Pulse-Doppler radar

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Pulse-Doppler_radar 

 

Airborne pulse-Doppler radar antenna

A pulse-Doppler radar is a radar system that determines the range to a target using pulse-timing techniques, and uses the Doppler effect of the returned signal to determine the target object's velocity. It combines the features of pulse radars and continuous-wave radars, which were formerly separate due to the complexity of the electronics.

The first operational Pulse Doppler radar was in the CIM-10 Bomarc, an American long range supersonic missile powered by ramjet engines, and which was armed with a W40 nuclear weapon to destroy entire formations of attacking enemy aircraft. Pulse-Doppler systems were first widely used on fighter aircraft starting in the 1960s. Earlier radars had used pulse-timing in order to determine range and the angle of the antenna (or similar means) to determine the bearing. However, this only worked when the radar antenna was not pointed down; in that case the reflection off the ground overwhelmed any returns from other objects. As the ground moves at the same speed but opposite direction of the aircraft, Doppler techniques allow the ground return to be filtered out, revealing aircraft and vehicles. This gives pulse-Doppler radars "look-down/shoot-down" capability. A secondary advantage in military radar is to reduce the transmitted power while achieving acceptable performance for improved safety of stealthy radar.

Pulse-Doppler techniques also find widespread use in meteorological radars, allowing the radar to determine wind speed from the velocity of any precipitation in the air. Pulse-Doppler radar is also the basis of synthetic aperture radar used in radar astronomy, remote sensing and mapping. In air traffic control, they are used for discriminating aircraft from clutter. Besides the above conventional surveillance applications, pulse-Doppler radar has been successfully applied in healthcare, such as fall risk assessment and fall detection, for nursing or clinical purposes.

History

The earliest radar systems failed to operate as expected. The reason was traced to Doppler effects that degrade performance of systems not designed to account for moving objects. Fast-moving objects cause a phase-shift on the transmit pulse that can produce signal cancellation. Doppler has maximum detrimental effect on moving target indicator systems, which must use reverse phase shift for Doppler compensation in the detector.

Doppler weather effects (precipitation) were also found to degrade conventional radar and moving target indicator radar, which can mask aircraft reflections. This phenomenon was adapted for use with weather radar in the 1950s after declassification of some World War II systems.

Pulse-Doppler radar was developed during World War II to overcome limitations by increasing pulse repetition frequency. This required the development of the klystron, the traveling wave tube, and solid state devices. Early pulse-dopplers were incompatible with other high power microwave amplification devices that are not coherent, but more sophisticated techniques were developed that record the phase of each transmitted pulse for comparison to returned echoes.

Early examples of military systems includes the AN/SPG-51B developed during the 1950s specifically for the purpose of operating in hurricane conditions with no performance degradation.

The Hughes AN/ASG-18 Fire Control System was a prototype airborne radar/combination system for the planned North American XF-108 Rapier interceptor aircraft for the United States Air Force, and later for the Lockheed YF-12. The US's first pulse-Doppler radar, the system had look-down/shoot-down capability and could track one target at a time.

Weather, chaff, terrain, flying techniques, and stealth are common tactics used to hide aircraft from radar. Pulse-Doppler radar eliminates these weaknesses.

It became possible to use pulse-Doppler radar on aircraft after digital computers were incorporated in the design. Pulse-Doppler provided look-down/shoot-down capability to support air-to-air missile systems in most modern military aircraft by the mid 1970s.

Principle

Principle of pulse-Doppler radar

Range measurement

Principle of pulsed radar

Pulse-Doppler systems measure the range to objects by measuring the elapsed time between sending a pulse of radio energy and receiving a reflection of the object. Radio waves travel at the speed of light, so the distance to the object is the elapsed time multiplied by the speed of light, divided by two - there and back.

Velocity measurement

Change of wavelength caused by motion of the source

Pulse-Doppler radar is based on the Doppler effect, where movement in range produces frequency shift on the signal reflected from the target.

${\displaystyle {\text{Doppler frequency}}={\frac {2\times {\text{transmit frequency}}\times {\text{radial velocity}}}{C}}.}$

Radial velocity is essential for pulse-Doppler radar operation. As the reflector moves between each transmit pulse, the returned signal has a phase difference, or phase shift, from pulse to pulse. This causes the reflector to produce Doppler modulation on the reflected signal.

Pulse-Doppler radars exploit this phenomenon to improve performance.

The amplitude of the successively returning pulse from the same scanned volume is

${\displaystyle I=I_{0}\sin \left({\frac {4\pi (x_{0}+v\Delta t)}{\lambda }}\right)=I_{0}\sin(\Theta _{0}+\Delta \Theta ),}$

where

${\displaystyle x_{0}}$ is the distance radar to target,

${\displaystyle \lambda }$ is the radar wavelength,

${\displaystyle \Delta t}$ is the time between two pulses.

So

${\displaystyle \Delta \Theta ={\frac {4\pi v\Delta t}{\lambda }}.}$

This allows the radar to separate the reflections from multiple objects located in the same volume of space by separating the objects using a spread spectrum to segregate different signals:

${\displaystyle v={\text{target speed}}={\frac {\lambda \Delta \Theta }{4\pi \Delta t}},}$

where ${\displaystyle \Delta \Theta }$ is the phase shift induced by range motion.

