GPS signals

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Artist's conception of GPS Block II-F satellite in Earth orbit

Civilian GPS receiver ("GPS navigation device") in a marine application

GPS signals are broadcast by Global Positioning System satellites to enable satellite navigation. Receivers on or near the Earth's surface can determine location, time, and velocity using this information. The GPS satellite constellation is operated by the 2nd Space Operations Squadron (2SOPS) of Space Delta 8, United States Space Force.

GPS signals include ranging signals, used to measure the distance to the satellite, and navigation messages. The navigation messages include ephemeris data, used in trilateration to calculate the position of each satellite in orbit, and information about the time and status of the entire satellite constellation, called the almanac.

There are four GPS signal specifications designed for civilian use. In order of date of introduction, these are: L1 C/A, L2C, L5 and L1C.^[1] L1 C/A is also called the legacy signal and is broadcast by all currently operational satellites. L2C, L5 and L1C are modernized signals, and only broadcast by newer satellites (or not yet at all), and as of January 2021, none are yet considered to be fully operational for civilian use. In addition, there are restricted signals with published frequencies and chip rates but encrypted coding intended to be used only by authorized parties. Some limited use of restricted signals can still be made by civilians without decryption; this is called codeless and semi-codeless access, and is officially supported.

The interface to the User Segment (GPS receivers) is described in the Interface Control Documents (ICD). The format of civilian signals is described in the Interface Specification (IS) which is a subset of the ICD.

Common characteristics

The GPS satellites (called space vehicles in the GPS interface specification documents) transmit simultaneously several ranging codes and navigation data using binary phase-shift keying (BPSK). Only a limited number of central frequencies are used; satellites using the same frequency are distinguished by using different ranging codes; in other words, GPS uses code-division multiple access. The ranging codes are also called chipping codes (in reference to CDMA/DSSS), pseudorandom noise and pseudorandom binary sequences (in reference to the fact that it is predictable, but statistically it resembles noise).

Some satellites transmit several BPSK streams at the same frequency in quadrature, in a form of quadrature amplitude modulation. However, unlike typical QAM systems where a single bit stream is split in two half-symbol-rate bit streams to improve spectral efficiency, in GPS signals the in-phase and quadrature components are modulated by separate (but functionally related) bit streams.

Further information: List of GPS satellites § PRN to SVN history

Satellites are uniquely identified by a serial number called space vehicle number (SVN) which does not change during its lifetime. In addition, all operating satellites are numbered with a space vehicle identifier (SV ID) and pseudorandom noise number (PRN number) which uniquely identifies the ranging codes that a satellite uses. There is a fixed one-to-one correspondence between SV identifiers and PRN numbers described in the interface specification. Unlike SVNs, the SV ID/PRN number of a satellite may be changed (also changing the ranging codes it uses). At any point in time, any SV ID/PRN number is in use by at most a single satellite. A single SV ID/PRN number may have been used by several satellites at different points in time and a single satellite may have used different SV ID/PRN numbers at different points in time. The current SVNs and PRN numbers for the GPS constellation may be found at NAVCEN.

Legacy GPS signals

The original GPS design contains two ranging codes: the coarse/acquisition (C/A) code, which is freely available to the public, and the restricted precision (P) code, usually reserved for military applications.

Coarse/acquisition code

The C/A PRN codes are Gold codes with a period of 1023 chips transmitted at 1.023 Mchip/s, causing the code to repeat every 1 millisecond. They are exclusive-ored with a 50 bit/s navigation message and the result phase modulates the carrier as previously described. These codes only match up, or strongly autocorrelate when they are almost exactly aligned. Each satellite uses a unique PRN code, which does not correlate well with any other satellite's PRN code. In other words, the PRN codes are highly orthogonal to one another. The 1 ms period of the C/A code corresponds to 299.8 km of distance, and each chip corresponds to a distance of 293 m. (Receivers track these codes well within one chip of accuracy, so measurement errors are considerably smaller than 293 m.)

The C/A codes are generated by combining (using "exclusive or") 2-bit streams generated by maximal period 10 stage linear-feedback shift registers (LFSR). Different codes are obtained by selectively delaying one of those bit streams. Thus:

C/A_i(t) = A(t) ⊕ B(t-D_i)

where:

C/A_i is the code with PRN number i.

A is the output of the first LFSR whose generator polynomial is x → x¹⁰ + x³ + 1, and initial state is 1111111111₂.

B is the output of the second LFSR whose generator polynomial is x → x¹⁰ + x⁹ + x⁸ + x⁶ + x³ + x² + 1 and initial state is also 1111111111₂.

D_i is a delay (by an integer number of periods) specific to each PRN number i; it is designated in the GPS interface specification.

⊕ is exclusive or.

The arguments of the functions therein are the number of bits or chips since their epochs, starting at 0. The epoch of the LFSRs is the point at which they are at the initial state; and for the overall C/A codes it is the start of any UTC second plus any integer number of milliseconds. The output of LFSRs at negative arguments is defined consistent with the period which is 1,023 chips (this provision is necessary because B may have a negative argument using the above equation).

The delay for PRN numbers 34 and 37 is the same; therefore their C/A codes are identical and are not transmitted at the same time (it may make one or both of those signals unusable due to mutual interference depending on the relative power levels received on each GPS receiver).

Precision code

See also: Error analysis for the Global Positioning System § Anti-spoofing

The P-code is a PRN sequence much longer than the C/A code: 6.187104 x 10¹² chips. Even though the P-code chip rate (10.23 Mchip/s) is ten times that of the C/A code, it repeats only once per week, eliminating range ambiguity. It was assumed that receivers could not directly acquire such a long and fast code so they would first "bootstrap" themselves with the C/A code to acquire the spacecraft ephemerides, produce an approximate time and position fix, and then acquire the P-code to refine the fix.

Whereas the C/A PRNs are unique for each satellite, each satellite transmits a different segment of a master P-code sequence approximately 2.35 x 10¹⁴ chips long (235,000,000,000,000 chips). Each satellite repeatedly transmits its assigned segment of the master code, restarting every Sunday at 00:00:00 GPS time. (The GPS epoch was Sunday January 6, 1980 at 00:00:00 UTC, but GPS does not follow UTC leap seconds. So GPS time is ahead of UTC by an integer number of seconds.)

The P code is public, so to prevent unauthorized users from using or potentially interfering with it through spoofing, the P-code is XORed with W-code, a cryptographically generated sequence, to produce the Y-code. The Y-code is what the satellites have been transmitting since the anti-spoofing module was set to the "on" state. The encrypted signal is referred to as the P(Y)-code.

The details of the W-code are secret, but it is known that it is applied to the P-code at approximately 500 kHz, about 20 times slower than the P-code chip rate. This has led to semi-codeless approaches for tracking the P(Y) signal without knowing the W-code.

Navigation message

GPS message format

Sub- frame	Word	Description
1	1–2	Telemetry and handover words (TLM and HOW)
	3–10	Satellite clock, GPS time relationship
2–3	1–2	Telemetry and handover words (TLM and HOW)
	3–10	Ephemeris (precise satellite orbit)
4–5	1–2	Telemetry and handover words (TLM and HOW)
	3–10	Almanac component (satellite network synopsis, error correction)

In addition to the PRN ranging codes, a receiver needs to know the time and position of each active satellite. GPS encodes this information into the navigation message and modulates it onto both the C/A and P(Y) ranging codes at 50 bit/s. The navigation message format described in this section is called LNAV data (for legacy navigation).

The navigation message conveys information of three types:

• The GPS date and time and the satellite's status.
• The ephemeris: precise orbital information for the transmitting satellite.
• The almanac: status and low-resolution orbital information for every satellite.

An ephemeris is valid for only four hours; an almanac is valid with little dilution of precision for up to two weeks. The receiver uses the almanac to acquire a set of satellites based on stored time and location. As each satellite is acquired, its ephemeris is decoded so the satellite can be used for navigation.

The navigation message consists of 30-second frames 1,500 bits long, divided into five 6-second subframes of ten 30-bit words each. Each subframe has the GPS time in 6-second increments. Subframe 1 contains the GPS date (week number) and satellite clock correction information, satellite status and health. Subframes 2 and 3 together contain the transmitting satellite's ephemeris data. Subframes 4 and 5 contain page 1 through 25 of the 25-page almanac. The almanac is 15,000 bits long and takes 12.5 minutes to transmit.

A frame begins at the start of the GPS week and every 30 seconds thereafter. Each week begins with the transmission of almanac page 1.

There are two navigation message types: LNAV-L is used by satellites with PRN numbers 1 to 32 (called lower PRN numbers) and LNAV-U is used by satellites with PRN numbers 33 to 63 (called upper PRN numbers). The 2 types use very similar formats. Subframes 1 to 3 are the same while subframes 4 and 5 are almost the same. Each message type contains almanac data for all satellites using the same navigation message type, but not the other.

Each subframe begins with a Telemetry Word (TLM) that enables the receiver to detect the beginning of a subframe and determine the receiver clock time at which the navigation subframe begins. Next is the handover word (HOW) giving the GPS time (actually the time when the first bit of the next subframe will be transmitted) and identifies the specific subframe within a complete frame. The remaining eight words of the subframe contain the actual data specific to that subframe. Each word includes 6 bits of parity generated using an algorithm based on Hamming codes, which take into account the 24 non-parity bits of that word and the last 2 bits of the previous word.

After a subframe has been read and interpreted, the time the next subframe was sent can be calculated through the use of the clock correction data and the HOW. The receiver knows the receiver clock time of when the beginning of the next subframe was received from detection of the Telemetry Word thereby enabling computation of the transit time and thus the pseudorange.

Time

GPS time is expressed with a resolution of 1.5 seconds as a week number and a time of week count (TOW). Its zero point (week 0, TOW 0) is defined to be 1980-01-06T00:00Z. The TOW count is a value ranging from 0 to 403,199 whose meaning is the number of 1.5 second periods elapsed since the beginning of the GPS week. Expressing TOW count thus requires 19 bits (2¹⁹ = 524,288). GPS time is a continuous time scale in that it does not include leap seconds; therefore the start/end of GPS weeks may differ from that of the corresponding UTC day by an integer number of seconds.

In each subframe, each hand-over word (HOW) contains the most significant 17 bits of the TOW count corresponding to the start of the next following subframe. Note that the 2 least significant bits can be safely omitted because one HOW occurs in the navigation message every 6 seconds, which is equal to the resolution of the truncated TOW count thereof. Equivalently, the truncated TOW count is the time duration since the last GPS week start/end to the beginning of the next frame in units of 6 seconds.

