without a human driver.
Cartography: both civilian and military cartographers use GPS extensively.
Cellular telephony:
clock synchronization enables time transfer, which is critical for
synchronizing its spreading codes with other base stations to facilitate
inter-cell handoff and support hybrid GPS/cellular position detection
for mobile emergency calls and other applications. The first handsets with integrated GPS launched in the late 1990s. The U.S. Federal Communications Commission
(FCC) mandated the feature in either the handset or in the towers (for
use in triangulation) in 2002 so emergency services could locate
911 callers. Third-party software developers later gained access to GPS
APIs from Nextel upon launch, followed by Sprint in 2006, and Verizon soon thereafter.
GPS-equipped radiosondes and dropsondes: measure and calculate the atmospheric pressure, wind speed and direction up to 27 km (89,000 ft) from the Earth's surface.
Fleet tracking: used to identify, locate and maintain contact reports with one or more fleet vehicles in real-time.
Geodesy: determination of Earth orientation parameters including the daily and sub-daily polar motion, and length-of-day variabilities, Earth's center-of-mass - geocenter motion, and low-degree gravity field parameters.
Geofencing: vehicle tracking systems, person tracking systems, and pet tracking
systems use GPS to locate devices that are attached to or carried by a
person, vehicle, or pet. The application can provide continuous tracking
and send notifications if the target leaves a designated (or
"fenced-in") area.
Geotagging: applies location coordinates to digital objects such as photographs (in Exif data) and other documents for purposes such as creating map overlays with devices like Nikon GP-1
GPS for mining:
the use of RTK GPS has significantly improved several mining operations
such as drilling, shoveling, vehicle tracking, and surveying. RTK GPS
provides centimeter-level positioning accuracy.
GPS data mining:
It is possible to aggregate GPS data from multiple users to understand
movement patterns, common trajectories and interesting locations.
GPS tours: location determines what content to display; for instance, information about an approaching point of interest.
Mental health: tracking mental health functioning and sociability.
Navigation:
navigators value digitally precise velocity and orientation
measurements, as well as precise positions in real-time with a support
of orbit and clock corrections.
Reference frames: realization and densification of the terrestrial reference frames in the framework of Global Geodetic Observing System. Co-location in space between Satellite laser ranging and microwave observations for deriving global geodetic parameters.
Robotics: self-navigating, autonomous robots using GPS sensors, which calculate latitude, longitude, time, speed, and heading.
Sport: used in football and rugby for the control and analysis of the training load.
Surveying: surveyors use absolute locations to make maps and determine property boundaries.
Tectonics: GPS enables direct fault motion measurement of earthquakes. Between earthquakes GPS can be used to measure crustal motion and deformation to estimate seismic strain buildup for creating seismic hazard maps.
The
U.S. government controls the export of some civilian receivers. All GPS
receivers capable of functioning above 60,000 ft (18 km) above sea
level and 1,000 kn (500 m/s; 2,000 km/h; 1,000 mph), or designed or
modified for use with unmanned missiles and aircraft, are classified as munitions (weapons)—which means they require State Department export licenses.
This rule applies even to otherwise purely civilian units that only
receive the L1 frequency and the C/A (Coarse/Acquisition) code.
Disabling operation above these limits exempts the receiver from
classification as a munition. Vendor interpretations differ. The rule
refers to operation at both the target altitude and speed, but some
receivers stop operating even when stationary. This has caused problems
with some amateur radio balloon launches that regularly reach 30 km
(100,000 feet).
These limits only apply to units or components exported from the
United States. A growing trade in various components exists, including
GPS units from other countries. These are expressly sold as ITAR-free.
Navigation: Soldiers use GPS to find objectives, even in the
dark or in unfamiliar territory, and to coordinate troop and supply
movement. In the United States armed forces, commanders use the Commander's Digital Assistant and lower ranks use the Soldier Digital Assistant.
Target tracking: Various military weapons systems use GPS to track
potential ground and air targets before flagging them as hostile. These weapon systems pass target coordinates to precision-guided munitions to allow them to engage targets accurately. Military aircraft, particularly in air-to-ground roles, use GPS to find targets.
Missile and projectile guidance: GPS allows accurate targeting of various military weapons including ICBMs, cruise missiles, precision-guided munitions and artillery shells. Embedded GPS receivers able to withstand accelerations of 12,000 g or about 118 km/s2 (260,000 mph/s) have been developed for use in 155-millimeter (6.1 in) howitzer shells.
