* This chapter provides a survey of various interesting modern radar technologies.
* As mentioned earlier, phased-array radars have been around since World War II. In more recent times, they have been associated with large, long-range search radars. One of the more spectacular earlier examples of such large PARs were the big BMEWS surveillance radars, also mentioned earlier, which were more formally known as the "AN/FPS-50 detection radars". There were two such installations, one at Thule Air Force Base (AFB) in Greenland, and the other at Clear AFB in Alaska.
Each AN/FPS-50 consisted of three fixed vertical billboard antennas, with each billboard being 122 meters wide and 56.4 meters tall (400 x 185 feet). The array was driven by 12 huge klystron tubes, each about 3 meters (10 feet) tall, organized in pairs and producing 1.5 megawatts of continuous RF power apiece at 91.5 MHz / 2 meters. They were fed to the array through waveguides big enough to crawl through, connected to three buildings, one in front of each array. Each building contained an organ-pipe scanner. The beam was 40 degrees wide and was switched on a rotating basis through four quadrants, giving it a total field of view 160 degrees wide. The array was so powerful that the first time the Moon got in its line of sight, the operators were flooded with false returns. A gating scheme was quickly developed to blank out the Moon.
Of course, the AN/FPS-50 was not designed to be very accurate, and so it was backed up with an "AN/FPS-92" tracking radar, with a fully steerable antenna in a 42.7-meter (140-foot) dome. It could be steered rapidly from one location to another to handle multiple targets. A third BMEWS site was set up at Fylingdales in the UK without the AN/FPS-50 radar, using instead three steerable AN/FPS-92 radars. Some jokers said the British insisted on steerable arrays so they could be used to keep an eye on the French. All three BMEWS sites have been updated to more modern PARs.
* One of the better-known examples of a modern large phased-array radar is the COBRA DANE radar, which was built in the Aleutian island chain to observe Soviet missile tests. It can follow hundreds of targets scattered through a volume of space spanning 120 degrees in azimuth and about 80 degrees in elevation. COBRA DANE can detect a metallic object the size of a grapefruit at a distance of 1,850 kilometers (1,000 nautical miles).
The COBRA DANE radar is a big structure, with an array about 29 meters (95 feet) in diameter that consists of 15,360 radiating elements is linked to a three-bit phase shifter. The antenna is organized into 96 subarrays, or "blocks", of 160 radiating elements each. Each block is powered by a single klystron tube generating 160 kilowatts of power. Each klystron is 1.5 meters (5 feet) tall and operates as 40,000 volts. The radar's pulses provide 15 megawatts for a millisecond, with pulse compression used to give the effect of three terawatts for five nanoseconds.
Another well-known phased-array radar, the PAVE PAWS system, is designed to provide warning against sea-launched ballistic missiles from sites on Cape Cod and in California. These are solid-state radars, with each radiating element driven by four 100-watt transistors hooked in parallel. The PAR that replaced the old BMEWS AN/FPS-50 radar at Clear AFB, known as the "Solid State PAR (SSPAR)", was actually a relocated and updated PAVE PAWS radar. The BMEWS sites now have what is called the "Upgraded Early Warning Radar (UEWR)", featuring processor and software improvements to enhance capability.
The latest big phased-array radar is the Raytheon "Sea-Based X-band (SBX)" radar, which was developed for the US National Missile Defense system. The SBX is a huge system, mounted on a floating oceanic platform to allow it to be relocated. It has a single array mounted on an alt-azimuth mount.
In modern times, large PARS have also been mounted on warships, the best-known being the "AN/SPY-1" series used on Aegis-class cruisers and Arleigh Burke-class destroyers. The AN/SPY-1 uses four phased array antennas to cover the quadrants around the ship. It provides multimode operational capabilities, performing surveillance, tracking, plus guidance for the ship's Standard SAMs. It has been progressively upgraded, most significantly to provide missile intercept capabilities with the latest generation of Standard SAMs, with the latest derivatives featuring active-array technology.
BACK_TO_TOP* As mentioned, the pioneering AN/APS-20 AEW radar remained in service through the 1950s, but even the final models were crude by modern standards, in particular lacking any real capability to pick targets out of low-altitude clutter. In the 1960s, a new generation of airborne radar systems was developed that provided much more capability, with such "airborne warning and control system (AWACS)" aircraft not only carrying radar but command and control links to fighter aircraft and other "shooters" to provide directions to targets.