Each frame contains (in subframe 1) the 10 least significant bits of the corresponding GPS week number. Note that each frame is entirely within one GPS week because GPS frames do not cross GPS week boundaries. Since rollover occurs every 1,024 GPS weeks (approximately every 19.6 years; 1,024 is 2¹⁰), a receiver that computes current calendar dates needs to deduce the upper week number bits or obtain them from a different source. One possible method is for the receiver to save its current date in memory when shut down, and when powered on, assume that the newly decoded truncated week number corresponds to the period of 1,024 weeks that starts at the last saved date. This method correctly deduces the full week number if the receiver is never allowed to remain shut down (or without a time and position fix) for more than 1,024 weeks (~19.6 years).

Almanac

The almanac consists of coarse orbit and status information for each satellite in the constellation, an ionospheric model, and information to relate GPS derived time to Coordinated Universal Time (UTC). Each frame contains a part of the almanac (in subframes 4 and 5) and the complete almanac is transmitted by each satellite in 25 frames total (requiring 12.5 minutes). The almanac serves several purposes. The first is to assist in the acquisition of satellites at power-up by allowing the receiver to generate a list of visible satellites based on stored position and time, while an ephemeris from each satellite is needed to compute position fixes using that satellite. In older hardware, lack of an almanac in a new receiver would cause long delays before providing a valid position, because the search for each satellite was a slow process. Advances in hardware have made the acquisition process much faster, so not having an almanac is no longer an issue. The second purpose is for relating time derived from the GPS (called GPS time) to the international time standard of UTC. Finally, the almanac allows a single-frequency receiver to correct for ionospheric delay error by using a global ionospheric model. The corrections are not as accurate as GNSS augmentation systems like WAAS or dual-frequency receivers. However, it is often better than no correction, since ionospheric error is the largest error source for a single-frequency GPS receiver.

Structure of subframes 4 and 5

LNAV-L frames 4 and 5 Sub- frame Page Description 4 1, 6, 11–12, 16, 19–24 Reserved 2–5, 7–10 Almanac data for SV 25–32 13 Navigation message correction table (NMCT) 14–15 Reserved for system use 17 Special messages 18 Ionospheric correction data and UTC 25 A-S flags for SV 1–32, health info. for SV 25–32 5 1–24 Almanac data for SV 1–24 25 Health info. for SV 1–24 almanac reference time

LNAV-U frames 4 and 5 Sub- frame Page Description 4 1, 6, 10–12, 16, 19–24 Reserved 2–5, 7–9 Almanac data for SV 89–95 13 Navigation message correction table (NMCT) 14–15 Reserved for system use 17 Special messages 18 Ionospheric correction data and UTC 25 A-S flags for PRN numbers 33–63, health info. for SV 89–95 5 1–24 Almanac data for SV 65–88 25 Health info. for SV 65–88 almanac reference time

Data updates

Satellite data is updated typically every 24 hours, with up to 60 days data loaded in case there is a disruption in the ability to make updates regularly. Typically the updates contain new ephemerides, with new almanacs uploaded less frequently. The Control Segment guarantees that during normal operations a new almanac will be uploaded at least every 6 days.

Satellites broadcast a new ephemeris every two hours. The ephemeris is generally valid for 4 hours, with provisions for updates every 4 hours or longer in non-nominal conditions. The time needed to acquire the ephemeris is becoming a significant element of the delay to first position fix, because as the receiver hardware becomes more capable, the time to lock onto the satellite signals shrinks; however, the ephemeris data requires 18 to 36 seconds before it is received, due to the low data transmission rate.

Frequency information

GPS broadcast signal

For the ranging codes and navigation message to travel from the satellite to the receiver, they must be modulated onto a carrier wave. In the case of the original GPS design, two frequencies are utilized; one at 1575.42 MHz (10.23 MHz × 154) called L1; and a second at 1227.60 MHz (10.23 MHz × 120), called L2.

The C/A code is transmitted on the L1 frequency as a 1.023 MHz signal using a bi-phase shift keying (BPSK) modulation technique. The P(Y)-code is transmitted on both the L1 and L2 frequencies as a 10.23 MHz signal using the same BPSK modulation, however the P(Y)-code carrier is in quadrature with the C/A carrier (meaning it is 90° out of phase).

Besides redundancy and increased resistance to jamming, a critical benefit of having two frequencies transmitted from one satellite is the ability to measure directly, and therefore remove, the ionospheric delay error for that satellite. Without such a measurement, a GPS receiver must use a generic model or receive ionospheric corrections from another source (such as the Wide Area Augmentation System or WAAS). Advances in the technology used on both the GPS satellites and the GPS receivers has made ionospheric delay the largest remaining source of error in the signal. A receiver capable of performing this measurement can be significantly more accurate and is typically referred to as a dual frequency receiver.

Modernization and additional GPS signals

See also: GPS Block III § New navigation signals

Having reached full operational capability on July 17, 1995 the GPS system had completed its original design goals. However, additional advances in technology and new demands on the existing system led to the effort to "modernize" the GPS system. Announcements from the Vice President and the White House in 1998 heralded the beginning of these changes and in 2000, the U.S. Congress reaffirmed the effort, referred to as GPS III.

The project involves new ground stations and new satellites, with additional navigation signals for both civilian and military users, and aims to improve the accuracy and availability for all users. A goal of 2013 was established with incentives offered to the contractors if they can complete it by 2011.

General features

A visual example of the 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, in this example at 45°N, changes with time.

Modernized GPS civilian signals have two general improvements over their legacy counterparts: a dataless acquisition aid and forward error correction (FEC) coding of the NAV message.

A dataless acquisition aid is an additional signal, called a pilot carrier in some cases, broadcast alongside the data signal. This dataless signal is designed to be easier to acquire than the data encoded and, upon successful acquisition, can be used to acquire the data signal. This technique improves acquisition of the GPS signal and boosts power levels at the correlator.

The second advancement is to use forward error correction (FEC) coding on the NAV message itself. Due to the relatively slow transmission rate of NAV data (usually 50 bits per second), small interruptions can have potentially large impacts. Therefore, FEC on the NAV message is a significant improvement in overall signal robustness.

L2C

One of the first announcements was the addition of a new civilian-use signal, to be transmitted on a frequency other than the L1 frequency used for the coarse/acquisition (C/A) signal. Ultimately, this became the L2C signal, so called because it is broadcast on the L2 frequency. Because it requires new hardware on board the satellite, it is only transmitted by the so-called Block IIR-M and later design satellites. The L2C signal is tasked with improving accuracy of navigation, providing an easy to track signal, and acting as a redundant signal in case of localized interference. L2C signals have been broadcast beginning in April 2014 on satellites capable of broadcasting it, but are still considered pre-operational. As of January 2021, L2C is broadcast on 23 satellites and is expected on 24 satellites by 2023.

Unlike the C/A code, L2C contains two distinct PRN code sequences to provide ranging information; the civil-moderate code (called CM), and the civil-long length code (called CL). The CM code is 10,230 chips long, repeating every 20 ms. The CL code is 767,250 chips long, repeating every 1,500 ms. Each signal is transmitted at 511,500 chips per second (chip/s); however, they are multiplexed together to form a 1,023,000-chip/s signal.

CM is modulated with the CNAV Navigation Message (see below), whereas CL does not contain any modulated data and is called a dataless sequence. The long, dataless sequence provides for approximately 24 dB greater correlation (~250 times stronger) than L1 C/A-code.

When compared to the C/A signal, L2C has 2.7 dB greater data recovery and 0.7 dB greater carrier-tracking, although its transmission power is 2.3 dB weaker.

The current status of the L2C signal as of June 9 2021 is:

• Pre-operational signal with message set "healthy"
• Broadcasting from 23 GPS satellites (as of January 9, 2021)
• Began launching in 2005 with GPS Block IIR-M
• Available on 24 GPS satellites with ground segment control capability by 2023 (as of Jan 2020)

CM and CL codes

The civil-moderate and civil-long ranging codes are generated by a modular LFSR which is reset periodically to a predetermined initial state. The period of the CM and CL is determined by this resetting and not by the natural period of the LFSR (as is the case with the C/A code). The initial states are designated in the interface specification and are different for different PRN numbers and for CM/CL. The feedback polynomial/mask is the same for CM and CL. The ranging codes are thus given by:

CM_i(t) = A(X_i,t mod 10 230)

CL_i(t) = A(Y_i,t mod 767 250)

where:

CM_i and CL_i are the ranging codes for PRN number i and their arguments are the integer number of chips elapsed (starting at 0) since start/end of GPS week, or equivalently since the origin of the GPS time scale (see § Time).

A(x, t) is the output of the LFSR when initialized with initial state x after being clocked t times.

X_i and Y_i are the initial states for CM and CL respectively. for PRN number ${\displaystyle i}$.

mod is the remainder of division operation.

t is the integer number of CM and CL chip periods since the origin of GPS time or equivalently, since any GPS second (starting from 0).

The initial states are described in the GPS interface specification as numbers expressed in octal following the convention that the LFSR state is interpreted as the binary representation of a number where the output bit is the least significant bit, and the bit where new bits are shifted in is the most significant bit. Using this convention, the LFSR shifts from most significant bit to least significant bit and when seen in big endian order, it shifts to the right. The states called final state in the IS are obtained after 10229 cycles for CM and after 767249 cycles for LM (just before reset in both cases).

CNAV navigation message

Message structure
(common fields)

Bits	Information
1–8	Preamble
9–14	PRN of transmitting satellite
15–20	Message type ID
21–37	Truncated TOW count
38	Alert flag
277–300	Cyclic redundancy check

Message types

Type ID	Description
10–11	Ephemeris and health
12, 31, 37	Almanac parameters
13–14, 34	Differential correction
15, 36	Text messages
30	Ionospheric and group delay correction
32	Earth orientation parameters
33	UTC parameters
35	GPS/GNSS time offset

The CNAV data is an upgraded version of the original NAV navigation message. It contains higher precision representation and nominally more accurate data than the NAV data. The same type of information (time, status, ephemeris, and almanac) is still transmitted using the new CNAV format; however, instead of using a frame / subframe architecture, it uses a new pseudo-packetized format made of 12-second 300-bit messages analogous to LNAV frames. While LNAV frames have a fixed information content, CNAV messages may be of one of several defined types. The type of a frame determines its information content. Messages do not follow a fixed schedule regarding which message types will be used, allowing the Control Segment some versatility. However, for some message types there are lower bounds on how often they will be transmitted.