Search and rescue.
Reconnaissance: Patrol movement can be managed more closely.
GPS satellites carry a set of nuclear detonation detectors consisting of an optical sensor called a bhangmeter, an X-ray sensor, a dosimeter, and an electromagnetic pulse (EMP) sensor (W-sensor), that form a major portion of the United States Nuclear Detonation Detection System.General William Shelton has stated that future satellites may drop this feature to save money.
GPS type navigation was first used in war in the 1991 Persian Gulf War, before GPS was fully developed in 1995, to assist Coalition Forces to navigate and perform maneuvers in the war. The war also demonstrated the vulnerability of GPS to being jammed,
when Iraqi forces installed jamming devices on likely targets that
emitted radio noise, disrupting reception of the weak GPS signal.
GPS's vulnerability to jamming is a threat that continues to grow as jamming equipment and experience grows.
GPS signals have been reported to have been jammed many times over the
years for military purposes. Russia seems to have several objectives for
this behavior, such as intimidating neighbors while undermining
confidence in their reliance on American systems, promoting their
GLONASS alternative, disrupting Western military exercises, and
protecting assets from drones. China uses jamming to discourage US surveillance aircraft near the contested Spratly Islands. North Korea
has mounted several major jamming operations near its border with South
Korea and offshore, disrupting flights, shipping and fishing
operations. Iranian Armed Forces disrupted the civilian airliner plane Flight PS752's GPS when it shot down the aircraft.
Timekeeping
Leap seconds
While most clocks derive their time from Coordinated Universal Time (UTC), the atomic clocks on the satellites are set to GPS time. The difference is that GPS time is not corrected to match the rotation of the Earth, so it does not contain new leap seconds
or other corrections that are periodically added to UTC. GPS time was
set to match UTC in 1980, but has since diverged. The lack of
corrections means that GPS time remains at a constant offset with International Atomic Time
(TAI) (TAI - GPS = 19 seconds). Periodic corrections are performed to
the on-board clocks to keep them synchronized with ground clocks.
The GPS navigation message includes the difference between GPS time and UTC. As of January 2017, GPS time is 18 seconds ahead of UTC because of the leap second added to UTC on December 31, 2016.
Receivers subtract this offset from GPS time to calculate UTC and
specific time zone values. New GPS units may not show the correct UTC
time until after receiving the UTC offset message. The GPS-UTC offset
field can accommodate 255 leap seconds (eight bits).
Accuracy
GPS time is theoretically accurate to about 14 nanoseconds, due to the clock drift relative to International Atomic Time that the atomic clocks in GPS transmitters experience Most receivers lose some accuracy in their interpretation of the signals and are only accurate to about 100 nanoseconds.
Relativistic corrections
The
GPS implements two major corrections to its time signals for
relativistic effects: one for relative velocity of satellite and
receiver, using the special theory of relativity, and one for the
difference in gravitational potential between satellite and receiver,
using general relativity. The acceleration of the satellite could also
be computed independently as a correction, depending on purpose, but
normally the effect is already dealt with in the first two corrections.
As opposed to the year, month, and day format of the Gregorian calendar, the GPS date is expressed as a week number and a seconds-into-week number. The week number is transmitted as a ten-bit
field in the C/A and P(Y) navigation messages, and so it becomes zero
again every 1,024 weeks (19.6 years). GPS week zero started at
00:00:00 UTC (00:00:19 TAI) on January 6, 1980, and the week number
became zero again for the first time at 23:59:47 UTC on August 21, 1999
(00:00:19 TAI on August 22, 1999). It happened the second time at
23:59:42 UTC on April 6, 2019. To determine the current Gregorian date, a
GPS receiver must be provided with the approximate date (to within
3,584 days) to correctly translate the GPS date signal. To address this
concern in the future the modernized GPS civil navigation (CNAV) message
will use a 13-bit field that only repeats every 8,192 weeks
(157 years), thus lasting until 2137 (157 years after GPS week zero).
The navigational signals transmitted by GPS satellites encode a
variety of information including satellite positions, the state of the
internal clocks, and the health of the network. These signals are
transmitted on two separate carrier frequencies that are common to all
satellites in the network. Two different encodings are used: a public
encoding that enables lower resolution navigation, and an encrypted
encoding used by the U.S. military.