Benefits

Rejection speed is selectable on pulse-Doppler aircraft-detection systems so nothing below that speed will be detected. A one degree antenna beam illuminates millions of square feet of terrain at 10 miles (16 km) range, and this produces thousands of detections at or below the horizon if Doppler is not used.

Pulse-Doppler radar uses the following signal processing criteria to exclude unwanted signals from slow-moving objects. This is also known as clutter rejection. Rejection velocity is usually set just above the prevailing wind speed (10 to 100 mile/hour or 15 to 150 km/hour). The velocity threshold is much lower for weather radar.

${\displaystyle \left\vert {\frac {{\text{Doppler frequency}}\times C}{2\times {\text{transmit frequency}}}}\right\vert >{\text{velocity threshold}}.}$

In airborne pulse-Doppler radar, the velocity threshold is offset by the speed of the aircraft relative to the ground.

${\displaystyle \left\vert {\frac {{\text{Doppler frequency}}\times C}{2\times {\text{transmit frequency}}}}-{\text{ground speed}}\times \cos \Theta \right\vert >{\text{velocity threshold}},}$

where ${\displaystyle \Theta }$ is the angle offset between the antenna position and the aircraft flight trajectory.

Surface reflections appear in almost all radar. Ground clutter generally appears in a circular region within a radius of about 25 miles (40 km) near ground-based radar. This distance extends much further in airborne and space radar. Clutter results from radio energy being reflected from the earth surface, buildings, and vegetation. Clutter includes weather in radar intended to detect and report aircraft and spacecraft.

Clutter creates a vulnerability region in pulse-amplitude time-domain radar. Non-Doppler radar systems cannot be pointed directly at the ground due to excessive false alarms, which overwhelm computers and operators. Sensitivity must be reduced near clutter to avoid overload. This vulnerability begins in the low-elevation region several beam widths above the horizon, and extends downward. This also exists throughout the volume of moving air associated with weather phenomenon.

Pulse-Doppler radar corrects this as follows.

• Allows the radar antenna to be pointed directly at the ground without overwhelming the computer and without reducing sensitivity.
• Fills in the vulnerability region associated with pulse-amplitude time-domain radar for small object detection near terrain and weather.
• Increases detection range by 300% or more in comparison to moving target indication (MTI) by improving sub-clutter visibility.

Clutter rejection capability of about 60 dB is needed for look-down/shoot-down capability, and pulse-Doppler is the only strategy that can satisfy this requirement. This eliminates vulnerabilities associated with the low-elevation and below-horizon environment.

Pulse compression, and moving target indicator (MTI) provide up to 25 dB sub-clutter visibility. MTI antenna beam is aimed above horizon to avoid excessive false alarm rate, which renders systems vulnerable. Aircraft and some missiles exploit this weakness using a technique called flying below the radar to avoid detection (Nap-of-the-earth). This flying technique is ineffective against pulse-Doppler radar.

Pulse-Doppler provides an advantage when attempting to detect missiles and low observability aircraft flying near terrain, sea surface, and weather.

Audible Doppler and target size support passive vehicle type classification when identification friend or foe is not available from a transponder signal. Medium pulse repetition frequency (PRF) reflected microwave signals fall between 1,500 and 15,000 cycle per second, which is audible. This means a helicopter sounds like a helicopter, a jet sounds like a jet, and propeller aircraft sound like propellers. Aircraft with no moving parts produce a tone. The actual size of the target can be calculated using the audible signal.

Detriments

Maximum range from reflectivity (red) and unambiguous Doppler velocity range (blue) with a fix pulse repetition rate.

Ambiguity processing is required when target range is above the red line in the graphic, which increases scan time.

Scan time is a critical factor for some systems because vehicles moving at or above the speed of sound can travel one mile (1.6 km) every few seconds, like the Exocet, Harpoon, Kitchen, and Air-to-air missile. The maximum time to scan the entire volume of the sky must be on the order of a dozen seconds or less for systems operating in that environment.

Pulse-Doppler radar by itself can be too slow to cover the entire volume of space above the horizon unless fan beam is used. This approach is used with the AN/SPS 49(V)5 Very Long Range Air Surveillance Radar, which sacrifices elevation measurement to gain speed.

Pulse-Doppler antenna motion must be slow enough so that all the return signals from at least 3 different PRFs can be processed out to the maximum anticipated detection range. This is known as dwell time. Antenna motion for pulse-Doppler must be as slow as radar using MTI.

Search radar that include pulse-Doppler are usually dual mode because best overall performance is achieved when pulse-Doppler is used for areas with high false alarm rates (horizon or below and weather), while conventional radar will scan faster in free-space where false alarm rate is low (above horizon with clear skies).

The antenna type is an important consideration for multi-mode radar because undesirable phase shift introduced by the radar antenna can degrade performance measurements for sub-clutter visibility.