In CNAV, at least 1 out of every 4 packets are ephemeris data and the same lower bound applies for clock data packets. The design allows for a wide variety of packet types to be transmitted. With a 32-satellite constellation, and the current requirements of what needs to be sent, less than 75% of the bandwidth is used. Only a small fraction of the available packet types have been defined; this enables the system to grow and incorporate advances without breaking compatibility.

There are many important changes in the new CNAV message:

• It uses forward error correction (FEC) provided by a rate 1/2 convolutional code, so while the navigation message is 25-bit/s, a 50-bit/s signal is transmitted.
• Messages carry a 24-bit CRC, against which integrity can be checked.
• The GPS week number is now represented as 13 bits, or 8192 weeks, and only repeats every 157.0 years, meaning the next return to zero won't occur until the year 2137. This is longer compared to the L1 NAV message's use of a 10-bit week number, which returns to zero every 19.6 years.
• There is a packet that contains a GPS-to-GNSS time offset. This allows better interoperability with other global time-transfer systems, such as Galileo and GLONASS, both of which are supported.
• The extra bandwidth enables the inclusion of a packet for differential correction, to be used in a similar manner to satellite based augmentation systems and which can be used to correct the L1 NAV clock data.
• Every packet contains an alert flag, to be set if the satellite data can not be trusted. This means users will know within 12 seconds if a satellite is no longer usable. Such rapid notification is important for safety-of-life applications, such as aviation.
• Finally, the system is designed to support 63 satellites, compared with 32 in the L1 NAV message.

CNAV messages begin and end at start/end of GPS week plus an integer multiple of 12 seconds. Specifically, the beginning of the first bit (with convolution encoding already applied) to contain information about a message matches the aforesaid synchronization. CNAV messages begin with an 8-bit preamble which is a fixed bit pattern and whose purpose is to enable the receiver to detect the beginning of a message.

Forward error correction code

The convolutional code used to encode CNAV is described by:

${\displaystyle \begin{array}{rl}{X}_1(t)& =d(t)\oplus d(t-2)\oplus d(t-3)\oplus d(t-5)\oplus d(t-6)\\ X_2(t)& =d(t)\oplus d(t-1)\oplus d(t-2)\oplus d(t-3)\oplus d(t-6)\\ d^{\prime }(t^{\prime })& =\{\begin{array}{ll}{X}_1\left(\frac{t^{\prime }}{2}\right)& \text{if }{t}^{\prime }\equiv 0(\mathrm{mod}2)\\ X_2\left(\frac{t^{\prime }-1}{2}\right)& \text{if }{t}^{\prime }\equiv 1(\mathrm{mod}2)\end{array}\end{array}}$

where:

${\displaystyle X_1}$ and ${\displaystyle X_2}$ are the unordered outputs of the convolutional encoder

${\displaystyle d}$ is the raw (non FEC encoded) navigation data, consisting of the simple concatenation of the 300-bit messages.

${\displaystyle t}$ is the integer number of non FEC encoded navigation data bits elapsed since an arbitrary point in time (starting at 0).

${\displaystyle d^{\prime }}$ is the FEC encoded navigation data.

${\displaystyle t^{\prime }}$ is the integer number of FEC encoded navigation data bits elapsed since the same epoch than ${\displaystyle t}$ (likewise starting at 0).

Since the FEC encoded bit stream runs at 2 times the rate than the non FEC encoded bit as already described, then ${\displaystyle t=\lfloor \frac{t^{\prime }}{2}\rfloor }$. FEC encoding is performed independently of navigation message boundaries; this follows from the above equations.

L2C frequency information

An immediate effect of having two civilian frequencies being transmitted is the civilian receivers can now directly measure the ionospheric error in the same way as dual frequency P(Y)-code receivers. However, users utilizing the L2C signal alone, can expect 65% more position uncertainty due to ionospheric error than with the L1 signal alone.

Military (M-code)

A major component of the modernization process is a new military signal. Called the Military code, or M-code, it was designed to further improve the anti-jamming and secure access of the military GPS signals.

Very little has been published about this new, restricted code. It contains a PRN code of unknown length transmitted at 5.115 MHz. Unlike the P(Y)-code, the M-code is designed to be autonomous, meaning that a user can calculate their position using only the M-code signal. From the P(Y)-code's original design, users had to first lock onto the C/A code and then transfer the lock to the P(Y)-code. Later, direct-acquisition techniques were developed that allowed some users to operate autonomously with the P(Y)-code.

MNAV navigation message

A little more is known about the new navigation message, which is called MNAV. Similar to the new CNAV, this new MNAV is packeted instead of framed, allowing for very flexible data payloads. Also like CNAV it can utilize Forward Error Correction (FEC) and advanced error detection (such as a CRC).

M-code frequency information

The M-code is transmitted in the same L1 and L2 frequencies already in use by the previous military code, the P(Y)-code. The new signal is shaped to place most of its energy at the edges (away from the existing P(Y) and C/A carriers). It does not work at every satellite, and M-code was switched off for SVN62/PRN25 on 05 April 2011.

In a major departure from previous GPS designs, the M-code is intended to be broadcast from a high-gain directional antenna, in addition to a full-Earth antenna. This directional antenna's signal, called a spot beam, is intended to be aimed at a specific region (several hundred kilometers in diameter) and increase the local signal strength by 20 dB, or approximately 100 times stronger. A side effect of having two antennas is that the GPS satellite will appear to be two GPS satellites occupying the same position to those inside the spot beam. While the whole Earth M-code signal is available on the Block IIR-M satellites, the spot beam antennas will not be deployed until the Block III satellites are deployed, which began in December 2018.

An interesting side effect of having each satellite transmit four separate signals is that the MNAV can potentially transmit four different data channels, offering increased data bandwidth.

The modulation method is binary offset carrier, using a 10.23 MHz subcarrier against the 5.115 MHz code. This signal will have an overall bandwidth of approximately 24 MHz, with significantly separated sideband lobes. The sidebands can be used to improve signal reception.

L5

The L5 signal provides a means of radionavigation secure and robust enough for life critical applications, such as aircraft precision approach guidance. The signal is broadcast in a frequency band protected by the ITU for aeronautical radionavigation services. It was first demonstrated from satellite USA-203 (Block IIR-M), and is available on all satellites from GPS IIF and GPS III. L5 signals have been broadcast beginning in April 2014 on satellites that support it. As of January 2021, 16 GPS satellites are broadcasting L5 signals, and the signals are considered pre-operational, scheduled to reach 24 satellites by approximately 2027.

The L5 band provides additional robustness in the form of interference mitigation, the band being internationally protected, redundancy with existing bands, geostationary satellite augmentation, and ground-based augmentation. The added robustness of this band also benefits terrestrial applications.

Two PRN ranging codes are transmitted on L5 in quadrature: the in-phase code (called I5-code) and the quadrature-phase code (called Q5-code). Both codes are 10,230 chips long, transmitted at 10.23 Mchip/s (1 ms repetition period), and are generated identically (differing only in initial states). Then, I5 is modulated (by exclusive-or) with navigation data (called L5 CNAV) and a 10-bit Neuman-Hofman code clocked at 1 kHz. Similarly, the Q5-code is then modulated but with only a 20-bit Neuman-Hofman code that is also clocked at 1 kHz.

Compared to L1 C/A and L2, these are some of the changes in L5:

• Improved signal structure for enhanced performance
• Higher transmitted power than L1/L2 signal (~3 dB, or 2× as powerful)
• Wider bandwidth provides a 10× processing gain, provides sharper autocorrelation (in absolute terms, not relative to chip time duration) and requires a higher sampling rate at the receiver.
• Longer spreading codes (10× longer than C/A)
• Uses the Aeronautical Radionavigation Services band

The current status of the L5 signal as of June 26 2022 is:

• Pre-operational signal with message set "unhealthy" until sufficient monitoring capability established
• Broadcasting from 17 GPS satellites (as of June 26, 2022)
• Began launching in 2010 with GPS Block IIF
• Available on 24 GPS satellites ~2027 (as of Jan 2020)

I5 and Q5 codes

The I5-code and Q5-code are generated using the same structure but with different parameters. These codes are the combination (by exclusive-or) of the output of 2 differing linear-feedback shift registers (LFSRs) which are selectively reset.

5_i(t) = U(t) ⊕ V_i(t)

U(t) = XA((t mod 10 230) mod 8 190)

V_i(t) = XB_i(X_i, t mod 10 230)

where:

i is an ordered pair (P, n) where P ∈ {I, Q} for in-phase and quadrature-phase, and n a PRN number; both phases and a single PRN are required for the L5 signal from a single satellite.

5_i is the ranging codes for i; also denoted as I5_n and Q5_n.

U and V_i are intermediate codes, with U not depending on phase or PRN.

The output of two 13-stage LFSRs with clock state t' is used:

XA(x,t') has feedback polynomial x¹³ + x¹² + x¹⁰ + x⁹ + 1, and initial state 1111111111111₂.

XB_i(x,t') has feedback polynomial x¹³ + x¹² + x⁸ + x⁷ + x⁶ + x⁴ + x³ + x + 1, and initial state X_i.

X_i is the initial state specified for the phase and PRN number given by i (designated in the IS).

t is the integer number of chip periods since the origin of GPS time or equivalently, since any GPS second (starting from 0).

A and B are maximal length LFSRs. The modulo operations correspond to resets. Note that both are reset each millisecond (synchronized with C/A code epochs). In addition, the extra modulo operation in the description of A is due to the fact it is reset 1 cycle before its natural period (which is 8,191) so that the next repetition becomes offset by 1 cycle with respect to B (otherwise, since both sequences would repeat, I5 and Q5 would repeat within any 1 ms period as well, degrading correlation characteristics).

L5 navigation message

The L5 CNAV data includes SV ephemerides, system time, SV clock behavior data, status messages and time information, etc. The 50 bit/s data is coded in a rate 1/2 convolution coder. The resulting 100 symbols per second (sps) symbol stream is modulo-2 added to the I5-code only; the resultant bit-train is used to modulate the L5 in-phase (I5) carrier. This combined signal is called the L5 Data signal. The L5 quadrature-phase (Q5) carrier has no data and is called the L5 Pilot signal. The format used for L5 CNAV is very similar to that of L2 CNAV. One difference is that it uses 2 times the data rate. The bit fields within each message, message types, and forward error correction code algorithm are the same as those of L2 CNAV. L5 CNAV messages begin and end at start/end of GPS week plus an integer multiple of 6 seconds (this applies to the beginning of the first bit to contain information about a message, as is the case for L2 CNAV).

L5 frequency information

Broadcast on the L5 frequency (1176.45 MHz, 10.23 MHz × 115), which is an aeronautical navigation band. The frequency was chosen so that the aviation community can manage interference to L5 more effectively than L2.