Each GPS satellite continuously broadcasts a navigation message on L1 (C/A and P/Y) and L2 (P/Y) frequencies at a rate of 50 bits per second (see bitrate). Each complete message takes 750 seconds (12+1⁄2
minutes) to complete. The message structure has a basic format of a
1500-bit-long frame made up of five subframes, each subframe being 300
bits (6 seconds) long. Subframes 4 and 5 are subcommutated
25 times each, so that a complete data message requires the
transmission of 25 full frames. Each subframe consists of ten words,
each 30 bits long. Thus, with 300 bits in a subframe times 5 subframes
in a frame times 25 frames in a message, each message is 37,500 bits
long. At a transmission rate of 50-bit/s, this gives 750 seconds to
transmit an entire almanac message (GPS). Each 30-second frame begins precisely on the minute or half-minute as indicated by the atomic clock on each satellite.
The first subframe of each frame encodes the week number and the time within the week, as well as the data about the health of the satellite. The second and the third subframes contain the ephemeris – the precise orbit for the satellite. The fourth and fifth subframes contain the almanac,
which contains coarse orbit and status information for up to 32
satellites in the constellation as well as data related to error
correction. Thus, to obtain an accurate satellite location from this
transmitted message, the receiver must demodulate the message from each
satellite it includes in its solution for 18 to 30 seconds. To collect
all transmitted almanacs, the receiver must demodulate the message for
732 to 750 seconds or 12+1⁄2 minutes.
All satellites broadcast at the same frequencies, encoding signals using unique code-division multiple access
(CDMA) so receivers can distinguish individual satellites from each
other. The system uses two distinct CDMA encoding types: the
coarse/acquisition (C/A) code, which is accessible by the general
public, and the precise (P(Y)) code, which is encrypted so that only the
U.S. military and other NATO nations who have been given access to the
encryption code can access it.
The ephemeris is updated every 2 hours and is sufficiently stable
for 4 hours, with provisions for updates every 6 hours or longer in
non-nominal conditions. The almanac is updated typically every 24 hours.
Additionally, data for a few weeks following is uploaded in case of
transmission updates that delay data upload.
Satellite frequencies
GPS frequency overview
Band
Frequency
Description
L1
1575.42 MHz
Coarse-acquisition (C/A) and encrypted precision (P(Y)) codes, plus the L1 civilian (L1C) and military (M) codes on Block III and newer satellites.
L2
1227.60 MHz
P(Y) code, plus the L2C and military codes on the Block IIR-M and newer satellites.
L3
1381.05 MHz
Used for nuclear detonation (NUDET) detection.
L4
1379.913 MHz
Being studied for additional ionospheric correction.
L5
1176.45 MHz
Used as a civilian safety-of-life (SoL) signal on Block IIF and newer satellites.
All satellites broadcast at the same two frequencies, 1.57542 GHz (L1
signal) and 1.2276 GHz (L2 signal). The satellite network uses a CDMA
spread-spectrum technique where the low-bitrate message data is encoded with a high-rate pseudo-random
(PRN) sequence that is different for each satellite. The receiver must
be aware of the PRN codes for each satellite to reconstruct the actual
message data. The C/A code, for civilian use, transmits data at
1.023 million chips
per second, whereas the P code, for U.S. military use, transmits at
10.23 million chips per second. The actual internal reference of the
satellites is 10.22999999543 MHz to compensate for relativistic effects
that make observers on the Earth perceive a different time reference
with respect to the transmitters in orbit. The L1 carrier is modulated
by both the C/A and P codes, while the L2 carrier is only modulated by
the P code.
The P code can be encrypted as a so-called P(Y) code that is only
available to military equipment with a proper decryption key. Both the
C/A and P(Y) codes impart the precise time-of-day to the user.
The L3 signal at a frequency of 1.38105 GHz is used to transmit
data from the satellites to ground stations. This data is used by the
United States Nuclear Detonation (NUDET) Detection System (USNDS) to
detect, locate, and report nuclear detonations (NUDETs) in the Earth's
atmosphere and near space. One usage is the enforcement of nuclear test ban treaties.
The L4 band at 1.379913 GHz is being studied for additional ionospheric correction.
The L5 frequency band at 1.17645 GHz was added in the process of GPS modernization.
