Types of Radars (2024)

Radar originally was developed to meet the needs ofthe military services, and it continues to have criticalapplications for national defense purposes. For instance,radars are used to detect aircraft, missiles, artillery andmortar projectiles, ships, land vehicles, and satellites. In addition, radar controls and guides weapons; allowsone class of target to be distinguished from another; aidsin the navigation of aircraft and ships; and assists inreconnaissance and damage assessment.

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

Some of the more prominent types of radars are described below. These descriptionsare not precise, for each of these radar types usually employa characteristic waveform and signal processing thatdifferentiate it from other radars.

Land-Based Air Defense Radars. These radars coverall fixed, mobile, and transportable 2-D and 3-D systemsused in the air defense mission.

Battlefield, Missile Control, and Ground SurveillanceRadars. 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 NavigationRadars. These radars consist of shipborne surface searchand air search radars (2-D and 3-D) as well as land-basedcoastal surveillance radars.

Naval Fire-Control Radars. These are shipborne radarsthat are part of a radar-based fire-control and weaponsguidance systems.

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

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

Spaceborne Radar Systems. Considerable efforthas been applied to spaceborne radar (SBR) research forintelligence, surveillance, and reconnaissance missionsover the last 30 years. The Department of Defense (DOD)seems to be expressing new interest in SBR.

Military Air Traffic Control (ATC), Instrumentationand Ranging Radars. These include both land-basedand shipborne ATC radar systems used for assisting aircraftlanding, and supporting test and evaluation activities ontest ranges. See Appendix B for descriptions of shipborneATC radars.

Simple Pulse Radar: This type is the most typical radarwith a waveform consisting of repetitive short-durationpulses. Typical examples are long-range air and maritimesurveillance radars, test range radars, and weather radars. There are two types of pulse radars that uses the Dopplerfrequency shift of the received signal to detect movingtargets, such as aircraft, and to reject the large unwantedechoes from stationary clutter that do not have a Dopplershift. One is called moving-target indication (MTI) radarand the other is called pulse Doppler radar. Users of pulseradars include the Army, Navy, Air Force, FAA, USCG,NASA, Department of Commerce (DOC), Departmentof Energy (DOE), U.S. Department of Agriculture (USDA),Department of the Interior (DOI), National ScienceFoundation (NSF), and Department of Treasury.

Moving-Target Indication (MTI) Radar: By sensingDoppler frequencies, an MTI radar can differentiate echoesof a moving target from stationary objects and clutter,and reject the clutter. Its waveform is a train of pulseswith a low PRR to avoid range ambiguities. What thismeans is that range measurement at the low PRR is goodwhile speed measurement is less accurate than at a highPRR's. Almost all ground-based aircraft search andsurveillance radar systems use some form of MTI. TheArmy, Navy, Air Force, FAA, USCG, NASA, and DOCare large users of MTI radars.

Airborne Moving-Target Indication (AMTI) Radar: An MTI radar in an aircraft encounters problems not foundin a ground-based system of the same kind because thelarge undesired clutter echoes from the ground and thesea have a Doppler frequency shift introduced by the motionof 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 thoughthe radar unit itself is in motion. AMTI radars are primarilyused by the Army, Navy, Air Force, and the USCG.

Pulse Doppler Radar: As with the MTI system, thepulse Doppler radar is a type of pulse radar that utilizesthe Doppler frequency shift of the echo signal to rejectclutter and detect moving aircraft. However, it operateswith a much higher PRR than the MTI radar. (A high-PRRpulse Doppler radar, for example, might have a PRR of100 kHz, as compared to an MTI radar with PRR of perhaps300 Hz) The difference of PRR's gives rise to distinctlydifferent behavior. The MTI radar uses a low PRR in orderto obtain an unambiguous range measurement. This causesthe measurement of the target's radial velocity (as derivedfrom the Doppler frequency shift) to be highly ambiguousand can result in missing some target detections. On theother hand, the pulse Doppler radar operates with a highPRR so as to have no ambiguities in the measurement ofradial velocity. A high PRR, however, causes a highlyambiguous range measurement. The true range is resolvedby transmitting multiple waveforms with different PRR's.(3)

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

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

Pulse-Compression Radar: This radar is similar toa high-range resolution radar but overcomes peak powerand long-range limitations by obtaining the resolution ofa short pulse but with the energy of a long pulse. It doesthis by modulating either the frequency or the phase ofa long, high-energy pulse. The frequency or phase modulationallows the long pulse to be compressed in the receiverby an amount equal to the reciprocal of the signal bandwidth. The Army, Navy, Air Force, NASA, and DOE are usersof pulse-compression radars.