L1C

"L1C" redirects here. For other uses, see L1C (disambiguation).

L1C is a civilian-use signal, to be broadcast on the L1 frequency (1575.42 MHz), which contains the C/A signal used by all current GPS users. The L1C signals will be broadcast from GPS III and later satellites, the first of which was launched in December 2018. As of January 2021, L1C signals are not yet broadcast, and only four operational satellites are capable of broadcasting them. L1C is expected on 24 GPS satellites in the late 2020s.

L1C consists of a pilot (called L1C_P) and a data (called L1C_D) component. These components use carriers with the same phase (within a margin of error of 100 milliradians), instead of carriers in quadrature as with L5. The PRN codes are 10,230 chips long and transmitted at 1.023 Mchip/s, thus repeating in 10 ms. The pilot component is also modulated by an overlay code called L1C_O (a secondary code that has a lower rate than the ranging code and is also predefined, like the ranging code). Of the total L1C signal power, 25% is allocated to the data and 75% to the pilot. The modulation technique used is BOC(1,1) for the data signal and TMBOC for the pilot. The time multiplexed binary offset carrier (TMBOC) is BOC(1,1) for all except 4 of 33 cycles, when it switches to BOC(6,1).

• Implementation will provide C/A code to ensure backward compatibility
• Assured of 1.5 dB increase in minimum C/A code power to mitigate any noise floor increase
• Data-less signal component pilot carrier improves tracking compared with L1 C/A
• Enables greater civil interoperability with Galileo L1

The current status of the L1C signal as of June 10 2021 is:

• Developmental signal with message set "unhealthy" and no navigation data
• Broadcasting from 4 GPS satellites (as of January 9, 2021)
• Began launching in 2018 with GPS III
• Available on 24 GPS satellites in late 2020s

L1C ranging code

The L1C pilot and data ranging codes are based on a Legendre sequence with length 10223 used to build an intermediate code (called a Weil code) which is expanded with a fixed 7-bit sequence to the required 10,230 bits. This 10,230-bit sequence is the ranging code and varies between PRN numbers and between the pilot and data components. The ranging codes are described by:

${\displaystyle \begin{array}{rl}{\text{L1C}}_i(t)& ={\text{L1C}}^{\prime }(tmod10230)\\ {\text{L1C}}_i^{\prime }(t^{\prime })& =\{\begin{array}{ll}{W}_i(t^{\prime })& \text{ if }{t}^{\prime }<p_i^{\prime }\\ S(t^{\prime }-p_i^{\prime })& \text{ if }{p}_i^{\prime }\le t^{\prime }<p_i^{\prime }+7\\ W_i(t^{\prime }-7)& \text{ if }{t}^{\prime }\ge p_i^{\prime }+7\end{array}\\ S& =(0,1,1,0,1,0,0)\\ W_i(n)& =L(n)\oplus L((n+w_i)mod10223)\\ L(n)& =\{\begin{array}{ll}1& \text{ if }n\ne 0\text{ and there is an integer }m\text{ such that }n\equiv m^2(\mathrm{mod}10223)\\ 0& \text{ otherwise}\end{array}\end{array}}$

where:

${\displaystyle {\text{L1C}}_i}$ is the ranging code for PRN number and component ${\displaystyle i}$.

${\displaystyle {\text{L1C}}_i^{\prime }}$ represents a period of ${\displaystyle {\text{L1C}}_i}$; it is introduced only to allow a more clear notation. To obtain a direct formula for ${\displaystyle \text{L1C}}$ start from the right side of the formula for ${\displaystyle {\text{L1C}}^{\prime }}$ and replace all instances of ${\displaystyle t^{\prime }}$ with ${\displaystyle tmod10230}$.

${\displaystyle t}$ is the integer number of L1C chip periods (which is 1⁄1.023 µs) since the origin of GPS time or equivalently, since any GPS second (starting from 0).

${\displaystyle i}$ is an ordered pair identifying a PRN number and a code (L1C_P or L1C_D) and is of the form ${\displaystyle (\text{P},n)}$ or ${\displaystyle (\text{D},n)}$ where ${\displaystyle n}$ is the PRN number of the satellite, and ${\displaystyle \text{P, D}}$ are symbols (not variables) that indicate the L1C_P code or L1C_D code, respectively.

${\displaystyle L}$ is an intermediate code: a Legendre sequence whose domain is the set of integers ${\displaystyle n}$ for which ${\displaystyle 0\le n\le 10222}$.

${\displaystyle W_i}$ is an intermediate code called Weil code, with the same domain as ${\displaystyle L}$.

${\displaystyle S}$ is a 7-bit long sequence defined for 0-based indexes 0 to 6.

${\displaystyle p_i^{\prime }}$ is the 0-based insertion index of the sequence ${\displaystyle S}$ into the ranging code (specific for PRN number and code ${\displaystyle i}$). It is defined in the Interface Specification (IS) as a 1-based index ${\displaystyle p}$, therefore ${\displaystyle p_i^{\prime }=p_i-1}$.

${\displaystyle w_i}$ is the Weil index for PRN number and code ${\displaystyle i}$ designated in the IS.

${\displaystyle \mathrm{mod}}$ is the remainder of division (or modulo) operation, which differs to the notation in statements of modular congruence, also used in this article.

According to the formula above and the GPS IS, the first ${\displaystyle w_i}$ bits (equivalently, up to the insertion point of ${\displaystyle S}$) of ${\displaystyle {\text{L1C}}_i^{\prime }}$ and ${\displaystyle \text{L1C}}$ are the first bits the corresponding Weil code; the next 7 bits are ${\displaystyle S}$; the remaining bits are the remaining bits of the Weil code.

The IS asserts that ${\displaystyle 0\le p_i^{\prime }\le 10222}$. For clarity, the formula for ${\displaystyle {\text{L1C}}_i^{\prime }}$ does not account for the hypothetical case in which ${\displaystyle p_i^{\prime }>10222}$, which would cause the instance of ${\displaystyle S}$ inserted into ${\displaystyle {\text{L1C}}_i^{\prime }}$ to wrap from index 10229 to 0.

L1C overlay code

The overlay codes are 1,800 bits long and is transmitted at 100 bit/s, synchronized with the navigation message encoded in L1C_D.

For PRN numbers 1 to 63 they are the truncated outputs of maximal period LFSRs which vary in initial conditions and feedback polynomials.

For PRN numbers 64 to 210 they are truncated Gold codes generated by combining 2 LFSR outputs (${\displaystyle {\text{S1}}_i}$ and ${\displaystyle {\text{S2}}_i}$, where ${\displaystyle i}$ is the PRN number) whose initial state varies. ${\displaystyle {\text{S1}}_i}$ has one of the 4 feedback polynomials used overall (among PRN numbers 64–210). ${\displaystyle {\text{S2}}_i}$ has the same feedback polynomial for all PRN numbers in the range 64–210.

CNAV-2 navigation message

Subframes

Subframe	Bit count		Description
	Raw	Encoded
1	9	52	Time of interval (TOI)
2	576	1,200	Time correction and ephemeris data
3	250	548	Variable data

Subframe 3 pages

Page no.	Description
1	UTC & IONO
2	GGTO & EOP
3	Reduced almanac
4	Midi almanac
5	Differential correction
6	Text

The L1C navigation data (called CNAV-2) is broadcast in 1,800 bits long (including FEC) frames and is transmitted at 100 bit/s.

The frames of L1C are analogous to the messages of L2C and L5. While L2 CNAV and L5 CNAV use a dedicated message type for ephemeris data, all CNAV-2 frames include that information.

The common structure of all messages consists of 3 frames, as listed in the adjacent table. The content of subframe 3 varies according to its page number which is analogous to the type number of L2 CNAV and L5 CNAV messages. Pages are broadcast in an arbitrary order.

The time of messages (not to be confused with clock correction parameters) is expressed in a different format than the format of the previous civilian signals. Instead it consists of 3 components:

1. The week number, with the same meaning as with the other civilian signals. Each message contains the week number modulo 8,192 or equivalently, the 13 least significant bits of the week number, allowing direct specification of any date within a cycling 157-year range.
2. An interval time of week (ITOW): the integer number of 2 hour periods elapsed since the latest start/end of week. It has range 0 to 83 (inclusive), requiring 7 bits to encode.
3. A time of interval (TOI): the integer number of 18 second periods elapsed since the period represented by the current ITOW to the beginning of the next message. It has range 0 to 399 (inclusive) and requires 9 bits of data.

TOI is the only content of subframe 1. The week number and ITOW are contained in subframe 2 along with other information.

Subframe 1 is encoded by a modified BCH code. Specifically, the 8 least significant bits are BCH encoded to generate 51 bits, then combined using exclusive or with the most significant bit and finally the most significant bit is appended as the most significant bit of the previous result to obtain the final 52 bits. Subframes 2 and 3 are individually expanded with a 24-bit CRC, then individually encoded using a low-density parity-check code, and then interleaved as a single unit using a block interleaver.

Overview of frequencies

GPS Frequencies

Band	Frequency (MHz)	Phase	Original usage	Modernized usage
L1	1575.42 (10.23 × 154)	I	Encrypted precision P(Y) code
		Q	Coarse/acquisition (C/A) code	C/A, L1 Civilian (L1C), and Military (M) code
L2	1227.60 (10.23 × 120)	I	Encrypted precision P(Y) code
		Q	unmodulated carrier	L2 Civilian (L2C) code and Military (M) code
L3	1381.05 (10.23 × 135)		used by Nuclear Detonation (NUDET) Detection System Payload (NDS): signals nuclear detonations/ high-energy infrared events. Used to enforce nuclear test ban treaties.
L4	1379.9133... (10.23 × 1214/9)		—	being studied for additional ionospheric correction
L5	1176.45 (10.23 × 115)	I	—	Safety-of-Life (SoL) Data signal
		Q		Safety-of-Life (SoL) Pilot signal

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 noise (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 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 which 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.

Each composite signal (in-phase and quadrature phase) becomes:

${\displaystyle S(t)=\sqrt{P_{\mathrm{I}}}{X}_{\mathrm{I}}(t)\cos \left(\omega t+{\varphi }_0\right)-\sqrt{P_{\mathrm{Q}}}{X}_{\mathrm{Q}}(t)\underset{-\cos \left(\omega t+{\varphi }_0+\frac{\pi }{2}\right)}{\underbrace{\sin \left(\omega t+{\varphi }_0\right)}},}$

where ${\displaystyle P_{\mathrm{I}}}$ and ${\displaystyle P_{\mathrm{Q}}}$ represent signal powers; ${\displaystyle X_{\mathrm{I}}(t)}$ and ${\displaystyle X_{\mathrm{Q}}(t)}$ represent codes with/without data ${\displaystyle (=\pm 1)}$. This is a formula for the ideal case (which is not attained in practice) as it does not model timing errors, noise, amplitude mismatch between components or quadrature error (when components are not exactly in quadrature).