This frequency falls into an internationally protected range for
aeronautical navigation, promising little or no interference under all
circumstances. The first Block IIF satellite that provides this signal
was launched in May 2010. On February 5, 2016, the 12th and final Block IIF satellite was launched.
The L5 consists of two carrier components that are in phase quadrature
with each other. Each carrier component is bi-phase shift key (BPSK)
modulated by a separate bit train. "L5, the third civil GPS signal, will
eventually support safety-of-life applications for aviation and provide
improved availability and accuracy."
In 2011, a conditional waiver was granted to LightSquared
to operate a terrestrial broadband service near the L1 band. Although
LightSquared had applied for a license to operate in the 1525 to 1559
band as early as 2003 and it was put out for public comment, the FCC
asked LightSquared to form a study group with the GPS community to test
GPS receivers and identify issues that might arise due to the larger
signal power from the LightSquared terrestrial network. The GPS
community had not objected to the LightSquared (formerly MSV and
SkyTerra) applications until November 2010, when LightSquared applied
for a modification to its Ancillary Terrestrial Component (ATC)
authorization. This filing (SAT-MOD-20101118-00239) amounted to a
request to run several orders of magnitude more power in the same
frequency band for terrestrial base stations, essentially repurposing
what was supposed to be a "quiet neighborhood" for signals from space as
the equivalent of a cellular network. Testing in the first half of 2011
has demonstrated that the impact of the lower 10 MHz of spectrum is
minimal to GPS devices (less than 1% of the total GPS devices are
affected). The upper 10 MHz intended for use by LightSquared may have
some impact on GPS devices. There is some concern that this may
seriously degrade the GPS signal for many consumer uses. Aviation Week magazine reports that the latest testing (June 2011) confirms "significant jamming" of GPS by LightSquared's system.
Demodulation and decoding
Demodulating and Decoding GPS Satellite Signals using the Coarse/Acquisition Gold code
Because all of the satellite signals are modulated onto the same L1
carrier frequency, the signals must be separated after demodulation.
This is done by assigning each satellite a unique binary sequence known as a Gold code.
The signals are decoded after demodulation using addition of the Gold
codes corresponding to the satellites monitored by the receiver.
If the almanac information has previously been acquired, the
receiver picks the satellites to listen for by their PRNs, unique
numbers in the range 1 through 32. If the almanac information is not in
memory, the receiver enters a search mode until a lock is obtained on
one of the satellites. To obtain a lock, it is necessary that there be
an unobstructed line of sight from the receiver to the satellite. The
receiver can then acquire the almanac and determine the satellites it
should listen for. As it detects each satellite's signal, it identifies
it by its distinct C/A code pattern. There can be a delay of up to
30 seconds before the first estimate of position because of the need to
read the ephemeris data.
Processing of the navigation message enables the determination of
the time of transmission and the satellite position at this time. For
more information see Demodulation and Decoding, Advanced.
The receiver uses messages received from satellites to determine the satellite positions and time sent. The x, y, and z components of satellite position and the time sent (s) are designated as [xi, yi, zi, si] where the subscript i denotes the satellite and has the value 1, 2, ..., n, where n ≥ 4. When the time of message reception indicated by the on-board receiver clock is , the true reception time is , where b
is the receiver's clock bias from the much more accurate GPS clocks
employed by the satellites. The receiver clock bias is the same for all
received satellite signals (assuming the satellite clocks are all
perfectly synchronized). The message's transit time is , where si is the satellite time. Assuming the message traveled at the speed of light, c, the distance traveled is .
For n satellites, the equations to satisfy are:
where di is the geometric distance or range between receiver and satellite i (the values without subscripts are the x, y, and z components of receiver position):
Defining pseudoranges as , we see they are biased versions of the true range:
.
Since the equations have four unknowns [x, y, z, b]—the three
components of GPS receiver position and the clock bias—signals from at
least four satellites are necessary to attempt solving these equations.
They can be solved by algebraic or numerical methods. Existence and
uniqueness of GPS solutions are discussed by Abell and Chaffee. When n is greater than four, this system is overdetermined and a fitting method must be used.
The amount of error in the results varies with the received
satellites' locations in the sky, since certain configurations (when the
received satellites are close together in the sky) cause larger errors.