Signal processing

The signal processing enhancement of pulse-Doppler allows small high-speed objects to be detected in close proximity to large slow moving reflectors. To achieve this, the transmitter must be coherent and should produce low phase noise during the detection interval, and the receiver must have large instantaneous dynamic range.

• Pulse-Doppler signal processing detailed explanation

Pulse-Doppler signal processing also includes ambiguity resolution to identify true range and velocity.

• Ambiguity resolution detailed explanation

The received signals from multiple PRF are compared to determine true range using the range ambiguity resolution process.

• Range ambiguity resolution detailed explanation

The received signals are also compared using the frequency ambiguity resolution process.

• Frequency ambiguity resolution detailed explanation

Range resolution

The range resolution is the minimal range separation between two objects traveling at the same speed before the radar can detect two discrete reflections:

${\displaystyle {\text{range resolution}}={\frac {C}{{\text{PRF}}\times ({\text{number of samples between transmit pulses}})}}.}$

In addition to this sampling limit, the duration of the transmitted pulse could mean that returns from two targets will be received simultaneously from different parts of the pulse.

Velocity resolution

The velocity resolution is the minimal radial velocity difference between two objects traveling at the same range before the radar can detect two discrete reflections:

${\displaystyle {\text{velocity resolution}}={\frac {C\times {\text{PRF}}}{2\times {\text{transmit frequency}}\times {\text{filter size in transmit pulses}}}}.}$

Special consideration

Pulse-Doppler radar has special requirements that must be satisfied to achieve acceptable performance.

Pulse repetition frequency

Main article: Pulse repetition frequency

Pulse-Doppler typically uses medium pulse repetition frequency (PRF) from about 3 kHz to 30 kHz. The range between transmit pulses is 5 km to 50 km.

Range and velocity cannot be measured directly using medium PRF, and ambiguity resolution is required to identify true range and speed. Doppler signals are generally above 1 kHz, which is audible, so audio signals from medium-PRF systems can be used for passive target classification.

Angular measurement

Radar systems require angular measurement. Transponders are not normally associated with pulse-Doppler radar, so sidelobe suppression is required for practical operation.

Tracking radar systems use angle error to improve accuracy by producing measurements perpendicular to the radar antenna beam. Angular measurements are averaged over a span of time and combined with radial movement to develop information suitable to predict target position for a short time into the future.

The two angle error techniques used with tracking radar are monopulse and conical scan.

• Monopulse radar
• Conical scanning

Coherency

Pulse-Doppler radar requires a coherent oscillator with very little noise. Phase noise reduces sub-clutter visibility performance by producing apparent motion on stationary objects.

Cavity magnetron and crossed-field amplifier are not appropriate because noise introduced by these devices interfere with detection performance. The only amplification devices suitable for pulse-Doppler are klystron, traveling wave tube, and solid state devices.

Scalloping

Main article: Radar scalloping

Pulse-Doppler signal processing introduces a phenomenon called scalloping. The name is associated with a series of holes that are scooped-out of the detection performance.

Scalloping for pulse-Doppler radar involves blind velocities created by the clutter rejection filter. Every volume of space must be scanned using 3 or more different PRF. A two PRF detection scheme will have detection gaps with a pattern of discrete ranges, each of which has a blind velocity.

Windowing

Ringing artifacts pose a problem with search, detection, and ambiguity resolution in pulse-Doppler radar.

Ringing is reduced in two ways.

First, the shape of the transmit pulse is adjusted to smooth the leading edge and trailing edge so that RF power is increased and decreased without an abrupt change. This creates a transmit pulse with smooth ends instead of a square wave, which reduces ringing phenomenon that is otherwise associated with target reflection.

Second, the shape of the receive pulse is adjusted using a window function that minimizes ringing that occurs any time pulses are applied to a filter. In a digital system, this adjusts the phase and/or amplitude of each sample before it is applied to the fast Fourier transform. The Dolph-Chebyshev window is the most effective because it produces a flat processing floor with no ringing that would otherwise cause false alarms.

Antenna

Pulse-Doppler radar is generally limited to mechanically aimed antennas and active phase array.

Mechanical RF components, such as wave-guide, can produce Doppler modulation due to phase shift induced by vibration. This introduces a requirement to perform full spectrum operational tests using shake tables that can produce high power mechanical vibration across all anticipated audio frequencies.

Doppler is incompatible with most electronically steered phase-array antenna. This is because the phase-shifter elements in the antenna are non-reciprocal and the phase shift must be adjusted before and after each transmit pulse. Spurious phase shift is produced by the sudden impulse of the phase shift, and settling during the receive period between transmit pulses places Doppler modulation onto stationary clutter. That receive modulation corrupts the measure of performance for sub-clutter visibility. Phase shifter settling time on the order of 50ns is required. Start of receiver sampling needs to be postponed at least 1 phase-shifter settling time-constant (or more) for each 20 dB of sub-clutter visibility.

Most antenna phase shifters operating at PRF above 1 kHz introduce spurious phase shift unless special provisions are made, such as reducing phase shifter settling time to a few dozen nanoseconds.

The following gives the maximum permissible settling time for antenna phase shift modules.