Demodulation and decoding

Demodulating and Decoding GPS Satellite Signals using the Coarse/Acquisition Gold code.

A GPS receiver processes the GPS signals received on its antenna to determine position, velocity and/or timing. The signal at antenna is amplified, down converted to baseband or intermediate frequency, filtered (to remove frequencies outside the intended frequency range for the digital signal that would alias into it) and digitalized; these steps may be chained in a different order. Note that aliasing is sometimes intentional (specifically, when undersampling is used) but filtering is still required to discard frequencies not intended to be present in the digital representation.

For each satellite used by the receiver, the receiver must first acquire the signal and then track it as long as that satellite is in use; both are performed in the digital domain in by far most (if not all) receivers.

Acquiring a signal is the process of determining the frequency and code phase (both relative to receiver time) when it was previously unknown. Code phase must be determined within an accuracy that depends on the receiver design (especially the tracking loop); 0.5 times the duration of code chips (approx. 0.489 µs) is a representative value.

Tracking is the process of continuously adjusting the estimated frequency and phase to match the received signal as close as possible and therefore is a phase locked loop. Note that acquisition is performed to start using a particular satellite, but tracking is performed as long as that satellite is in use.

In this section, one possible procedure is described for L1 C/A acquisition and tracking, but the process is very similar for the other signals. The described procedure is based on computing the correlation of the received signal with a locally generated replica of the ranging code and detecting the highest peak or lowest valley. The offset of the highest peak or lowest valley contains information about the code phase relative to receiver time. The duration of the local replica is set by receiver design and is typically shorter than the duration of navigation data bits, which is 20 ms.

Acquisition

Acquisition of a given PRN number can be conceptualized as searching for a signal in a bidimensional search space where the dimensions are (1) code phase, (2) frequency. In addition, a receiver may not know which PRN number to search for, and in that case a third dimension is added to the search space: (3) PRN number.

Frequency space

The frequency range of the search space is the band where the signal may be located given the receiver knowledge. The carrier frequency varies by roughly 5 kHz due to the Doppler effect when the receiver is stationary; if the receiver moves, the variation is higher. The code frequency deviation is 1/1,540 times the carrier frequency deviation for L1 because the code frequency is 1/1,540 of the carrier frequency (see § Frequencies used by GPS). The down conversion does not affect the frequency deviation; it only shifts all the signal frequency components down. Since the frequency is referenced to the receiver time, the uncertainty in the receiver oscillator frequency adds to the frequency range of the search space.

Code phase space

The ranging code has a period of 1,023 chips each of which lasts roughly 0.977 µs (see § Coarse/acquisition code). The code gives strong autocorrelation only at offsets less than 1 in magnitude. The extent of the search space in the code phase dimension depends on the granularity of the offsets at which correlation is computed. It is typical to search for the code phase within a granularity of 0.5 chips or finer; that means 2,046 offsets. There may be more factors increasing the size of the search space of code phase. For example, a receiver may be designed so as to examine 2 consecutive windows of the digitalized signal, so that at least one of them does not contain a navigation bit transition (which worsens the correlation peak); this requires the signal windows to be at most 10 ms long.

PRN number space

The lower PRN numbers range from 1 to 32 and therefore there are 32 PRN numbers to search for when the receiver does not have information to narrow the search in this dimension. The higher PRN numbers range from 33 to 66. See § Navigation message.

If the almanac information has previously been acquired, the receiver picks which satellites to listen for by their PRNs. If the almanac information is not in memory, the receiver enters a search mode and cycles through the PRN numbers 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 decode 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.

Simple correlation

The simplest way to acquire the signal (not necessarily the most effective or least computationally expensive) is to compute the dot product of a window of the digitalized signal with a set of locally generated replicas. The locally generated replicas vary in carrier frequency and code phase to cover all the already mentioned search space which is the Cartesian product of the frequency search space and the code phase search space. The carrier is a complex number where real and imaginary components are both sinusoids as described by Euler's formula. The replica that generates the highest magnitude of dot product is likely the one that best matches the code phase and frequency of the signal; therefore, if that magnitude is above a threshold, the receiver proceeds to track the signal or further refine the estimated parameters before tracking. The threshold is used to minimize false positives (apparently detecting a signal when there is in fact no signal), but some may still occur occasionally.

Using a complex carrier allows the replicas to match the digitalized signal regardless of the signal's carrier phase and to detect that phase (the principle is the same used by the Fourier transform). The dot product is a complex number; its magnitude represents the level of similarity between the replica and the signal, as with an ordinary correlation of real-valued time series. The argument of the dot product is an approximation of the corresponding carrier in the digitalized signal.

As an example, assume that the granularity for the search in code phase is 0.5 chips and in frequency is 500 Hz, then there are 1,023/0.5 = 2,046 code phases and 10,000 Hz/500 Hz = 20 frequencies to try for a total of 20×2,046 = 40,920 local replicas. Note that each frequency bin is centered on its interval and therefore covers 250 Hz in each direction; for example, the first bin has a carrier at −4.750 Hz and covers the interval −5,000 Hz to −4,500 Hz. Code phases are equivalent modulo 1,023 because the ranging code is periodic; for example, phase −0.5 is equivalent to phase 1,022.5.

The following table depicts the local replicas that would be compared against the digitalized signal in this example. "•" means a single local replica while "..." is used for elided local replicas:

Carrier freq. deviation	Code phase (in chips)
	0.0	0.5	(more phases)	1,022.0	1,022.5
−4,750 Hz	•	•	...	•	•
−4,250 Hz	•	•	...	•	•
(more frequencies)	...	...	...	...	...
4,250 Hz	•	•	...	•	•
4,750 Hz	•	•	...	•	•

Fourier transform

As an improvement over the simple correlation method, it is possible to implement the computation of dot products more efficiently with a Fourier transform. Instead of performing one dot product for each element in the Cartesian product of code and frequency, a single operation involving FFT and covering all frequencies is performed for each code phase; each such operation is more computationally expensive, but it may still be faster overall than the previous method due to the efficiency of FFT algorithms, and it recovers carrier frequency with a higher accuracy, because the frequency bins are much closely spaced in a DFT.

Specifically, for all code phases in the search space, the digitalized signal window is multiplied element by element with a local replica of the code (with no carrier), then processed with a discrete Fourier transform.

Given the previous example to be processed with this method, assume real-valued data (as opposed to complex data, which would have in-phase and quadrature components), a sampling rate of 5 MHz, a signal window of 10 ms, and an intermediate frequency of 2.5 MHz. There will be 5 MHz×10 ms = 50,000 samples in the digital signal, and therefore 25,001 frequency components ranging from 0 Hz to 2.5 MHz in steps of 100 Hz (note that the 0 Hz component is real because it is the average of a real-valued signal and the 2.5 MHz component is real as well because it is the critical frequency). Only the components (or bins) within 5 kHz of the central frequency are examined, which is the range from 2.495 MHz to 2.505 MHz, and it is covered by 51 frequency components. There are 2,046 code phases as in the previous case, thus in total 51×2,046 = 104,346 complex frequency components will be examined.

Circular correlation with Fourier transform

Likewise, as an improvement over the simple correlation method, it is possible to perform a single operation covering all code phases for each frequency bin. The operation performed for each code phase bin involves forward FFT, element-wise multiplication in the frequency domain. inverse FFT, and extra processing so that overall, it computes circular correlation instead of circular convolution. This yields more accurate code phase determination than the simple correlation method in contrast with the previous method, which yields more accurate carrier frequency determination than the previous method.

Tracking and navigation message decoding

Since the carrier frequency received can vary due to Doppler shift, the points where received PRN sequences begin may not differ from O by an exact integral number of milliseconds. Because of this, carrier frequency tracking along with PRN code tracking are used to determine when the received satellite's PRN code begins. Unlike the earlier computation of offset in which trials of all 1,023 offsets could potentially be required, the tracking to maintain lock usually requires shifting of half a pulse width or less. To perform this tracking, the receiver observes two quantities, phase error and received frequency offset. The correlation of the received PRN code with respect to the receiver generated PRN code is computed to determine if the bits of the two signals are misaligned. Comparisons of the received PRN code with receiver generated PRN code shifted half a pulse width early and half a pulse width late are used to estimate adjustment required. The amount of adjustment required for maximum correlation is used in estimating phase error. Received frequency offset from the frequency generated by the receiver provides an estimate of phase rate error. The command for the frequency generator and any further PRN code shifting required are computed as a function of the phase error and the phase rate error in accordance with the control law used. The Doppler velocity is computed as a function of the frequency offset from the carrier nominal frequency. The Doppler velocity is the velocity component along the line of sight of the receiver relative to the satellite.

As the receiver continues to read successive PRN sequences, it will encounter a sudden change in the phase of the 1,023-bit received PRN signal. This indicates the beginning of a data bit of the navigation message. This enables the receiver to begin reading the 20 millisecond bits of the navigation message. The TLM word at the beginning of each subframe of a navigation frame enables the receiver to detect the beginning of a subframe and determine the receiver clock time at which the navigation subframe begins. The HOW word then enables the receiver to determine which specific subframe is being transmitted. 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 before computing the intersections of sphere surfaces.

After a subframe has been read and interpreted, the time the next subframe was sent can be calculated through the use of the clock correction data and the HOW. The receiver knows the receiver clock time of when the beginning of the next subframe was received from detection of the Telemetry Word thereby enabling computation of the transit time and thus the pseudorange. The receiver is potentially capable of getting a new pseudorange measurement at the beginning of each subframe or every 6 seconds.

Then the orbital position data, or ephemeris, from the navigation message is used to calculate precisely where the satellite was at the start of the message. A more sensitive receiver will potentially acquire the ephemeris data more quickly than a less sensitive receiver, especially in a noisy environment.

Alpha Magnetic Spectrometer

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Alpha_Magnetic_Spectrometer 

 

Alpha Magnetic Spectrometer

AMS-02 on the truss, as viewed during an Expedition 50 spacewalk
Module statistics
Part of	International Space Station
Launch date	16 May 2011 13:56:28 UTC
Launch vehicle	Space Shuttle Endeavour
Berthed	May 19, 2011
Mass	6,717 kg (14,808 lb)

AMS-02 logo

 

Computer rendering

The Alpha Magnetic Spectrometer (AMS-02) is a particle physics experiment module that is mounted on the International Space Station (ISS). The experiment is a recognized CERN experiment (RE1). The module is a detector that measures antimatter in cosmic rays; this information is needed to understand the formation of the Universe and search for evidence of dark matter.