Receivers usually calculate a running estimate of the error in the
calculated position. This is done by multiplying the basic resolution of
the receiver by quantities called the geometric dilution of position (GDOP) factors, calculated from the relative sky directions of the satellites used. The receiver location is expressed in a specific coordinate system, such as latitude and longitude using the WGS 84geodetic datum or a country-specific system.
Geometric interpretation
The
GPS equations can be solved by numerical and analytical methods.
Geometrical interpretations can enhance the understanding of these
solution methods.
The measured ranges, called pseudoranges, contain clock errors. In a
simplified idealization in which the ranges are synchronized, these true
ranges represent the radii of spheres, each centered on one of the
transmitting satellites. The solution for the position of the receiver
is then at the intersection of the surfaces of these spheres; see trilateration
(more generally, true-range multilateration). Signals from at minimum
three satellites are required, and their three spheres would typically
intersect at two points.
One of the points is the location of the receiver, and the other moves
rapidly in successive measurements and would not usually be on Earth's
surface.
In practice, there are many sources of inaccuracy besides clock
bias, including random errors as well as the potential for precision
loss from subtracting numbers close to each other if the centers of the
spheres are relatively close together. This means that the position
calculated from three satellites alone is unlikely to be accurate
enough. Data from more satellites can help because of the tendency for
random errors to cancel out and also by giving a larger spread between
the sphere centers. But at the same time, more spheres will not
generally intersect at one point. Therefore, a near intersection gets
computed, typically via least squares. The more signals available, the
better the approximation is likely to be.
Hyperboloids
Three
satellites (labeled as "stations" A, B, C) have known locations. The
true times it takes for a radio signal to travel from each satellite to
the receiver are unknown, but the true time differences are known. Then,
each time difference locates the receiver on a branch of a hyperbola
focused on the satellites. The receiver is then located at one of the
two intersections.
If the pseudorange between the receiver and satellite i and the pseudorange between the receiver and satellite j are subtracted, pi − pj, the common receiver clock bias (b) cancels out, resulting in a difference of distances di − dj. The locus of points having a constant difference in distance to two points (here, two satellites) is a hyperbola on a plane and a hyperboloid of revolution (more specifically, a two-sheeted hyperboloid) in 3D space (see Multilateration).
Thus, from four pseudorange measurements, the receiver can be placed at
the intersection of the surfaces of three hyperboloids each with foci
at a pair of satellites. With additional satellites, the multiple
intersections are not necessarily unique, and a best-fitting solution is
sought instead.
Inscribed sphere
A smaller circle (red) inscribed and tangent to other circles (black), that need not necessarily be mutually tangent
The receiver position can be interpreted as the center of an inscribed sphere (insphere) of radius bc, given by the receiver clock bias b (scaled by the speed of light c). The insphere location is such that it touches other spheres. The circumscribing spheres are centered at the GPS satellites, whose radii equal the measured pseudoranges pi.
This configuration is distinct from the one described above, in which
the spheres' radii were the unbiased or geometric ranges di.
Hypercones
The
clock in the receiver is usually not of the same quality as the ones in
the satellites and will not be accurately synchronized to them. This
produces pseudoranges
with large differences compared to the true distances to the
satellites. Therefore, in practice, the time difference between the
receiver clock and the satellite time is defined as an unknown clock
bias b. The equations are then solved simultaneously for the receiver position and the clock bias. The solution space [x, y, z, b] can be seen as a four-dimensional spacetime, and signals from at minimum four satellites are needed. In that case each of the equations describes a hypercone (or spherical cone),
with the cusp located at the satellite, and the base a sphere around
the satellite. The receiver is at the intersection of four or more of
such hypercones.
Solution methods
Least squares
When
more than four satellites are available, the calculation can use the
four best, or more than four simultaneously (up to all visible
satellites), depending on the number of receiver channels, processing
capability, and geometric dilution of precision (GDOP).
Using more than four involves an over-determined system of equations with no unique solution; such a system can be solved by a least-squares or weighted least squares method.
Iterative
Both
the equations for four satellites, or the least squares equations for
more than four, are non-linear and need special solution methods. A
common approach is by iteration on a linearized form of the equations,
such as the Gauss–Newton algorithm.
The GPS was initially developed assuming use of a numerical
least-squares solution method—i.e., before closed-form solutions were
found.
Closed-form
One closed-form solution to the above set of equations was developed by S. Bancroft. Its properties are well known; in particular, proponents claim it is superior in low-GDOP situations, compared to iterative least squares methods.