The first was the US Navy E-2C Hawkeye twin-turboprop aircraft, developed by Grumman and carrying a GE air and sea surveillance system. The original "AN/APS-115" radar has been improved through several generations of technology up to the current "AN/APS-145" system. It operates at relatively low UHF frequencies to reduce sea clutter and features an array of Yagi-style endfire antennas in a rotating dome or "rotodome" on top of the aircraft. It is a monopulse radar that uses DPCA with a clutter cancellation system, plus a chirped pulse, switching between three PRFs to eliminate range and velocity ambiguities. The radar system has a maximum range of 650 kilometers (350 NMI) and can track 2,000 targets at once.
The second was the US Air Force E-3 Sentry, a militarized Boeing 707 four-jet airliner carrying a Westinghouse "AN/APY-1" and later "AN/APY-2" airborne warning radar system. The AN/APY-2 operates at medium S-band frequencies, around 3 GHz, and features a slotted planar array with dimensions of 7.3 x 1.5 meters (24 by 5 feet). The planar array is fitted into a rotodome on top of the aircraft that spins at 6 RPM. Vertical scanning is performed electronically. The radar shares the radome with a large IFF antenna facing to the rear.
The AN/APY-2 provides four main modes of operation:
The E-2C and E-3 both carry extensive processing power and have sensor systems in addition to radar. Both the E-2C and E-3 have been adopted by a number of US allies, and the Japanese Self-Defense Forces have acquired a similar system based on the Boeing 767 twinjet airliner. The Soviets found them such a threat that they developed very long range AAMs to engage and destroy them. The Soviets developed their own answer to the E-3, the "A-50", with the NATO reporting name of "Mainstay", based on their Ilyushin Il-76 cargolifter.
Boeing has developed a next-generation AWACS based on the smaller 737 jetliner originally for Australia named the "Wedgetail", which is gradually replacing the E-3. The Wedgetail is built around a Northrop Grumman "multimode AESA (MESA)" radar in a surfboard-like "top hat" antenna structure on the back of the aircraft. The MESA has 288 transmit-receive modules, arranged around the top hat structure to give 360 degree coverage, with IFF integrated into the radar view. The Wedgetail also has an ESM system, a defensive countermeasures suite, and a communications system including a satcom datalink.
* Other nations have developed their own AEW systems. One of the more interesting examples is the E-99 AEW platform operated by the Brazilian Air Force, which is a Brazilian EMBRAER ERJ-145 small jetliner carrying the Swedish Ericsson Erieye AEW radar. The Erieye is an AESA system, featuring a long antenna carried on the back of the aircraft, with the antenna consisting of 192 transmit-receive modules.
The Erieye is capable of tracking hundreds of targets at once at a maximum radius of about 460 kilometers (286 miles), and can deal with both airborne and water-borne targets, using four different waveform modes. It is capable of wide-area search, or closeup examination of targets with a high-power narrow beam. The E-99 carries a suite of SIGINT gear along with the radar. The Swedish Air Force operates the Erieye radar as well, but carried on a SAAB 2000 twin-turboprop airliner instead of an ERJ-145.
The Elta subsidiary of Israeli Military Industries (IMI) has also developed an AWACS based on a business jet with an AESA system. The "Model EL/W-2085" or "Eitam (Sea Eagle)" is based on the Gulfstream 550 executive jet, with radome fairings in its nose and tail and arrays along the fuselage, as well as ESM antennas under the wingtips and a communications antenna fairing at the tip of the tailfin. It can accommodate from two to six mission specialists, with a line-of-sight (LOS) or satellite communications (satcom) datalink to provide inputs to ground systems. The platform was obtained by both the Israeli and Singapore air forces.
The Elta AESA uses four flat arrays, with a 1x1 meter array in the nose and tail fairings and a 1.5x6.5 meter array in each side fairing. The AESA provides radar and IFF coverage over 360 degrees of horizon. The nose and tail arrays are S-band, with coverage of 40 and 50 degrees respectively. The side-mounted arrays are L-band, with longer range and better resolution, with coverage of 135 degrees. Elta officials claim the AESA system is highly reliable and requires little maintenance. They also claim that two mission specialists are all that's required, since they would basically be performing housekeeping while the end users on the ground obtained the data, filtered by the platform's automated systems. In fact, the next generation Elta AWACS machine may be an unpiloted drone.
BACK_TO_TOP* Secondary radars, which involve sending a radio pulse to a transponder that then shoots back a response, remain in widespread use. One of the important uses is in the form of a commercial aircraft navigation tool, as part of the "VOR/DME" stations generally found at commercial airports.