The principal investigator is Nobel laureate particle physicist Samuel Ting. The launch of Space Shuttle Endeavour flight STS-134 carrying AMS-02 took place on May 16, 2011, and the spectrometer was installed on May 19, 2011. By April 15, 2015, AMS-02 had recorded over 60 billion cosmic ray events and 90 billion after five years of operation since its installation in May 2011.

In March 2013, Professor Ting reported initial results, saying that AMS had observed over 400,000 positrons, with the positron to electron fraction increasing from 10 GeV to 250 GeV. (Later results have shown a decrease in positron fraction at energies over about 275 GeV). There was "no significant variation over time, or any preferred incoming direction. These results are consistent with the positrons originating from the annihilation of dark matter particles in space, but not yet sufficiently conclusive to rule out other explanations." The results have been published in Physical Review Letters. Additional data are still being collected.

History

The alpha magnetic spectrometer was proposed in 1995 by the Antimatter Study Group, led by MIT particle physicist Samuel Ting, not long after the cancellation of the Superconducting Super Collider. The original name for the instrument was Antimatter Spectrometer, with the stated objective to search for primordial antimatter, with a target resolution of antimatter/matter ≈10⁻⁹. The proposal was accepted and Ting became the principal investigator.

AMS-01

AMS-01 flew in space in June 1998 aboard the Space Shuttle Discovery on STS-91. It is visible near the rear of the payload bay.

 

A detail view of the AMS-01 module (center) mounted in the shuttle payload bay for the STS-91 mission.

An AMS prototype designated AMS-01, a simplified version of the detector, was built by the international consortium under Ting's direction and flown into space aboard the Space Shuttle Discovery on STS-91 in June 1998. By not detecting any antihelium the AMS-01 established an upper limit of 1.1×10⁻⁶ for the antihelium to helium flux ratio and proved that the detector concept worked in space. This shuttle mission was the last shuttle flight to the Mir Space Station.

AMS-02

AMS-02 during integration and testing at CERN near Geneva.

After the flight of the prototype, the group, now labelled the AMS Collaboration, began the development of a full research system designated AMS-02. This development effort involved the work of 500 scientists from 56 institutions and 16 countries organized under United States Department of Energy (DOE) sponsorship.

The instrument which eventually resulted from a long evolutionary process has been called "the most sophisticated particle detector ever sent into space", rivaling very large detectors used at major particle accelerators, and has cost four times as much as any of its ground-based counterparts. Its goals have also evolved and been refined over time. As built it is a more comprehensive detector which has a better chance of discovering evidence of dark matter along other goals.

The power requirements for AMS-02 were thought to be too great for a practical independent spacecraft. So AMS-02 was designed to be installed as an external module on the International Space Station and use power from the ISS. The post-Space Shuttle Columbia plan was to deliver AMS-02 to the ISS by space shuttle in 2005 on station assembly mission UF4.1, but technical difficulties and shuttle scheduling issues added more delays.

AMS-02 successfully completed final integration and operational testing at CERN in Geneva, Switzerland which included exposure to energetic proton beams generated by the CERN SPS particle accelerator. AMS-02 was then shipped by specialist haulier to ESA's European Space Research and Technology Centre (ESTEC) facility in the Netherlands where it arrived February 16, 2010. Here it underwent thermal vacuum, electromagnetic compatibility and electromagnetic interference testing. AMS-02 was scheduled for delivery to the Kennedy Space Center in Florida, United States. in late May 2010. This was however postponed to August 26, as AMS-02 underwent final alignment beam testing at CERN.

AMS-02 during final alignment testing at CERN just days before being airlifted to Cape Canaveral.

 

Beamline from SPS feeding 20 GeV positrons to AMS for alignment testing at the time of the picture.

A cryogenic, superconducting magnet system was developed for the AMS-02. When the Obama administration extended International Space Station operations beyond 2015, the decision was made by AMS management to exchange the AMS-02 superconducting magnet for the non-superconducting magnet previously flown on AMS-01. Although the non-superconducting magnet has a weaker field strength, its on-orbit operational time at ISS is expected to be 10 to 18 years versus only three years for the superconducting version. In December 2018 it was announced that funding for the ISS had been extended to 2030.

In 1999, after the successful flight of AMS-01, the total cost of the AMS program was estimated to be $33 million, with AMS-02 planned for flight to the ISS in 2003. After the Space Shuttle Columbia disaster in 2003, and after a number of technical difficulties with the construction of AMS-02, the cost of the program ballooned to an estimated $2 billion.

Installation on the International Space Station

A computer generated image showing AMS-02 mounted to the ISS S3 Upper Inboard Payload Attach Site.

 

Location of the AMS on the International Space Station (upper left).

AMS-02 installed on the ISS.

For several years it was uncertain if AMS-02 would ever be launched because it was not manifested to fly on any of the remaining Space Shuttle flights. After the 2003 Columbia disaster NASA decided to reduce shuttle flights and retire the remaining shuttles by 2010. A number of flights were removed from the remaining manifest including the flight for AMS-02. In 2006 NASA studied alternative ways of delivering AMS-02 to the space station, but they all proved to be too expensive.

In May 2008 a bill was proposed to launch AMS-02 to ISS on an additional shuttle flight in 2010 or 2011. The bill was passed by the full House of Representatives on June 11, 2008. The bill then went before the Senate Commerce, Science and Transportation Committee where it also passed. It was then amended and passed by the full Senate on September 25, 2008, and was passed again by the House on September 27, 2008. It was signed by President George W. Bush on October 15, 2008. The bill authorized NASA to add another space shuttle flight to the schedule before the space shuttle program was discontinued. In January 2009 NASA restored AMS-02 to the shuttle manifest. On August 26, 2010, AMS-02 was delivered from CERN to the Kennedy Space Center by a Lockheed C-5 Galaxy.

It was delivered to the International Space Station on May 19, 2011, as part of station assembly flight ULF6 on shuttle flight STS-134, commanded by Mark Kelly. It was removed from the shuttle cargo bay using the shuttle's robotic arm and handed off to the station's robotic arm for installation. AMS-02 is mounted on top of the Integrated Truss Structure, on USS-02, the zenith side of the S3-element of the truss.

Operations, condition and repairs

ESA astronaut Luca Parmitano, attached to the Canadarm2 robotic arm, carries the new thermal pump system for AMS

By April 2017 only one of the 4 redundant coolant pumps for the silicon trackers was fully working, and repairs were being planned, despite AMS-02 not being designed to be serviced in space. By 2019, the last one was being operated intermittently. In November 2019, after four years of planning, special tools and equipment were sent to the ISS for in-situ repairs that may require four or five EVAs. Liquid carbon dioxide coolant was also replenished.

The repairs were conducted by the ISS crew of Expedition 61. The spacewalkers were the expedition commander and ESA astronaut Luca Parmitano, and NASA astronaut Andrew Morgan. Both of them were assisted by NASA astronauts Christina Koch and Jessica Meir who operated the Canadarm2 robotic arm from inside the Station. The spacewalks were described as the "most challenging since [the last] Hubble repairs".

The entire spacewalk campaign was a central feature of the Disney+ docuseries Among The Stars.

First spacewalk

The first spacewalk was conducted on November 15, 2019. The spacewalk began with the removal of the debris shield covering AMS, which was jettisoned to burn up in the atmosphere. The next task was to install three handrails in the vicinity of AMS to prepare for the next spacewalks and remove zip ties on the AMS' vertical support strut. This was followed by the "get ahead" tasks: Luca Parmitano removed the screws from a carbon-fibre cover under the insulation and passed the cover to Andrew Morgan to jettison. The spacewalkers also removed the vertical support beam cover. The duration of the spacewalk was 6 hours and 39 minutes.

Second spacewalk

The second spacewalk was conducted on November 22, 2019. Parmitano and Morgan cut a total of eight stainless steel tubes, including one that vented the remaining carbon dioxide from the old cooling pump. The crew members also prepared a power cable and installed a mechanical attachment device in advance of installing the new cooling system. The duration of the spacewalk was 6 hours and 33 minutes.

Third spacewalk

The third spacewalk was conducted on December 2, 2019. The crew completed the primary task of installing the upgraded cooling system, called the upgraded tracker thermal pump system (UTTPS), completed the power and data cable connections for the system, and connected all eight cooling lines from the AMS to the new system. The intricate connection work required making a clean cut for each existing stainless steel tube connected to the AMS, then connecting it to the new system through swaging.

The astronauts also completed an additional task to install an insulating blanket on the nadir side of the AMS to replace the heat shield and blanket they removed during the first spacewalk to begin the repair work. The flight control team on Earth initiated power-up of the system and confirmed its reception of power and data.

The duration of the spacewalk was 6 hours and 2 minutes.

Fourth spacewalk

The final spacewalk was conducted on January 25, 2020. The astronauts conducted leak checks for the cooling system on the AMS and opened a valve to pressurize the system. Parmitano found a leak in one of the AMS's cooling lines. The leak was fixed during the spacewalk. Preliminary testing showed the AMS was responding as expected.

Ground teams are working to fill the new AMS thermal control system with carbon dioxide, allow the system to stabilize, and power on the pumps to verify and optimize their performance. The tracker, one of several detectors on the AMS, began collecting science data again before the end of the week after the spacewalk.

The astronauts also completed an additional task to remove degraded lens filters on two high-definition video cameras.

The duration of the spacewalk was 6 hours and 16 minutes.

Specifications

• Mass: 7,500 kilograms (16,500 lb)
• Structural material: Stainless steel
• Power: 2,500 W
• Internal data rate: 7 Gbit/s
• Data rate to ground: 2 Mbit/s (typical, average)
• Primary mission duration: 10 to 18 years
• Design life: 3 years.
• Magnetic field intensity: 0.15 teslas produced by a 1,200 kilograms (2,600 lb) permanent neodymium magnet
• Original superconducting magnet: 2 coils of niobium-titanium at 1.8 K producing a central field of 0.87 teslas (Not used in the actual device)
• AMS-02 flight magnet changed to non-superconducting AMS-01 version to extend experiment life and to solve reliability problems in the operation of the superconducting system

About 1,000 cosmic rays are recorded by the instrument per second, generating about one GB/sec of data. This data is filtered and compressed to about 300 kbit/s for download to the operation center POCC at CERN.