Bancroft's method is algebraic, as opposed to numerical, and can
be used for four or more satellites. When four satellites are used, the
key steps are inversion of a 4x4 matrix and solution of a
single-variable quadratic equation. Bancroft's method provides one or
two solutions for the unknown quantities. When there are two (usually
the case), only one is a near-Earth sensible solution.
When a receiver uses more than four satellites for a solution, Bancroft uses the generalized inverse
(i.e., the pseudoinverse) to find a solution. A case has been made that
iterative methods, such as the Gauss–Newton algorithm approach for
solving over-determined non-linear least squares problems, generally provide more accurate solutions.
Leick et al. (2015) states that "Bancroft's (1985) solution is a very early, if not the first, closed-form solution."
Other closed-form solutions were published afterwards, although their adoption in practice is unclear.
GPS error analysis examines error sources in GPS results and the
expected size of those errors. GPS makes corrections for receiver clock
errors and other effects, but some residual errors remain uncorrected.
Error sources include signal arrival time measurements, numerical
calculations, atmospheric effects (ionospheric/tropospheric delays), ephemeris
and clock data, multipath signals, and natural and artificial
interference. Magnitude of residual errors from these sources depends on
geometric dilution of precision. Artificial errors may result from
jamming devices and threaten ships and aircraft
or from intentional signal degradation through selective availability,
which limited accuracy to ≈ 6–12 m (20–40 ft), but has been switched off
since May 1, 2000.
Accuracy enhancement and surveying
GNSS enhancement
refers to techniques used to improve the accuracy of positioning
information provided by the Global Positioning System or other global navigation satellite systems in general, a network of satellites used for navigation.
Enhancement methods of improving accuracy rely on external information
being integrated into the calculation process. There are many such
systems in place and they are generally named or described based on how
the GPS sensor receives the information. Some systems transmit
additional information about sources of error (such as clock drift,
ephemeris, or ionospheric delay),
others provide direct measurements of how much the signal was off in
the past, while a third group provides additional navigational or
vehicle information to be integrated into the calculation process.
In the United States, GPS receivers are regulated under the Federal Communications Commission's (FCC) Part 15
rules. As indicated in the manuals of GPS-enabled devices sold in the
United States, as a Part 15 device, it "must accept any interference
received, including interference that may cause undesired operation".
With respect to GPS devices in particular, the FCC states that GPS
receiver manufacturers "must use receivers that reasonably discriminate
against reception of signals outside their allocated spectrum".
For the last 30 years, GPS receivers have operated next to the Mobile
Satellite Service band, and have discriminated against reception of
mobile satellite services, such as Inmarsat, without any issue.
The spectrum allocated for GPS L1 use by the FCC is 1559 to
1610 MHz, while the spectrum allocated for satellite-to-ground use owned
by Lightsquared is the Mobile Satellite Service band. Since 1996, the FCC has authorized licensed use of the spectrum neighboring the GPS band of 1525 to 1559 MHz to the Virginia company LightSquared. On March 1, 2001, the FCC received an application from LightSquared's predecessor, Motient Services, to use their allocated frequencies for an integrated satellite-terrestrial service.
In 2002, the U.S. GPS Industry Council came to an out-of-band-emissions
(OOBE) agreement with LightSquared to prevent transmissions from
LightSquared's ground-based stations from emitting transmissions into
the neighboring GPS band of 1559 to 1610 MHz.
In 2004, the FCC adopted the OOBE agreement in its authorization for
LightSquared to deploy a ground-based network ancillary to their
satellite system – known as the Ancillary Tower Components (ATCs) – "We
will authorize MSS ATC subject to conditions that ensure that the added
terrestrial component remains ancillary to the principal MSS offering.
We do not intend, nor will we permit, the terrestrial component to
become a stand-alone service." This authorization was reviewed and approved by the U.S. Interdepartment Radio Advisory Committee, which includes the U.S. Department of Agriculture, U.S. Space Force, U.S. Army, U.S. Coast Guard, Federal Aviation Administration, National Aeronautics and Space Administration (NASA), U.S. Department of the Interior, and U.S. Department of Transportation.
In January 2011, the FCC conditionally authorized LightSquared's wholesale customers—such as Best Buy, Sharp, and C Spire—to
only purchase an integrated satellite-ground-based service from
LightSquared and re-sell that integrated service on devices that are
equipped to only use the ground-based signal using LightSquared's
allocated frequencies of 1525 to 1559 MHz.