${\displaystyle T={\frac {1}{e^{\frac {\text{SCV}}{20}}\times S\times {\text{PRF}}}},}$

where

T = phase shifter settling time,

SCV = sub-clutter visibility in dB,

S = number of range samples between each transmit pulse,

PRF = maximal design pulse repetition frequency.

The antenna type and scan performance is a practical consideration for multi-mode radar systems.

Diffraction

Choppy surfaces, like waves and trees, form a diffraction grating suitable for bending microwave signals. Pulse-Doppler can be so sensitive that diffraction from mountains, buildings or wave tops can be used to detect fast moving objects otherwise blocked by solid obstruction along the line of sight. This is a very lossy phenomenon that only becomes possible when radar has significant excess sub-clutter visibility.

Refraction and ducting use transmit frequency at L-band or lower to extend the horizon, which is very different from diffraction. Refraction for over-the-horizon radar uses variable density in the air column above the surface of the earth to bend RF signals. An inversion layer can produce a transient troposphere duct that traps RF signals in a thin layer of air like a wave-guide.

Subclutter visibility

Subclutter visibility involves the maximum ratio of clutter power to target power, which is proportional to dynamic range. This determines performance in heavy weather and near the earth surface.

${\displaystyle {\text{dynamic range}}=\min {\begin{cases}{\tfrac {\text{carrier power}}{\text{noise power}}}&{\text{transmit noise, where bandwidth is }}{\tfrac {\text{PRF}}{\text{filter size}}}\\2^{{\text{sample bits}}+{\text{filter size}}}&{\text{receiver dynamic range}}\end{cases}}.}$

Subclutter visibility is the ratio of the smallest signal that can be detected in the presence of a larger signal.

${\displaystyle {\text{subclutter visibility}}={\frac {\text{dynamic range}}{\text{CFAR detection threshold}}}.}$

A small fast-moving target reflection can be detected in the presence of larger slow-moving clutter reflections when the following is true:

${\displaystyle {\text{target power}}>{\frac {\text{clutter power}}{\text{subclutter visibility}}}.}$

Performance

The pulse-Doppler radar equation can be used to understand trade-offs between different design constraints, like power consumption, detection range, and microwave safety hazards. This is a very simple form of modeling that allows performance to be evaluated in a sterile environment.

The theoretical range performance is as follows.

${\displaystyle R=\left({\frac {P_{t}G_{t}A_{r}\sigma FD}{16\pi ^{2}K_{b}TBN}}\right)^{\frac {1}{4}},}$

where

R = distance to the target,

P_t = transmitter power,

G_t = gain of the transmitting antenna,

A_r = effective aperture (area) of the receiving antenna,

σ = radar cross section, or scattering coefficient, of the target,

F = antenna pattern propagation factor,

D = Doppler filter size (transmit pulses in each Fast Fourier transform),

K_b = Boltzmann's constant,

T = absolute temperature,

B = receiver bandwidth (band-pass filter),

N = noise figure.

This equation is derived by combining the radar equation with the noise equation and accounting for in-band noise distribution across multiple detection filters. The value D is added to the standard radar range equation to account for both pulse-Doppler signal processing and transmitter FM noise reduction.

Detection range is increased proportional to the fourth root of the number of filters for a given power consumption. Alternatively, power consumption is reduced by the number of filers for a given detection range.

Pulse-Doppler signal processing integrates all of the energy from all of the individual reflected pulses that enter the filter. This means a pulse-Doppler signal processing system with 1024 elements provides 30.103 dB of improvement due to the type of signal processing that must be used with pulse-Doppler radar. The energy of all of the individual pulses from the object are added together by the filtering process.

Signal processing for a 1024-point filter improves performance by 30.103 dB, assuming compatible transmitter and antenna. This corresponds to 562% increase in maximal distance.

These improvements are the reason pulse-Doppler is essential for military and astronomy.

Aircraft tracking uses

Pulse-Doppler radar for aircraft detection has two modes.

• Scan
• Track

Scan mode involves frequency filtering, amplitude thresholding, and ambiguity resolution. Once a reflection has been detected and resolved, the pulse-Doppler radar automatically transitions to tracking mode for the volume of space surrounding the track.

Track mode works like a phase-locked loop, where Doppler velocity is compared with the range movement on successive scans. Lock indicates the difference between the two measurements is below a threshold, which can only occur with an object that satisfies Newtonian mechanics. Other types of electronic signals cannot produce a lock. Lock exists in no other type of radar.

The lock criterion needs to be satisfied during normal operation.

• Pulse-Doppler signal processing § Lock

Lock eliminates the need for human intervention with the exception of helicopters and electronic jamming.

Weather phenomenon obey adiabatic process associated with air mass and not Newtonian mechanics, so the lock criterion is not normally used for weather radar.

Pulse-Doppler signal processing selectively excludes low-velocity reflections so that no detections occurs below a threshold velocity. This eliminates terrain, weather, biologicals, and mechanical jamming with the exception of decoy aircraft.