VOR/DME is actually two more or less independent systems that make up a single beacon, with the two subsystems using different radio bands. VOR stands for "VHF Omni-Range", and it is a type of "radio-compass" unit, broadcasting a directional signal that not only acts as a beacon but provides coded information that gives the compass angle of the signal between the aircraft and the ground station.
The DME or "Distance Measuring Equipment" component is the secondary radar. The DME unit in the aircraft shoots out a radio pulse to the DME transponder in the ground station, which immediately sends back an amplified response. The DME unit measures the total time delay to determine the distance to the ground station.
The military has a "Tactical Air Navigation (TACAN)" system that is very much like VOR/DME. In fact, TACAN uses the same DME scheme, the main difference being that TACAN's VOR equivalent uses the same radio band as DME instead of a different band. Some stations provide both VOR/DME and TACAN, and are known as "VORTACS".
* The old IFF technology has evolved in the decades since World War II, leading to the current "Mark X/XII" IFF standard, and is an important element in both military and commercial service. An airport IFF interrogation unit operates in one of two modes, querying the IFF transponder in an aircraft to either obtain the identity of the aircraft or its altitude. The interrogator will send out two pulses modulated on a 1.03 GHz carrier. To obtain the aircraft identity, the interrogator performs a "Mode 3/A" interrogation, with the pulses 8 microseconds apart. To obtain the aircraft altitude, the interrogator performs a "Mode C" interrogation, with the pulses 21 microseconds apart.
The aircraft transponder replies on 1.09 GHz with 12 pulses giving the requested information, with each pulse providing a "0" or "1" bit, giving a total of 4,096 possibilities. The aircraft identity code is assigned when the aircraft departs from an airport, with the code entered by the flight crew. Light civil aircraft operating under daylight flight rules will always respond with a "1200" code. There are also three reserved reply codes, including "we have an emergency", "we have been hijacked", and "our radio is broken". The altitude is given in multiples of 100 feet (30 meters).
The 3/A and C modes are common to both civil and military aircraft, but three modes are reserved for military use:
* The current commercial IFF modes are basically manual in operation, and so the US Federal Aviation Administration (FAA) approved a new "Mode S" that is replacing Mode 3/A and Mode C. Mode S uses the same frequencies as the old modes, but the challenge and response formats are much more elaborate. In particular, each aircraft will have its own unique, permanently assigned IFF code, with more than 16 million possibilities available. The response will also include altitude and other relevant data. The Mode S scheme is much more convenient for automated systems.
The military has also investigated advanced IFF modes providing a level of capability along the lines of Mode S, with a higher level of security by using "low probability of intercept" features such as wideband spread spectrum communications. Work on an advanced "Mark XV" IFF bogged down and was abandoned, but efforts continue, focusing on more modest enhancements of the Mark X/XII technology.
* With Mode S, IFF begins to seem much more like a communications technology than a radar technology. However, IFF is based on radar technology and can still be used very much as a radar technology. A number of large airports use "multilateration" or "multistatic dependent surveillance" systems to track aircraft on the runways; such multilateration systems consist of a network of ground-based sensors that triangulate the position of aircraft by comparing the time of arrival of signals from aircraft IFF transponders. Multilateration is less complicated and less expensive than radar, consumes less power, and is easier to maintain because it doesn't require rotating antennas. The basic idea is far from new, but it wasn't practical until low-cost computing hardware that could perform the triangulations became available.
"Wide area multilateration (WAM)" systems that track traffic in flight are in increasing use as well. For example, the airport at Innsbruck, Austria, installed a WAM system to track incoming and outgoing flights. The airport is small and couldn't afford an expensive radar system, but the approach to the airstrip is bordered by mountains and very hazardous, particularly in low-visibility conditions. Innsbruck controllers had relied on radar tracking from Munich, 100 kilometers (62 miles) away, but for final approach the local controllers were effectively blind. Only one aircraft could be brought in at a time, with the others remaining in line of sight of Munich.
The Innsbruck system features two transmitters and eight antennas, with three receive & transmit antennas and five receive-only antennas. Six of the antennas are sited in the surrounding mountains and two are sited at the airport. It can provide three-dimensional locations with accuracies of 30 meters (100 feet) or better, with position updates once a second. It can be used with any aircraft with a standard IFF transponder, and can also be used to keep track of ground vehicles fitted with transponders to prevent runway collisions.
Since the installation of the Innsbruck system, dozens of other small airports have also obtained WAM systems. WAM has also been used for tracking air traffic over military firing ranges. However, WAM has little or no use for frontline combat operations, since it requires that the aircraft being tracked cooperate with the scheme.