Synthetic Aperture Radar (SAR): This radar is employedon an aircraft or satellite and generally its antenna beamis oriented perpendicular to its direction of travel. TheSAR achieves high resolution in angle (cross range) bystoring the sequentially received signals in memory overa period of time and then adding them as if they were froma large array antenna. The output is a high-resolution imageof a scene. The Army, Navy, Air Force, NASA, and NOAAare primary users of SAR radars.

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

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

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

Tracking Radar: This kind of radar continuously followsa single target in angle (azimuth and elevation) and rangeto determine its path or trajectory, and to predict its futureposition. The single-target tracking radar provides targetlocation almost continuously. A typical tracking radarmight measure the target location at a rate of 10 times persecond. Range instrumentation radars are typical trackingradars. Military tracking radars employ sophisticated signalprocessing to estimate target size or identify specificcharacteristics before a weapon system is activated againstthem. These radars are sometimes referred to as fire-controlradars. Tracking radars are primarily used by the Army,Navy, Air Force, NASA, and DOE.

Track-While-Scan (TWS) Radar: There are two differentTWS radars. One is more or less the conventional airsurveillance radar with a mechanically rotating antenna. Target tracking is done from observations made from onerotation to another. The other TWS radar is a radar whoseantenna rapidly scans a small angular sector to extractthe angular location of a target. The Army, Navy, AirForce, NASA, and FAA are primary user of TWS radars.

3-D Radar: Conventional air surveillance radar measuresthe 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 airsurveillance radar that measures range in a conventionalmanner but that has an antenna which is mechanicallyor electronically rotated about a vertical axis to obtainthe azimuth angle of a target and which has either fixedmultiple beams in elevation or a scanned pencil beam tomeasure its elevation angle. There are other types of radar(such as electronically scanned phased arrays and trackingradars) that measure the target location in three dimensions,but a radar that is properly called 3-D is an air surveillancesystem that measures the azimuth and elevation anglesas just described. The use of 3-D radars is primarily bythe Army, Navy, Air Force, NASA, FAA, USCG, and DOE.

Electronically Scanned Phased-Array Radar: Anelectronically scanned phased-array antenna can positionits beam rapidly from one direction to another withoutmechanical movement of large antenna structures. Agile,rapid beam switching permits the radar to track many targetssimultaneously and to perform other functions as required. The Army, Navy, and Air Force are the primary users ofelectronically scanned phased-array radars.

Continuous-Wave (CW) Radar: Since a CW radartransmits and receives at the same time, it must dependon the Doppler frequency shift produced by a moving targetto separate the weak echo signal from the strong transmittedsignal. A simple CW radar can detect targets, measuretheir radial velocity (from the Doppler frequency shift),and determine the direction of arrival of the received signal.However, a more complicated waveform is required forfinding the range of the target. Almost all Federal agenciesused some type of CW radar for applications ranging fromtarget tracking to weapons fire-control to vehicle-speeddetection.

Frequency-modulated Continuous-wave (FM-CW)Radar: If the frequency of a CW radar is continuallychanged with time, the frequency of the echo signal willdiffer from that transmitted and the difference will beproportional to the range of the target. Accordingly,measuring the difference between the transmitted andreceived frequencies gives the range to the target. In sucha frequency-modulated continuous-wave radar, the frequencyis generally changed in a linear fashion, so that there isan up-and-down alternation in frequency. The most commonform of FM-CW radar is the radar altimeter used on aircraftor a satellite to determine their height above the surfaceof the Earth. Phase modulation, rather than frequencymodulation, of the CW signal has also been used to obtainrange measurement. The primary users of these radarsare the Army, Navy, Air Force, NASA, and USCG.

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

Scatterometer: This radar is employed on an aircraftor satellite and generally its antenna beam is oriented atvarious aspects to the sides of its track vertically beneathit. The scatterometer uses the measurement of the returnecho power variation with aspect angle to determine thewind direction and speed of the Earth's ocean surfaces.

Precipitation Radar: This radar is employed on an aircraftor satellite and generally its antenna beam is scanning atan angle optimum to its flight path to measure radar returnsfrom rainfall to determine rainfall rate.

Cloud Profile Radar: Usually employed aboard an aircraftor satellite. The radar beam is oriented at nadir measuringthe radar returns from clouds to determine the cloudreflectivity profile over the Earth's surface.