Design

The detector module consists of a series of detectors that are used to determine various characteristics of the radiation and particles as they pass through. Characteristics are determined only for particles that pass through from top to bottom. Particles that enter the detector at any other angles are rejected. From top to bottom the subsystems are identified as:

• Transition radiation detector measures the velocities of the highest energy particles;
• Upper time of flight counter, along with the lower time of flight counter, measures the velocities of lower energy particles;
• Star tracker determines the orientation of the module in space;
• Silicon tracker (9 disks among 6 locations) measures the coordinates of charged particles in the magnetic field;
  • Has 4 redundant coolant pumps
• Permanent magnet bends the path of charged particles so they can be identified;
• Anti-coincidence counter rejects stray particles that enter through the sides;
• Ring imaging Cherenkov detector measures velocity of fast particles with extreme accuracy;
• Electromagnetic calorimeter measures the total energy of the particles.

Scientific goals

The AMS-02 will use the unique environment of space to advance knowledge of the Universe and lead to the understanding of its origin by searching for antimatter, dark matter and measuring cosmic rays.

Antimatter

See also: Antimatter

Experimental evidence indicates that our galaxy is made of matter; however, scientists believe there are about 100–200 billion galaxies in the observable Universe and some versions of the Big Bang theory of the origin of the Universe require equal amounts of matter and antimatter. Theories that explain this apparent asymmetry violate other measurements. Whether or not there is significant antimatter is one of the fundamental questions of the origin and nature of the Universe. Any observations of an antihelium nucleus would provide evidence for the existence of antimatter in space. In 1999, AMS-01 established a new upper limit of 10⁻⁶ for the antihelium/helium flux ratio in the Universe. AMS-02 was designed to search with a sensitivity of 10⁻⁹, an improvement of three orders of magnitude over AMS-01, sufficient to reach the edge of the expanding Universe and resolve the issue definitively.

Dark matter

See also: Dark matter

The visible matter in the Universe, such as stars, adds up to less than 5 percent of the total mass that is known to exist from many other observations. The other 95 percent is dark, either dark matter, which is estimated at 20 percent of the Universe by weight, or dark energy, which makes up the balance. The exact nature of both still is unknown. One of the leading candidates for dark matter is the neutralino. If neutralinos exist, they should be colliding with each other and giving off an excess of charged particles that can be detected by AMS-02. Any peaks in the background positron, antiproton, or gamma ray flux could signal the presence of neutralinos or other dark matter candidates, but would need to be distinguished from poorly known confounding astrophysical signals.

Strangelets

See also: Strangelet

Six types of quarks (up, down, strange, charm, bottom and top) have been found experimentally; however, the majority of matter on Earth is made up of only up and down quarks. It is a fundamental question whether there exists stable matter made up of strange quarks in combination with up and down quarks. Particles of such matter are known as strangelets. Strangelets might have extremely large mass and very small charge-to-mass ratios. It would be a totally new form of matter. AMS-02 may determine whether this extraordinary matter exists in our local environment.

Space radiation environment

Cosmic radiation during transit is a significant obstacle to sending humans to Mars. Accurate measurements of the cosmic ray environment are needed to plan appropriate countermeasures. Most cosmic ray studies are done by balloon-borne instruments with flight times that are measured in days; these studies have shown significant variations. AMS-02 is operative on the ISS, gathering a large amount of accurate data and allowing measurements of the long term variation of the cosmic ray flux over a wide energy range, for nuclei from protons to iron. In addition to understanding the radiation protection required for astronauts during interplanetary flight, this data will allow the interstellar propagation and origins of cosmic rays to be identified.

Results

In July 2012, it was reported that AMS-02 had observed over 18 billion cosmic rays.

In February 2013, Samuel Ting reported that in its first 18 months of operation AMS had recorded 25 billion particle events including nearly eight billion fast electrons and positrons. The AMS paper reported the positron-electron ratio in the mass range of 0.5 to 350 GeV, providing evidence about the weakly interacting massive particle (WIMP) model of dark matter.

On March 30, 2013, the first results from the AMS experiment were announced by the CERN press office. The first physics results were published in Physical Review Letters on April 3, 2013. A total of 6.8×10⁶ positron and electron events were collected in the energy range from 0.5 to 350 GeV. The positron fraction (of the total electron plus positron events) steadily increased from energies of 10 to 250  GeV, but the slope decreased by an order of magnitude above 20 GeV, even though the fraction of positrons still increased. There was no fine structure in the positron fraction spectrum, and no anisotropies were observed. The accompanying Physics Viewpoint said that "The first results from the space-borne Alpha Magnetic Spectrometer confirm an unexplained excess of high-energy positrons in Earth-bound cosmic rays." These results are consistent with the positrons originating from the annihilation of dark matter particles in space, but not yet sufficiently conclusive to rule out other explanations. Ting said "Over the coming months, AMS will be able to tell us conclusively whether these positrons are a signal for dark matter, or whether they have some other origin."

On September 18, 2014, new results with almost twice as much data were presented in a talk at CERN and published in Physical Review Letters. A new measurement of positron fraction up to 500 GeV was reported, showing that positron fraction peaks at a maximum of about 16% of total electron+positron events, around an energy of 275 ± 32 GeV. At higher energies, up to 500 GeV, the ratio of positrons to electrons begins to fall again.

AMS presented for 3 days at CERN in April 2015, covering new data on 300 million proton events and helium flux. It revealed in December 2016 that it had discovered a few signals consistent with antihelium nuclei amidst several billion helium nuclei. The result remains to be verified, and the team is currently trying to rule out contamination.

A study from 2019, using data from NASA's Fermi Gamma-ray Space Telescope discovered a halo around the nearby pulsar Geminga. The accelerated electrons and positrons collide with nearby starlight. The collision boosts the light up to much higher energies. Geminga alone could be responsible for as much as 20% of the high-energy positrons seen by the AMS-02 experiment.

The AMS-02 on the ISS has, as of 2021, recorded eight events that seem to indicate the detection of antihelium-3.

As of 2023, the AMS-02 has collected more than 215 billion cosmic ray events.

Cybernetics in the Soviet Union

From Wikipedia, the free encyclopedia

Cybernetics in the Soviet Union had its own particular characteristics, as the study of cybernetics came into contact with the dominant scientific ideologies of the Soviet Union and the nation's economic and political reforms: from the unmitigated anti-Americanist criticism of cybernetics in the early 1950s; its legitimization after Stalin's death and up to 1961; its total saturation of Soviet academia in the 1960s; and its eventual decline through the 1970s and 1980s.

Initially, from 1950-1954, the reception of cybernetics by the Soviet Union establishment was exclusively negative. The Soviet Department for Agitation and Propaganda had called for anti-Americanism to be intensified in Soviet media, and in an attempt to fill the Department's quotas, Soviet journalists latched on to cybernetics as an American "reactionary pseudoscience" to denounce and mock. This attack was interpreted as a signal of an official attitude to cybernetics, so, under Joseph Stalin's premiership, cybernetics was inflated into "a full embodiment of imperialist ideology" by Soviet writers. Upon Stalin's death, the wide-reaching reforms of Nikita Khrushchev's premiership allowed cybernetics to legitimize itself as "a serious, important science", and in 1955, articles on cybernetics were published in the state philosophical organ, Voprosy Filosofii, after a group of Soviet scientists realized the potential of this new science.

Under the formerly suppressive scientific culture of the Soviet Union, cybernetics began to serve as an umbrella term for previously maligned areas of Soviet science, such as structural linguistics and genetics. Under the leadership of academician Aksel Berg, the Council of Cybernetics was formed, an umbrella organization dedicated to providing funding for these new lights of Soviet science. By the 1960s, this fast legitimization put cybernetics in fashion, as "cybernetics" became a buzzword among career-minded scientists. Additionally, Berg's administration left many of the original cyberneticians of the organization disgruntled; complaints were made that he seemed more focused on administration than scientific research, citing Berg's grand plans to expand the council to subsume "practically all of Soviet science". By the 1980s, cybernetics had lost relevance in Soviet scientific culture, as its terminology and political function were succeeded by those of informatics in the Soviet Union and, eventually, post-Soviet states.

Official criticism: 1950–1954

Cybernetics: a reactionary pseudoscience that appeared in the U.S.A. after World War II and also spread through other capitalist countries. Cybernetics clearly reflects one of the basic features of the bourgeois worldview—its inhumanity, striving to transform workers into an extension of the machine, into a tool of production, and an instrument of war. At the same time, for cybernetics an imperialistic utopia is characteristic—replacing living, thinking man, fighting for his interests, by a machine, both in industry and in war. The instigators of a new world war use cybernetics in their dirty, practical affairs.

"Cybernetics" in the Short Philosophical Dictionary, 1954

The initial reception of cybernetics in the stifling scientific culture of Soviet state-sanctioned media and academic publication was exclusively negative. Under the plans of the Soviet Department for Agitation and Propaganda, Soviet anti-American propaganda was to be intensified, in order "to show the decay of bourgeois culture and morals" and "debunk the myths of American propaganda" in the wake of the formation of NATO. This imperative put Soviet newspaper editors in a frantic search for topics to criticize, in order to fill these propagandistic quotas.

The first to latch onto Cybernetics was science journalist, Boris Agapov, following the post-war American interest in the developments in computer technology. The cover of the January 23, 1950, issue of Time had boasted an anthropomorphic cartoon of a Harvard Mark III under the slogan "Can Man Build a Superman?". On 4 May 1950, Agapov published an article in the Literaturnaya Gazeta entitled "Mark III, a Calculator", ridiculing this American excitement at the "sweet dream" of the military and industrial uses of these new "thinking machines", and criticizing cybernetics originator Norbert Wiener as an example of the "charlatans and obscurantists, whom capitalists substitute for genuine scientists".

Though it was not commissioned by any Soviet authority and never mentioned the science by name, Agapov's article was taken as a signal of an official critical attitude towards cybernetics; editions of Wiener's Cybernetics were removed from library circulation, and several other periodicals followed suit, denouncing cybernetics as a "reactionary pseudoscience". In 1951, Mikhail Yaroshevsky [ru], of the Institute of Philosophy, led a public campaign against the philosophy of "semantic idealism", characterizing Wiener, and cybernetics as a whole, as a part of this "reactionary philosophy". In 1952, another more explicitly anti-cybernetic article was published in the Literaturnaya Gazeta, definitively starting the campaign and leading the way for a flurry of popular titles denouncing the topic. At the zenith of this criticism, an article in the October 1953 issue of the state ideological organ, Voprosy Filosofii, was published under the pseudonym "Materialist", entitled "Whom Does Cybernetics Serve?"; it condemned cybernetics as a "misanthropic pseudo-theory" consisting of "mechanicism turning into idealism", pointing to the American military as the "god whom cybernetics served".

