Chapter 1

Radars and Federal Government Applications

Introduction

In today's life style, many U.S. citizens are assisted in their personal life by information garnered from land-based, airborne, shipborne, and spaceborne radars. We hear daily weather broadcasts from television and radio stations where the weather radar is mentioned, providing significant detailed information of the weather situation. Air travelers are generally aware that air traffic control radars help with the safe and efficient movement of commercial and military aircraft, although the radars are not always visible to the traveler. Many automobile drivers know of radar speed guns and their use by law enforcement units to help curb speeding drivers. Many ocean-going boats and ships employ maritime surveillance and weather radars to help in piloting during adverse weather conditions.

Historically, the military is primarily credited with developing the radar. The term RADAR is derived from the description of its first primary role as a RAdio Detection And Ranging system. Originally, it was developed as a means of detecting approaching aircraft at long ranges to enable military defenses to react in sufficient time to counter incoming threats. Most natural and man-made objects reflect radio frequency waves and, depending on the radar's purpose, information can be obtained from objects such as aircraft, ships, ground vehicles, terrain features, weather phenomenon, and even insects. The determination of the object's position, velocity and other characteristics, or the obtaining of information relating to these parameters by the transmission of radio waves and reception of their return is sometimes referred to as radiodetermination.(1)

Radar Fundamentals

In most cases, a basic radar operates by generating pulses of radio frequency energy and transmitting these pulses via a directional antenna. When a pulse impinges on a object(2) in its path, a small portion of the energy is reflected back to the antenna. The radar is in the receive mode in between the transmitted pulses, and receives the reflected pulse if it is strong enough. The radar indicates the range to the object as a function of the elapsed time of the pulse traveling to the object and returning. The radar indicates the direction of the object by the direction of the antenna at the time the reflected pulse was received.

The "radar equation" mathematically describes the process and may be used to determine maximum range as a function of the pulse width (PW) and the pulse repetition rate (PRR). In most cases, narrow pulses with a high PRR are used for short-range, high-resolution systems, while wide PW's with a low PRR may be used for long-range search.

In general, a higher gain (larger aperture) antenna will give better angular resolution, and a narrower pulse width will give better range resolution. Changing the parameters of radars to satisfy a particular mission requires radar designers to have a variety of frequencies to choose from so that the system can be optimized for the mission and the radar platform.

Radar Developments

Radar as a means of detection has been around for over 60 years, and although technology has become immensely more sophisticated than it was in the 1930's, the basic requirement remains the same--to measure the range, bearing, and other attributes of a target. Regardless of whether the system is land-based, shipborne, airborne, or spaceborne, this remains true since whatever the target may be, aircraft, ship, land vehicles, pedestrians, land masses, precipitation, oceans--all provide returns of the transmitted radar energy. What has changed dramatically is the system design, the method and speed of processing the return radar signals, the amount of information which can be obtained, and the way that the information is displayed to the operator. The key to modern radar systems is the digital computer and its data processing capability which can extract a vast amount of information from the raw radar signals and present this information in a variety of graphic and alphanumeric ways on displays as well as feeding it direct to weapon systems. It also enables the systems to carry out many more tasks such as target tracking and identification. In addition, modern signal processors provide adaptive operation by matching the waveform to the environment in which the radar is operating.

Much of the development effort over the past 50 years has been aimed at a number of operational requirements: improvements in the extraction of return signals from the background of noise, provision of more information to the operator, improvement of displays, and increased automation. Other developments have responded to the increasing operational requirements for radars to operate in a hostile electromagnetic environment. It is no longer enough to provide only bearing and range information; to this must be added altitude information, the ability to track a large number of moving targets, including airborne targets at supersonic and hypersonic speeds and to carry out normal surveillance at the same time. The latest shipborne surveillance and tracking radars, and some land-based systems, are designed to allocate the threat priority to incoming targets and guide weapons against them on this basis. Many radars are specifically designed for fire-control of missiles and guns, and also for use in missile guidance and homing systems, which entails packing the system into a very small space. In the airborne role, systems have to be packaged into a relatively small space with units sometimes scattered around the airborne vehicle.

Radars have seen significant use in the Earth exploration-satellite service (EESS) especially with the deployment of airborne and spaceborne synthetic aperture radars (SAR's). Significant contributions in the areas of Earth observations, assisting in natural resource monitoring, hazard monitoring, and other global benefits can be attributed in part to the use of radars. The general categories of the active spaceborne sensors used in the EESS include SAR's, altimeters, scatterometers, precipitation, and cloud profile radars.