The target Doppler signal from the detection is converted from frequency domain back into time domain sound for the operator in track mode on some radar systems. The operator uses this sound for passive target classification, such as recognizing helicopters and electronic jamming.

Helicopters

Special consideration is required for aircraft with large moving parts because pulse-Doppler radar operates like a phase-locked loop. Blade tips moving near the speed of sound produce the only signal that can be detected when a helicopter is moving slow near terrain and weather.

Helicopters appears like a rapidly pulsing noise emitter except in a clear environment free from clutter. An audible signal is produced for passive identification of the type of airborne object. Microwave Doppler frequency shift produced by reflector motion falls into the audible sound range for human beings (20 – 20,000 Hz), which is used for target classification in addition to the kinds of conventional radar display used for that purpose, like A-scope, B-scope, C-scope, and RHI indicator. The human ear may be able to tell the difference better than electronic equipment.

A special mode is required because the Doppler velocity feedback information must be unlinked from radial movement so that the system can transition from scan to track with no lock.

Similar techniques are required to develop track information for jamming signals and interference that cannot satisfy the lock criterion.

Multi-mode

Pulse-Doppler radar must be multi-mode to handle aircraft turning and crossing trajectory.

Once in track mode, pulse-Doppler radar must include a way to modify Doppler filtering for the volume of space surrounding a track when radial velocity falls below the minimum detection velocity. Doppler filter adjustment must be linked with a radar track function to automatically adjust Doppler rejection speed within the volume of space surrounding the track.

Tracking will cease without this feature because the target signal will otherwise be rejected by the Doppler filter when radial velocity approaches zero because there is no change in frequency.

Multi-mode operation may also include continuous wave illumination for semi-active radar homing.

National Register of Historic Places

From Wikipedia, the free encyclopedia

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

National Register of Historic Places

Agency overview
Formed	1966; 56 years ago
Jurisdiction	United States
Annual budget	$16.8 million (2018)
Agency executive	Sherry A. Frear, Chief, National Register of Historic Places/National Historic Landmarks Program and Deputy Keeper of the National Register of Historic Places
Parent department	National Park Service
Website	www.nps.gov/subjects/nationalregister

The National Register of Historic Places (NRHP) is the United States federal government's official list of districts, sites, buildings, structures and objects deemed worthy of preservation for their historical significance. A property listed in the National Register, or located within a National Register Historic District, may qualify for tax incentives derived from the total value of expenses incurred in preserving the property.

The passage of the National Historic Preservation Act (NHPA) in 1966 established the National Register and the process for adding properties to it. Of the more than one and a half million properties on the National Register, 95,000 are listed individually. The remainder are contributing resources within historic districts.

For most of its history, the National Register has been administered by the National Park Service (NPS), an agency within the United States Department of the Interior. Its goals are to help property owners and interest groups, such as the National Trust for Historic Preservation, as well as to coordinate, identify and protect historic sites in the United States. While National Register listings are mostly symbolic, their recognition of significance provides some financial incentive to owners of listed properties. Protection of the property is not guaranteed. During the nomination process, the property is evaluated in terms of the four criteria for inclusion on the National Register of Historic Places. The application of those criteria has been the subject of criticism by academics of history and preservation, as well as the public and politicians.

Occasionally, historic sites outside the country proper, but associated with the United States (such as the American Legation in Tangier) are also listed. Properties can be nominated in a variety of forms, including individual properties, historic districts and multiple property submissions (MPS). The Register categorizes general listings into one of five types of properties: district, site, structure, building or object.

National Register Historic Districts are defined geographical areas consisting of contributing and non-contributing properties. Some properties are added automatically to the National Register when they become administered by the National Park Service. These include National Historic Landmarks (NHL), National Historic Sites (NHS), National Historical Parks, National Military Parks, National Memorials and some National Monuments. (Federal properties can be proclaimed National Monuments under the Antiquities Act because of either their historical or natural significance. They are managed by multiple agencies. Only monuments that are historic in character and managed by the National Park Service are listed administratively in the National Register.)

History

Main article: History of the National Register of Historic Places

 

Old Slater Mill, a historic district in Pawtucket, Rhode Island, was the first property listed in the National Register, on November 13, 1966.

 

George B. Hartzog Jr., director of the National Park Service from January 8, 1964, until December 31, 1972

On October 15, 1966, the Historic Preservation Act created the National Register of Historic Places and the corresponding State Historic Preservation Offices (SHPO). Initially, the National Register consisted of the National Historic Landmarks designated before the Register's creation, as well as any other historic sites in the National Park system. Approval of the act, which was amended in 1980 and 1992, represented the first time the United States had a broad-based historic preservation policy. The 1966 act required those agencies to work in conjunction with the SHPO and an independent federal agency, the Advisory Council on Historic Preservation (ACHP), to confront adverse effects of federal activities on historic preservation.

U.S. Secretary of the Interior (1977–1981) Cecil Andrus removed the National Register from the jurisdiction of the National Park Service in 1978.