During this period, Stalin himself never engaged in this rabid criticism of cybernetics, with the head of the Soviet Department of Sciences, Iurii Zhdanov, recalling that "he never opposed cybernetics" and made every effort "to advance computer technology" in order to give the USSR the technological advantage. Though the scale of this campaign was modest, with only around 10 anti-cybernetic publications being produced, Valery Shilov has argued it constituted a "strict directive to action" from the "central ideological organs", a universal declaration of cybernetics as a bourgeois pseudoscience to be criticized and destroyed.

Few of these critics had any access to primary sources on cybernetics. Agapov's sources were limited to the January 1950 issue of Time; the Institute's criticisms were based on the 1949 volume of ETC: A Review of General Semantics; and, among Soviet articles on cybernetics, only the "Materialist" quoted Wiener's Cybernetics directly. Select sensational quotes of Wiener and speculations based "exclusively on the basis of other [Soviet] books already written on the same or similar subject", were used to characterize Wiener as both an idealist and a mechanicist, criticizing his supposed reduction of scientific and sociological ideas to mere "mechanical model[s]". Wiener's gloomy speculations on the "second industrial revolution" and the "assembly line without human agents" were distorted to brand him as a "technocrat", wishing for "the process of production realized without workers, only with machines controlled by the gigantic brain of the computer" with "no strikes or strike movements, and moreover no revolutionary insurrections". According to Slava Gerovitch, "each critic carried criticism one step further, gradually inflating the significance of cybernetics until it was seen as a full embodiment of imperialist ideology".

Legitimization and rise: 1954–1961

Joseph Stalin and Nikita Khrushchev, 1936; the death of Stalin (right) and accession of Khrushchev (left) in 1953, alongside the following political thaw, allowed cybernetics to be legitimized in the Soviet Union.

The reformed academic culture of the Soviet Union, after the death of Stalin and reforms of the Khrushchev era, allowed cybernetics to tear down its previous ideological criticisms and redeem itself in the public view. To Soviet scientists, cybernetics emerged as a possible vector of escape from the ideological traps of Stalinism, replacing it with the computational objectivity of cybernetics.

Military computer scientist Anatoly Kitov recalled stumbling onto Cybernetics in the secret library of the Special Construction Bureau and realizing instantly that "cybernetics was not a bourgeois pseudo-science, as official publications considered it at the time, but the opposite—a serious, important science". He joined with the dissident mathematician Alexey Lyapunov, and, in 1952, presented a pro-cybernetic paper to Voprosy Filosofii, which the journal tacitly endorsed, though the Communist Party required that Lyapunov and Kitov present public lectures on cybernetics before its publication, with 121 seminars produced in total from 1954 until 55.

A very different academic, the Soviet philosopher and former ideological watchdog Ernst Kolman, also joined this rehabilitation. In November 1954, Kolman presented a lecture at the Academy of Social Sciences, condemning this stifling of cybernetics to a shocked audience, who had expected a lecture rehearsing previous Stalinist criticisms, and marched down to the office of Voprosy Filosofii to have his lecture published.

The beginning of a Soviet cybernetic movement was therefore first signalled by two articles, published together in the July–August 1955 volume of Voprosy Filosofii: "The Main Features of Cybernetics" by Sergei Sobolev, Alexey Lyapunov, and Anatoly Kitov, and "What is Cybernetics" by Ernst Kolman. According to Benjamin Peters, these "two Soviet articles set the stage for the revolution of cybernetics in the Soviet Union".

The first article—authored by three Soviet military scientists—attempted to present the tenets of cybernetics as a coherent scientific theory, retooling it for Soviet use; they purposely avoided any discussion of philosophy, and presented Wiener as an American anti-capitalist, in order to avoid any politically dangerous confrontation. They asserted cybernetics' main tenets as:

1. information theory,
2. the theory of automatic high-speed electronic calculating machines as a theory of self-organizing logical processes,
3. the theory of automatic control systems (particularly, the theory of feedback).

In juxtaposition, Kolman's defense of cybernetics mirrored the Stalinist criticisms it had endured. Kolman created a spurious historiography of cybernetics (which inevitably found its origins in Soviet science) and corrected the supposed "deviations" of the anti-cybernetic philosophers, employing well-placed quotes from Marxist authorities and philosophical epithets (e.g. "idealist" or "vitalist"), implying cybernetics' opponents fell into the same philosophical errors Marx and Lenin had criticized decades earlier, within their dialectical materialist framework.

With this, Soviet cybernetics began its journey towards legitimization. Academician Aksel Berg, at the time Deputy Minister of Defense, authored secret reports beleaguering the deficient state of information science in the USSR, pointing towards the suppression of cybernetics as the prime culprit. Party officials allowed a small Soviet delegation to be sent to the First International Congress on Cybernetics in June 1956, and they informed the Party of the extent to which USSR was "lagging behind the developed countries" in computer technology. Unfavorable descriptions of cybernetics were removed from official literature, and in 1958, the first Russian translations of Wiener were published.

The publishing of the first Soviet journal on cybernetics, Проблемы кибернетики [Problems of Cybernetics], was launched with Lyapunov as its editor. For the 1960 First International Federation of Automatic Control, Wiener came to Russia to lecture on cybernetics at the Polytechnic Museum. He arrived to see the booked hall swarmed with scientists eager to hear his lecture, some of whom sat on aisles and stairs to hear him speak; several Soviet publications, including the formerly anti-cybernetic Voprosy Filosofii, crammed in to get interviews from Wiener. In the Krushchev Thaw, Soviet cybernetics had not only been legitimized as a science, but had entered the vogue in Soviet academia.

On 10 April 1959, Berg sent a report edited by Lyapunov to a presidium of the Academy of Sciences, recommending the establishment of an organization dedicated to advancing cybernetics. The presidium determined that the Council on Cybernetics would be formed, with Berg as the chairman (due to his strong administrative connections) and Lyapunov his deputy. This council was wide-reaching, subsuming as many as 15 disciplines as of 1967, from "cybernetic linguistics" to "legal cybernetics". During Khrushchev's relaxation of scientific culture, the Council on Cybernetics served as an umbrella organization for formerly suppressed research, including such subjects as non-Pavlovian physiology ("physiological cybernetics"), structural linguistics ("cybernetic linguistics"), and genetics ("biological cybernetics").

Thanks to Lyapunov, a further, 20-person Department of Cybernetics was created to solicit official funding for cybernetic research. Even with these institutions, Lyapunov still lamented that "the field of cybernetics in our country is not organized", and, from 1960–61, worked with the Department to establish an official Institute of Cybernetics. Lyapunov joined forces with the structural linguists, who had been authorized to create the Institute of Semiotics directed by Andrey Markov Jr., and, in June 1961, together planned to create an Institute of Cybernetics. Despite these efforts, Lyapunov lost faith in the project after Krushchev's refusal to build more Moscow scientific institutes, and the Institute never emerged, settling with the Council of Cybernetics instead gaining the formal powers of an institute, without any expansion of staff.

Peak and decline: 1961–1980s

See also: Academset, OGAS, and VNIIPAS

Number of authors engaged in each discipline of the Institute of Automation and Remote Control, from 1950–69.

Berg continued with his campaign for Soviet cybernetics into the 1960s, as cybernetics entered the Soviet mainstream. Berg's council sponsored pro-cybernetic programs in Soviet media. 20-minute radio broadcasts, entitled "Cybernetics in Our Lives", were produced; a series of broadcasts on Moscow TV detailed advances in computer technology; and hundreds of lectures were given before various party members and workers on the subject of cybernetics. In 1961, the council produced an official volume proffering cybernetics as a socialist science: entitled Cybernetics—in the Service of Communism.

The work of the council was rewarded when, at the 22nd Party Congress, cybernetics was declared one of the "major tools of the creation of a communist society". Khrushchev declared the development of cybernetics an "imperative" in Soviet science. According to Gerovitch, this put cybernetics "in fashion" as "many career-minded scientists began using 'cybernetics' as a buzzword" and the movement swelled with its new membership. The CIA reported that the July 1962 'Conference on the Philosophical Problems of Cybernetics' received "approximately 1000 specialists, mathematicians, philosophers, physicists, economists, psychologists, biologists, engineers, linguists, physicians". American intelligence apparently bought into the hype, though it confused institutional enthusiasm with Soviet government policy. Special Assistant Arthur Schlesinger Jr warned President John F. Kennedy that the Soviet commitment to cybernetics provided them "a tremendous advantage" in technology and economic productivity; in the absence of any complementary American program, Schlesinger wrote, "we are finished".

In July 1962, Berg created a plan for the radical restructuring of the Council such that it covered "practically all of Soviet science". This was met with cold reception from many of the researchers of the Council, with one cybernetician complaining, in a letter to Lyapunov, that "[t]here are almost no results from the Council. Berg only demands paperwork and strives for the expansion of the Council." Lyapunov, disgruntled with Berg and the non-academic direction of cybernetics, refused to write for Cybernetics—in the Service of Communism and gradually lost his influence in cybernetics. As one memoirist put it, this resignation meant that "the center that had unified cybernetics disappeared, and cybernetics [would] naturally split into numerous branches." While the old guard of cyberneticians complained, the cybernetics movement, as a whole, was exploding; with the council subsuming 170 projects and 29 institutions by 1962, and 500 projects and 150 institutions by 1967.

According to Gerovitch, "by the early 1970s, the cybernetics movement [...] no longer challenged the orthodoxy; instead, tactical uses of cyberspeak overshadowed the original reformist goals that aspired the first Soviet cyberneticians." The ideas which were once seen as controversial, and huddled under the umbrella organization of cybernetics, now entered the scientific mainstream, leaving cybernetics as a loose and incoherent ideological patchwork. Some cyberneticians, whose dissident styles had been sheltered by the cybernetics movement, now felt persecuted, and some, such as Valentin Turchin, Alexander Lerner, and Igor Mel'čuk emigrated to escape this newfound scientific atmosphere. By the 1980s, cybernetics had lost its cultural relevance, being replaced in Soviet scientific culture with the concepts of 'informatics'.

Notable Soviet cyberneticists

• Aksel Berg (1893–1979) Deputy Minister of Defense of the Soviet Union (September 1953–November 1957)
• Yuri Gastev (1928–1993) dissident who emigrated in 1981
• Victor Glushkov (1923–1982) Soviet mathematician and founding father of Soviet cybernetics
• Anatoly Kitov (1920–2005)
• Andrey Kolmogorov (1903-1987)
• Leonid Kraizmer (1912–2002)
• Alexey Lyapunov (1911–1973)
• Sergei Sobolev (1908–1989)