Radar Propagation Limitations

There are numerous radio frequency bands allocated to support radar operations in the United States. TABLE 1 below presents the broad categories of the radio frequency spectrum and why geophysical and mechanical limitations make one region of the spectrum more attractive for a particular radar application. These limitations are some of the reasons why operational compromises are necessary for today's multi-role, multi-function radars.

General Applications-Federal Government Radars

Over the years, radar has been used for many and varied military and non-military purposes. Most Federal Government radars are functionally classified as either surveillance or tracking radars, or some combination of the two. A surveillance radar is designed to continuously search for and detect new targets. The basic surveillance radar function has a 2-dimensional (2-D) plot showing the target object position in degrees from North (azimuth) and range (distance) from the radar. Radars that can determine azimuth, distance, and elevation are called 3-dimensional (3-D) radars. A tracking radar calculates a path for individual targets by using radar return echoes from one scan to the next, and are usually 3-D radars. Radars that perform both surveillance and tracking are loosely called multi-mode radars. A sampling of some of the more significant Federal Government radar applications are described as follows:

Air Traffic Control

Radar is an important tool in the safe and efficient management of the U.S. national airspace system (NAS) which is the largest, busiest, most complex, and technologically advanced aviation operation in the world. Safe and efficient air travel involves radars for short-range surveillance of air traffic and weather in the vicinity of airports, the long-range surveillance and tracking of aircraft and weather on routes between airports, and the surveillance of aircraft and ground vehicles on the airport surface and runways.

Aeronautical Radionavigation

Radar is employed by many of today's pilots to assist in aircraft navigation. For instance, the aircraft height above ground is determined by radar altimeters that assist in safe and efficient flight. Airborne Doppler navigation radars measure the vector velocity of the aircraft and determines the distance traveled. Weather avoidance radars identify dangerous weather phenomena and assist the pilot to avoid them. Some military pilots, who train at low altitudes, use terrain-following and terrain-avoidance radars that allow the pilot to closely fly over the ground and fly over or around other obstacles in its path.

TABLE 1

Frequency Bands and Radar OperationalPropagation Limitations

LF

30-300 kHz

Allocations are provided in the frequency range but no radar usage or applications have been identified.
MF

300-3000 kHz

Used by continuous wave (CW) radar systems for accurate position location. Very high noise levels are characteristic of this band.
HF

3-30 MHz

Refractive properties of the ionosphere make frequencies in this band attractive for long-range radar observations of areas such as over oceans at ranges of approximately 500-2000 nautical miles. Only a few radar applications occur in this frequency range because its limitations frequently outweigh its advantages: very large system antennas are needed, available bandwidths are narrow, the spectrum is extremely congested with other users, and the external noise (both natural noise and noise due to other transmitters) is high.
VHF

30-300 MHz

For reasons similar to those cited above, this frequency band is not too popular for radar. However, long-range surveillance radars for either aircraft or satellite detection can be built in the VHF band more economically than at higher frequencies. Radar operations at such frequencies are not affected by rain clutter, but auroras and meteors produce large echoes that can interfere with target detection. There have not been many applications of radar in this frequency range because its limitations frequently outweigh its advantages.
UHF

300-3000 MHz

Larger antennas are required at the lower end than at the upper end of the UHF band. As compared to the above bands, obtaining larger bandwidths is less difficult, and external natural noise and weather effects are much less of a problem. At the lower end, long-range surveillance of aircraft, spacecraft, and ballistic missiles is particularly useful. The middle range of this band is used by airborne and spaceborne SAR's. The higher UHF end is well suited for short to medium-range surveillance radars.
SHF

3 GHz-30 GHz

Smaller antennas are generally used in this band than in the above bands. Because of the effects of atmospheric absorption, the lower SHF band is better for medium-range surveillance than the upper portions. This frequency band is better suited than the lower bands for recognition of individual targets and their attributes. In this band, Earth observation efforts employ radars such as SAR's, altimeters, scatterometers, and precipitation radars .
EHF

30-300 GHz

It is difficult to generate high power in this band. Rain clutter and atmospheric attenuation are the main factors in not using this frequency band. However, Earth observation efforts are made in this band employing radars such as altimeters, scatterometers, and cloud profile radars.

Ship Safety

U.S. Navy and U.S. Coast Guard (USCG) ships employ maritime navigation radars to assist in avoiding collisions, assisting in making landfall, and piloting in restricted waters. Radars are also used on shore for harbor surveillance supporting the vessel traffic system (VTS).

Space

NASA astronauts first used and continue to use spaceborne radars to assist in the rendezvous of their spacecraft with other spacecraft or space objects, as well as with the docking and landing of spacecraft. Spaceborne radars have also been used for altimetry, ocean observation, remote sensing, mapping, navigation, and weather forecasting.