To administer the newly created National Register of Historic Places, the National Park Service of the U.S. Department of the Interior, with director George B. Hartzog Jr., established an administrative division named the Office of Archeology and Historic Preservation (OAHP). Hartzog charged OAHP with creating the National Register program mandated by the 1966 law. Ernest Connally was the Office's first director. Within OAHP new divisions were created to deal with the National Register. The division administered several existing programs, including the Historic Sites Survey and the Historic American Buildings Survey, as well as the new National Register and Historic Preservation Fund.

The first official Keeper of the Register was William J. Murtagh, an architectural historian. During the Register's earliest years in the late 1960s and early 1970s, organization was lax and SHPOs were small, understaffed and underfunded. However, funds were still being supplied for the Historic Preservation Fund to provide matching grants-in-aid to listed property owners, first for house museums and institutional buildings, but later for commercial structures as well.

A few years later in 1979, the NPS history programs affiliated with both the U.S. National Parks system and the National Register were categorized formally into two "Assistant Directorates". Established were the Assistant Directorate for Archeology and Historic Preservation and the Assistant Directorate for Park Historic Preservation. From 1978 until 1981, the main agency for the National Register was the Heritage Conservation and Recreation Service (HCRS) of the United States Department of the Interior.

In February 1983, the two assistant directorates were merged to promote efficiency and recognize the interdependency of their programs. Jerry L. Rogers was selected to direct this newly merged associate directorate. He was described as a skilled administrator, who was sensitive to the need for the NPS to work with SHPOs, academia and local governments.

Although not described in detail in the 1966 act, SHPOs eventually became integral to the process of listing properties on the National Register. The 1980 amendments of the 1966 law further defined the responsibilities of SHPOs concerning the National Register. Several 1992 amendments of the NHPA added a category to the National Register, known as Traditional Cultural Properties: those properties associated with Native American or Hawaiian groups.

The National Register of Historic Places has grown considerably from its legislative origins in 1966. In 1986, citizens and groups nominated 3,623 separate properties, sites and districts for inclusion on the National Register, a total of 75,000 separate properties. Of the more than one and a half million properties on the National Register, 95,000 are listed individually. Others are listed as contributing members within historic districts.

Nomination process

It is hereby declared to be the policy of the United States Government that special effort should be made to preserve the natural beauty of the countryside and public park and recreation lands, wildlife and waterfowl refuges, and historic sites.

— (49 USC 303)

Any individual can prepare a National Register nomination, although historians and historic preservation consultants often are employed for this work. The nomination consists of a standard registration form (NPS 10-900) and contains basic information about a property's physical appearance and the type of significance embodied in the building, structure, object, site, or district.

The State Historic Preservation Office (SHPO) receives National Register nominations and provides feedback to the nominating individual or group. After preliminary review, the SHPO sends each nomination to the state's historic review commission, which then recommends whether the State Historic Preservation Officer should send the nomination to the Keeper of the National Register. For any non-Federally owned property, only the State Historic Preservation Officer may officially nominate a property for inclusion in the National Register. After the nomination is recommended for listing in the National Register by the SHPO, the nomination is sent to the National Park Service, which approves or denies the nomination.

If approved, the property is entered officially by the Keeper of the National Register into the National Register of Historic Places. Property owners are notified of the nomination during the review by the SHPO and state's historic review commission. If an owner objects to a nomination of private property, or in the case of a historic district, a majority of owners, then the property cannot be listed in the National Register of Historic Places.

Criteria

S. R. Crown Hall is listed under criteria B and C for its association with architect Ludwig Mies van der Rohe and modernist design.

For a property to be eligible for the National Register, it must meet at least one of the four National Register main criteria. Information about architectural styles, association with various aspects of social history and commerce and ownership are all integral parts of the nomination. Each nomination contains a narrative section that provides a detailed physical description of the property and justifies why it is significant historically with regard either to local, state, or national history. The four National Register of Historic Places criteria are the following.

• Criterion A, "Event", the property must make a contribution to the major pattern of American history.
• Criterion B, "Person", is associated with significant people of the American past.
• Criterion C, "Design/Construction", concerns the distinctive characteristics of the building by its architecture and construction, including having great artistic value or being the work of a master.
• Criterion D, "Information potential", is satisfied if the property has yielded or may be likely to yield information important to prehistory or history.

The criteria are applied differently for different types of properties; for instance, maritime properties have application guidelines different from those of buildings.

Exclusions

There are specific instances where properties usually do not merit listing in the National Register. As a general rule, cemeteries, birthplaces, graves of historical figures, properties owned by religious institutions or used for religious purposes, moved structures, reconstructed historic buildings, commemorative properties and properties that have achieved significance during the last fifty years are not qualified for listing on the Register. There are, however, exceptions to all the preceding; mitigating circumstances allow properties classified in one of those groups to be included.

Properties listed

See also: United States National Register of Historic Places listings and List of U.S. National Historic Landmarks by state

 

A typical plaque found on properties listed in the National Register of Historic Places

 

An alternate series of plaques. Buildings on the National Register are often listed in local historic societies as well.

A listing on the National Register of Historic Places is governmental acknowledgment of a historic district, site, building, or property. However, the Register is mostly "an honorary status with some federal financial incentives." The National Register of Historic Places automatically includes all National Historic Landmarks as well as all historic areas administered by the National Park Service.