In December 2010, GPS receiver manufacturers expressed concerns to the
FCC that LightSquared's signal would interfere with GPS receiver devices
although the FCC's policy considerations leading up to the January 2011
order did not pertain to any proposed changes to the maximum number of
ground-based LightSquared stations or the maximum power at which these
stations could operate. The January 2011 order makes final authorization
contingent upon studies of GPS interference issues carried out by a
LightSquared led working group along with GPS industry and Federal
agency participation. On February 14, 2012, the FCC initiated
proceedings to vacate LightSquared's Conditional Waiver Order based on
the NTIA's conclusion that there was currently no practical way to
mitigate potential GPS interference.
GPS receiver manufacturers design GPS receivers to use spectrum
beyond the GPS-allocated band. In some cases, GPS receivers are designed
to use up to 400 MHz of spectrum in either direction of the L1
frequency of 1575.42 MHz, because mobile satellite services in those
regions are broadcasting from space to ground, and at power levels
commensurate with mobile satellite services.
As regulated under the FCC's Part 15 rules, GPS receivers are not
warranted protection from signals outside GPS-allocated spectrum.
This is why GPS operates next to the Mobile Satellite Service band, and
also why the Mobile Satellite Service band operates next to GPS. The
symbiotic relationship of spectrum allocation ensures that users of both
bands are able to operate cooperatively and freely.
The FCC adopted rules in February 2003 that allowed Mobile
Satellite Service (MSS) licensees such as LightSquared to construct a
small number of ancillary ground-based towers in their licensed spectrum
to "promote more efficient use of terrestrial wireless spectrum".
In those 2003 rules, the FCC stated: "As a preliminary matter,
terrestrial [Commercial Mobile Radio Service ('CMRS')] and MSS ATC are
expected to have different prices, coverage, product acceptance and
distribution; therefore, the two services appear, at best, to be
imperfect substitutes for one another that would be operating in
predominantly different market segments ... MSS ATC is unlikely to
compete directly with terrestrial CMRS for the same customer base...".
In 2004, the FCC clarified that the ground-based towers would be
ancillary, noting: "We will authorize MSS ATC subject to conditions that
ensure that the added terrestrial component remains ancillary to the
principal MSS offering. We do not intend, nor will we permit, the
terrestrial component to become a stand-alone service."
In July 2010, the FCC stated that it expected LightSquared to use its
authority to offer an integrated satellite-terrestrial service to
"provide mobile broadband services similar to those provided by
terrestrial mobile providers and enhance competition in the mobile
broadband sector".
GPS receiver manufacturers have argued that LightSquared's licensed
spectrum of 1525 to 1559 MHz was never envisioned as being used for
high-speed wireless broadband based on the 2003 and 2004 FCC ATC rulings
making clear that the Ancillary Tower Component (ATC) would be, in
fact, ancillary to the primary satellite component.
To build public support of efforts to continue the 2004 FCC
authorization of LightSquared's ancillary terrestrial component vs. a
simple ground-based LTE service in the Mobile Satellite Service band,
GPS receiver manufacturer Trimble Navigation Ltd. formed the "Coalition To Save Our GPS".
The FCC and LightSquared have each made public commitments to
solve the GPS interference issue before the network is allowed to
operate. According to Chris Dancy of the Aircraft Owners and Pilots Association, airline pilots with the type of systems that would be affected "may go off course and not even realize it". The problems could also affect the Federal Aviation Administration upgrade to the air traffic control system, United States Defense Department guidance, and local emergency services including 911.
On February 14, 2012, the FCC moved to bar LightSquared's planned national broadband network after being informed by the National Telecommunications and Information Administration
(NTIA), the federal agency that coordinates spectrum uses for the
military and other federal government entities, that "there is no
practical way to mitigate potential interference at this time". LightSquared is challenging the FCC's action.
Following the United States' deployment of GPS, other countries have
also developed their own satellite navigation systems. These systems
include:
The Russian Global Navigation Satellite System (GLONASS) was developed at the same time as GPS, but suffered from incomplete coverage of the globe until the mid-2000s.
GLONASS reception in addition to GPS can be combined in a receiver
thereby allowing for additional satellites available to enable faster
position fixes and improved accuracy, to within two meters (6.6 ft).
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