Law Enforcement

The familiar police speed-gun radars are a well-known application of radar for law enforcement. Radars are also used by many Federal agencies for intrusion detection systems in various protected areas. Lately, land-based, airborne, and shipborne radars are being employed in Federal law enforcement efforts.

Environmental Monitoring

Radar is an important application for several Federal agencies involved in making weather observations, conducting geological surveys, and making Earth observations. In the meteorological aids service, radars are employed to detect precipitation, wind speeds, wind shear at airports, forming tornadoes, and hurricanes. Radars have been used in space to assist in weather forecasting (hurricanes, tropical storms, etc.), measure ocean characteristics and mineral resources on Earth. Finally, scientists are employing radars to help monitor bird migrations as well as insect migrations and their flight characteristics.

Instrumentation

Radar is used extensively by various Federal agencies on test ranges to track aircraft, unmanned aircraft, spacecraft, and missiles to measure and determining quantitatively the actions that take place during test flights. Federal geologists and surveyors use radar devices for precise position measurements associated with geological and water/shoreline boundary surveys.

National Defense

Radar originally was developed to meet the needs of the military services, and it continues to have critical applications for national defense purposes. For instance, radars are used to detect aircraft, missiles, artillery and mortar projectiles, ships, land vehicles, and satellites. In addition, radar controls and guides weapons; allows one class of target to be distinguished from another; aids in the navigation of aircraft and ships; and assists in reconnaissance and damage assessment.

Military radar systems can be divided into three main classes based on platform: land-based, shipborne, and airborne. Within these broad classes, there are several other categories based mainly on the operational use of the radar system. For the purposes of this report, the categories of military radars will be as described below, although there are some "gray" areas where some systems tend to cover more than one category. There is also a trend to develop multimode radar systems. In these cases, the radar category is based on the primary use of the radar.

Land-Based Air Defense Radars. These radars cover all fixed, mobile, and transportable 2-D and 3-D systems used in the air defense mission.

Battlefield, Missile Control, and Ground Surveillance Radars. These radars also include battlefield surveillance, tracking, fire-control, and weapons-locating radar systems, whether fixed, mobile, transportable, or man-portable.

Naval and Coastal Surveillance, and Navigation Radars. These radars consist of shipborne surface search and air search radars (2-D and 3-D) as well as land-based coastal surveillance radars.

Naval Fire-Control Radars. These are shipborne radars that are part of a radar-based fire-control and weapons guidance systems.

Airborne Surveillance Radars. These radar systems are designed for early warning, land and maritime surveillance, whether for fixed-wing aircraft, helicopters, or remotely piloted vehicles (RPV's).

Airborne Fire-Control Radars. Includes those airborne radar systems for weapons fire-control (missiles or guns) and weapons aiming.

Spaceborne Radar Systems. Considerable effort has been applied to spaceborne radar (SBR) research for intelligence, surveillance, and reconnaissance missions over the last 30 years. The Department of Defense (DOD) seems to be expressing new interest in SBR.

Military Air Traffic Control (ATC), Instrumentation and Ranging Radars. These include both land-based and shipborne ATC radar systems used for assisting aircraft landing, and supporting test and evaluation activities on test ranges. See Appendix B for descriptions of shipborne ATC radars.

Types of Radars

Some of the more prominent types of radars used by Federal agencies are described below. These descriptions are not precise, for each of these radar types usually employ a characteristic waveform and signal processing that differentiate it from other radars.

Simple Pulse Radar: This type is the most typical radar with a waveform consisting of repetitive short-duration pulses. Typical examples are long-range air and maritime surveillance radars, test range radars, and weather radars. There are two types of pulse radars that uses the Doppler frequency shift of the received signal to detect moving targets, such as aircraft, and to reject the large unwanted echoes from stationary clutter that do not have a Doppler shift. One is called moving-target indication (MTI) radar and the other is called pulse Doppler radar. Users of pulse radars include the Army, Navy, Air Force, FAA, USCG, NASA, Department of Commerce (DOC), Department of Energy (DOE), U.S. Department of Agriculture (USDA), Department of the Interior (DOI), National Science Foundation (NSF), and Department of Treasury.

Moving-Target Indication (MTI) Radar: By sensing Doppler frequencies, an MTI radar can differentiate echoes of a moving target from stationary objects and clutter, and reject the clutter. Its waveform is a train of pulses with a low PRR to avoid range ambiguities. What this means is that range measurement at the low PRR is good while speed measurement is less accurate than at a high PRR's. Almost all ground-based aircraft search and surveillance radar systems use some form of MTI. The Army, Navy, Air Force, FAA, USCG, NASA, and DOC are large users of MTI radars.