Landmarks such as these include National Historic Sites (NHS), National Historical Parks, National Military Parks/Battlefields, National Memorials and some National Monuments. Occasionally, historic sites outside the United States borders, but associated with the United States, such as the American Legation in Tangier, Morocco, also are listed.

Listing in the National Register does not restrict private property owners from the use of their property.

Some states and municipalities, however, may have laws that become effective when a property is listed in the National Register. If federal money or a federal permitting process is involved, Section 106 of the National Historic Preservation Act of 1966 is invoked. Section 106 requires the federal agency involved to assess the effect of its actions on historic resources. Statutorily, the Advisory Council on Historic Preservation (ACHP) has the most significant role by Section 106 of the National Historic Preservation Act. The section requires that the director of any federal agency with direct or indirect jurisdiction of a project that may affect a property listed or determined eligible for listing in the National Register of Historic Places must first report to the Advisory Council. The director of said agency is required to "take into account the effect of the undertaking" on the National Register property, as well as to afford the ACHP a reasonable opportunity to comment.

While Section 106 does not mandate explicitly that any federal agency director accept the advice of the ACHP, their advice has a practical influence, especially given the statutory obligations of the NHPA that require federal agencies to "take into account the effect of the undertaking."

In cases where the ACHP determines federal action will have an "adverse effect" on historic properties, mitigation is sought. Typically, a Memorandum of Agreement (MOA) is created by which the involved parties agree to a particular plan. Many states have laws similar to Section 106. In contrast to conditions relating to a federally designated historic district, municipal ordinances governing local historic districts often restrict certain kinds of changes to properties. Thus, they may protect the property more than a National Register listing does.

The Department of Transportation Act, passed on October 15, 1966, the same day as the National Historic Preservation Act, included provisions that addressed historic preservation. The DOT Act is much more general than Section 106 NHPA in that it refers to properties other than those listed in the Register.

The more general language has allowed more properties and parklands to enjoy status as protected areas by this legislation, a policy developed early in its history. The United States Supreme Court ruled in the 1971 case Citizens to Preserve Overton Park v. Volpe that parklands could have the same protected status as "historic sites."

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  Pecos Pueblo is one of a number of NRHP sites administered by the National Park Service.

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  The Illinois State Capitol is one of 44 U.S. state capitols listed on the NRHP.

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  College Hill Historic District is an example of a National Historic Landmark District.

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  The Robert C. Weaver Federal Building is an example of a modern building listed on the NRHP.

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  The American Legation, Tangier is the only site on the NRHP in a foreign nation.

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  Walden Pond is an example of a natural site listed on the NRHP.

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  The USS Wisconsin (BB-64) is one of a number of ships listed on the NRHP.

Multiple property submission

Round Barns in Illinois Thematic Resources is a Multiple Property Submission that includes 18 structures throughout the state.

A multiple property submission (MPS) is a thematic group listing of the National Register of Historic Places that consists of related properties that share a common theme and can be submitted as a group. Multiple property submissions must satisfy certain basic criteria for the group of properties to be included in the National Register.

The process begins with the multiple property documentation form which acts as a cover document rather than the nomination to the National Register of Historic Places. The purpose of the documentation form is to establish the basis of eligibility for related properties. The information of the multiple property documentation form can be used to nominate and register related historic properties simultaneously, or to establish criteria for properties that may be nominated in the future. Thus, additions to an MPS can occur over time.

The nomination of individual properties in an MPS is accomplished in the same manner as other nominations. The name of the "thematic group" denotes the historical theme of the properties. It is considered the "multiple property listing." Once an individual property or a group of properties is nominated and listed in the National Register, the multiple property documentation form, combined with the individual National Register of Historic Places nomination forms, constitute a multiple property submission.

Examples of MPS include the Lee County Multiple Property Submission, the Warehouses in Omaha, the Boundary Markers of the Original District of Columbia and the Illinois Carnegie Libraries. Before the term "Multiple Property Submission" was introduced in 1984, such listings were known as "Thematic Resources", such as the Operating Passenger Railroad Stations Thematic Resource, or "Multiple Resource Areas".

Types of properties

See also: National Register of Historic Places property types and Historic districts in the United States

 

From top to bottom: a building, a structure, an object and a site – all are examples of NRHP property types.

 

Example of a barn on the National Register of Historic Places; Cow Barn; Enfield Shaker Village, New Hampshire; built 1854.

Listed properties are generally in one of five broad categories, although there are special considerations for other types of properties that in any one, or into more specialized subcategories. The five general categories for National Register properties are: building, structure, site, district and object. In addition, historic districts consist of contributing and non-contributing properties.

Buildings, as defined by the National Register, are distinguished in the traditional sense. Examples include a house, barn, hotel, church, or similar construction. They are created primarily to shelter human activity. The term building, as in outbuilding, can be used to refer to historically and functionally related units, such as a courthouse and a jail or a barn and a house.

Structures differ from buildings in that they are functional constructions meant to be used for purposes other than sheltering human activity. Examples include an aircraft, a grain elevator, a gazebo and a bridge.