Airborne Moving-Target Indication (AMTI) Radar: An MTI radar in an aircraft encounters problems not found in a ground-based system of the same kind because the large undesired clutter echoes from the ground and the sea have a Doppler frequency shift introduced by the motion of the aircraft carrying the radar. The AMTI radar, however, compensates for the Doppler frequency shift of the clutter, making it possible to detect moving targets even though the radar unit itself is in motion. AMTI radars are primarily used by the Army, Navy, Air Force, and the USCG.

Pulse Doppler Radar: As with the MTI system, the pulse Doppler radar is a type of pulse radar that utilizes the Doppler frequency shift of the echo signal to reject clutter and detect moving aircraft. However, it operates with a much higher PRR than the MTI radar. (A high-PRR pulse Doppler radar, for example, might have a PRR of 100 kHz, as compared to an MTI radar with PRR of perhaps 300 Hz) The difference of PRR's gives rise to distinctly different behavior. The MTI radar uses a low PRR in order to obtain an unambiguous range measurement. This causes the measurement of the target's radial velocity (as derived from the Doppler frequency shift) to be highly ambiguous and can result in missing some target detections. On the other hand, the pulse Doppler radar operates with a high PRR so as to have no ambiguities in the measurement of radial velocity. A high PRR, however, causes a highly ambiguous range measurement. The true range is resolved by transmitting multiple waveforms with different PRR's.(3)

Pulse Doppler radars are used by the Army, Navy, Air Force, FAA, USCG, NASA, and DOC.

High-Range Resolution Radar: This is a pulse-type radar that uses very short pulses to obtain range resolution of a target the size ranging from less than a meter to several meters across. It is used to detect a fixed or stationary target in the clutter and for recognizing one type of target from another and works best at short ranges. The Army, Navy, Air Force, NASA, and DOE are users of high-range resolution radars.

Pulse-Compression Radar: This radar is similar to a high-range resolution radar but overcomes peak power and long-range limitations by obtaining the resolution of a short pulse but with the energy of a long pulse. It does this by modulating either the frequency or the phase of a long, high-energy pulse. The frequency or phase modulation allows the long pulse to be compressed in the receiver by an amount equal to the reciprocal of the signal bandwidth. The Army, Navy, Air Force, NASA, and DOE are users of pulse-compression radars.

Synthetic Aperture Radar (SAR): This radar is employed on an aircraft or satellite and generally its antenna beam is oriented perpendicular to its direction of travel. The SAR achieves high resolution in angle (cross range) by storing the sequentially received signals in memory over a period of time and then adding them as if they were from a large array antenna. The output is a high-resolution image of a scene. The Army, Navy, Air Force, NASA, and NOAA are primary users of SAR radars.

Inverse Synthetic Aperture Radar (ISAR): In many respects, an ISAR is similar to SAR, except that it obtains cross-range resolution by using Doppler frequency shift that results from target movements relative to the radar. It is usually used to obtain an image of a target. ISAR radars are used primarily by the Army, Navy, Air Force, and NASA.

Side-Looking Airborne Radar (SLAR): This variety of airborne radar employs a large side-looking antenna (i.e., one whose beam is perpendicular to the aircraft's line of flight) and is capable of high-range resolution. (The resolution in cross range is not as good as can be obtained with SAR, but it is simpler than the latter and is acceptable for some applications.) SLAR generates map-like images of the ground and permits detection of ground targets. This radar is used primarily by the Army, Navy, Air Force, NASA, and the USCG.

Imaging Radar: Synthetic aperture, inverse synthetic aperture, and side-looking airborne radar techniques are sometimes referred to as imaging radars. The Army, Navy, Air Force, and NASA are the primary users of imaging radars.

Tracking Radar: This kind of radar continuously follows a single target in angle (azimuth and elevation) and range to determine its path or trajectory, and to predict its future position. The single-target tracking radar provides target location almost continuously. A typical tracking radar might measure the target location at a rate of 10 times per second. Range instrumentation radars are typical tracking radars. Military tracking radars employ sophisticated signal processing to estimate target size or identify specific characteristics before a weapon system is activated against them. These radars are sometimes referred to as fire-control radars. Tracking radars are primarily used by the Army, Navy, Air Force, NASA, and DOE.

Track-While-Scan (TWS) Radar: There are two different TWS radars. One is more or less the conventional air surveillance radar with a mechanically rotating antenna. Target tracking is done from observations made from one rotation to another. The other TWS radar is a radar whose antenna rapidly scans a small angular sector to extract the angular location of a target. The Army, Navy, Air Force, NASA, and FAA are primary user of TWS radars.