Objects are usually artistic in nature, or small in scale compared to structures and buildings. Although objects may be movable, they are generally associated with a specific setting or environment. Examples of objects include monuments, sculptures and fountains.

Sites are the locations of significant events, which can be prehistoric or historic in nature and represent activities or buildings (standing, ruined, or vanished). When sites are listed, it is the locations themselves that are of historical interest. They possess cultural or archaeological value regardless of the value of any structures that currently exist at the locations. Examples of types of sites include shipwrecks, battlefields, campsites, natural features and rock shelters.

Historic districts possess a concentration, association, or continuity of the other four types of properties. Objects, structures, buildings and sites in a historic district are united historically or aesthetically, either by choice or by the nature of their development.

There are several other different types of historic preservation associated with the properties of the National Register of Historic Places that cannot be classified as either simple buildings and historic districts. Through the National Park Service, the National Register of Historic Places publishes a series of bulletins designed to aid in evaluating and applying the criteria for evaluation of different types of properties. Although the criteria are always the same, the manner they are applied may differ slightly, depending upon the type of property involved. The National Register bulletins describe the application of the criteria for aids to navigation, historic battlefields, archaeological sites, aviation properties, cemeteries and burial places, historic designed landscapes, mining sites, post offices, properties associated with significant persons, properties achieving significance within the last fifty years, rural historic landscapes, traditional cultural properties and vessels and shipwrecks.

Property owner incentives

A National Register of Historic Places plaque at the Robert E. Howard Museum in Cross Plains, Texas

Properties are not protected in any strict sense by the Federal listing. States and local zoning bodies may or may not choose to protect listed historic places. Indirect protection is possible, by state and local regulations on the development of National Register properties and by tax incentives. By contrast, the state of Colorado, for example, does not set any limits on owners of National Register properties.

Until 1976, federal tax incentives were virtually non-existent for buildings on the National Register. Before 1976 the federal tax code favored new construction rather than the reuse of existing, sometimes historical, structures. In 1976, the tax code was altered to provide tax incentives that promote the preservation of income-producing historic properties. The National Park Service was given the responsibility to ensure that only rehabilitations that preserved the historic character of a building would qualify for federal tax incentives. A qualifying rehabilitation is one that the NPS deems consistent with the Secretary of the Interior's Standards for Rehabilitation. Properties and sites listed in the Register, as well as those located in and contributing to the period of significance of National Register Historic Districts, became eligible for the federal tax benefits.

Owners of income-producing properties listed individually in the National Register of Historic Places or of properties that are contributing resources within a National Register Historic District may be eligible for a 20% investment tax credit for the rehabilitation of the historic structure. The rehabilitation may be of a commercial, industrial, or residential property, for rentals. The tax incentives program is operated by the Federal Historic Preservation Tax Incentives program, which is managed jointly by the National Park Service, individual State Historic Preservation Offices and the Internal Revenue Service.

Some property owners may also qualify for grants, like the now-defunct Save America's Treasures grants, which apply specifically to properties entered in the Register with national significance or designated as National Historic Landmarks.

The NHPA did not distinguish between properties listed in the National Register of Historic Places and those designated as National Historic Landmarks concerning qualification for tax incentives or grants. This was deliberate, as the authors of the act had learned from experience that distinguishing between categories of significance for such incentives caused the lowest category to become expendable. Essentially, this made the Landmarks a kind of "honor roll" of the most significant properties of the National Register of Historic Places.

Recent past

The Portland Building was added to the NRHP only 29 years after its opening.

50-year rule

In American historic preservation, the 50-year rule is the generally held belief that a property must be at least 50 years old to be listed in the National Register of Historic Places. Actually, there is no hard rule. As stated by John H. Sprinkle Jr., deputy director of the Federal Preservation Institute, "this 'rule' is only an exception to the criteria that shape listings within the National Register of Historic Places. Of the eight 'exceptions' [or criteria considerations], Consideration G, for properties that have achieved significance within the past fifty years, is probably the best-known, yet also misunderstood preservation principle in America." Each year, a new group of resources crosses the 50-year threshold. The preservation of these "underage" resources has gained attention in recent years.

Limitations

The demolition of the Jobbers Canyon Historic District marked the largest National Register historic district lost to date.

As of 1999, there have been 982 properties removed from the Register, most often due to being destroyed. Among the properties that were demolished or otherwise destroyed after their listing are the Jobbers Canyon Historic District in Omaha, Nebraska (listed in 1979, demolished in 1989), Pan-Pacific Auditorium in Los Angeles, California (listed in 1978, destroyed in a fire in 1989), Palace Amusements in Asbury Park, New Jersey (listed in 2000, demolished in 2004), The Balinese Room in Galveston, Texas (listed in 1997, destroyed by Hurricane Ike in 2008), seven of the nine buildings included in the University of Connecticut Historic District in Storrs, Connecticut (listed in 1989, demolished in 2017), and the Terrell Jacobs Circus Winter Quarters in Peru, Indiana (listed in 2012, demolished in 2021).