3-D Radar: Conventional air surveillance radar measures the location of a target in two dimensions-range and azimuth. The elevation angle, from which target height can be derived, also can be determined. The so-called 3-D radar is an air surveillance radar that measures range in a conventional manner but that has an antenna which is mechanically or electronically rotated about a vertical axis to obtain the azimuth angle of a target and which has either fixed multiple beams in elevation or a scanned pencil beam to measure its elevation angle. There are other types of radar (such as electronically scanned phased arrays and tracking radars) that measure the target location in three dimensions, but a radar that is properly called 3-D is an air surveillance system that measures the azimuth and elevation angles as just described. The use of 3-D radars is primarily by the Army, Navy, Air Force, NASA, FAA, USCG, and DOE.

Electronically Scanned Phased-Array Radar: An electronically scanned phased-array antenna can position its beam rapidly from one direction to another without mechanical movement of large antenna structures. Agile, rapid beam switching permits the radar to track many targets simultaneously and to perform other functions as required. The Army, Navy, and Air Force are the primary users of electronically scanned phased-array radars.

Continuous-Wave (CW) Radar: Since a CW radar transmits and receives at the same time, it must depend on the Doppler frequency shift produced by a moving target to separate the weak echo signal from the strong transmitted signal. A simple CW radar can detect targets, measure their radial velocity (from the Doppler frequency shift), and determine the direction of arrival of the received signal. However, a more complicated waveform is required for finding the range of the target. Almost all Federal agencies used some type of CW radar for applications ranging from target tracking to weapons fire-control to vehicle-speed detection.

Frequency-modulated Continuous-wave (FM-CW) Radar: If the frequency of a CW radar is continually changed with time, the frequency of the echo signal will differ from that transmitted and the difference will be proportional to the range of the target. Accordingly, measuring the difference between the transmitted and received frequencies gives the range to the target. In such a frequency-modulated continuous-wave radar, the frequency is generally changed in a linear fashion, so that there is an up-and-down alternation in frequency. The most common form of FM-CW radar is the radar altimeter used on aircraft or a satellite to determine their height above the surface of the Earth. Phase modulation, rather than frequency modulation, of the CW signal has also been used to obtain range measurement. The primary users of these radars are the Army, Navy, Air Force, NASA, and USCG.

High Frequency Over-the-Horizon (HF OTH) Radar: This radar operates in the high frequency (HF) portion of the electromagnetic spectrum (3-30 MHz) to take advantage of the refraction of radio waves by the ionosphere that allows OTH ranges of up to approximately 2,000 nautical miles. HF OTH can detect aircraft, ballistic missiles, ships, and ocean-wave effects. The Navy and Air Force use HF OTH radars.

Scatterometer: This radar is employed on an aircraft or satellite and generally its antenna beam is oriented at various aspects to the sides of its track vertically beneath it. The scatterometer uses the measurement of the return echo power variation with aspect angle to determine the wind direction and speed of the Earth's ocean surfaces.

Precipitation Radar: This radar is employed on an aircraft or satellite and generally its antenna beam is scanning at an angle optimum to its flight path to measure radar returns from rainfall to determine rainfall rate.

Cloud Profile Radar: Usually employed aboard an aircraft or satellite. The radar beam is oriented at nadir measuring the radar returns from clouds to determine the cloud reflectivity profile over the Earth's surface.

EnhancedSynthetic Vision Radars. These radars are under development for aviation use and will provide computer-generated visual scenes during approaches and landings. Radars operating at 34.7-35.2 and 92-100 GHz ranges will be used to provide the pilot with high-resolution video displays of the terrain and environment during low visibility conditions. The NASA, FAA, and DOD are involved in these efforts.


Endnotes: Chapter 1

1. In spectrum management, the term radiodetermination is defined as: "The determination of the position, velocity and/or other characteristics of an object, or the obtaining of information relating to these parameters, by means of the propagation properties of radio waves." Later in this report, the two sub-categories of radiodetermination, radiolocation and radionavigation, are discussed.

2. In the case of a radar in the meteorological aids service, the "object" may be rain, inhomogeneities in the atmosphere, etc.

3. A modified form of pulse Doppler radar that operates at a lower PRR (10 kHz, for example) than the above-mentioned high-PRR pulse Doppler system has both range and Doppler shift ambiguities. It is, however, better for detecting aircraft with low closing speeds than high-PRR pulse Doppler radar (which is better for detecting aircraft with high closing speeds). An airborne medium-PRR pulse Doppler radar might have to use seven or eight different PRR's in order to extract the target information without ambiguities.


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