BACK_TO_TOP* Radars are commonly used to track weather, with ground-based weather radars used by weathermen, and aircraft weather radars used by airliners and the like to avoid storm fronts.
One of the better-known of the ground-based systems is "Nexrad", a network of 160 radar stations that was set up in the 1990s by the US National Oceanographic & Atmospheric Administration (NOAA). The Nexrad radars, formally designated "WSR-8DD" (for "Weather Surveillance Radar 1988 Doppler") are mounted on towers ranging from 4.6 to 30.5 meters (15 to 100 feet) tall, topped by a rotating radar dish 8.5 meters (28 feet) in diameter inside a protective spherical fiberglass radome. The panels that make up the radome are irregularly shaped, giving the radome something of the look of a cracked-up eggshell, since a more regular pattern would interfere more with radio waves.
In clear weather, the dish performs five rotations in ten minutes, with the angle of the sweeps ranging from 0.5 to 4.5 degrees. In nasty weather, the dish picks up the pace, performing 14 sweeps in five minutes, with the angle of the sweeps ranging from 0.5 to 19.5 degrees. The radar doesn't work well at angles lower than 0.5 degrees, since ground clutter ruins the measurements, and there is a "cone of silence" above the radar sweeps that isn't observed.
Sweeps alternate between ranging or "reflectivity" sweeps that spot precipitation, and Doppler or "velocity" sweeps that determine wind speed and direction. A Nexrad radar operates in the S band, at a frequency of 3 GHz / 10 centimeters, sending out horizontally-polarized radio pulses with a PRF of 860 to 1,300 hertz. It is sensitive enough to pick up a single bee from a range of 29 kilometers (18 miles), and measurements can be thrown off by swarms of insects or flocks of birds.
From 2008, the Nexrad system underwent a "Super Resolution" upgrade to allow the system to obtain higher-resolution data; and from 2010 obtained a "dual polarization" upgrade with both horizontally and vertically polarized pulses to obtain more weather data. Developers are considering phased array radars for the next generation that do away with the rotating dish.
* Airliner weather radars are a well-established item of technology and continue to be refined. The Boeing C-17 cargolifter, for example, uses the Honeywell RDR-4000 weather radar, which is also flown on a number of civil airliners.
The RDR-4000 can map a three-dimensional "volumetric" block of sky to obtain rain and turbulence data up to 600 kilometers (320 NMI) ahead of the aircraft, from the ground to 18,300 meters (60,000 feet). This data is stored in a database, to be accessed and processed as needed. The RDR-4000 also provides terrain-mapping capabilities and stores a digital terrain database. 3D weather profiles can be displayed relative to the ground track, the flight plan route, or the current position and bearing. Both a 2D top and side view can be displayed simultaneously if that is preferred. In addition, the highly automated radar provides automatic warning of "wind shear" conditions ahead.
The RDR-4000 can be fitted with an antenna dish 30 or 45 centimeters (12 and 18 inches) in diameter, smaller than the antennas of previous airliner weather radars, and is also much more compact and lightweight than its predecessors, allowing it to be used in a wide range of aircraft. The RDR-4000 variant used by the C-17 also provides rendezvous tracking for mid-air refueling -- which, of course, is not seen in the civil airliner variants of the radar.
BACK_TO_TOP* One of the interesting side tracks of radar development in the modern age has been the use of radars in spacecraft. Civilian uses, both for Earth resources studies and planetary exploration, are well documented. The first radar-based Earth resources satellite was the US National Aeronautics & Space Administration's (NASA) "SeaSat" satellite, launched in 1978 to perform observations of both land and sea. It actually carried three radar systems, including a SAR using optical processing for mapping land areas; a radar altimeter to measure ocean surface heights; and a "microwave scatterometer" to measure wind speeds.
A microwave scatterometer is a specialized type of radar fitted with an arrangement of several different transmitters and receivers featuring polarizations along several axes. It observes the reflection and scattering of microwaves from surfaces in two or more directions. It is generally used to measure ocean wind speed and direction, since such winds generate wave patterns that result in characteristic microwave scattering patterns. Radar altimeters and scatterometers are often carried by ocean studies satellites. Earth resources satellites also generally carry microwave "radiometers", but these are not in any way radars, being instead passive receivers that measure microwave intensities and spectral patterns.
SARs are useful for Earth resources mapping since they operate day and night, in any weather, can cut through some level of vegetation and tree cover, and depending on the operational band can penetrate dry soils to some depth. SAR satellites have become very common since the flight of SeaSat; a more or less typical example was the Canadian "RadarSat 1", which was launched in 1995. It carried a SAR also operating in the C band around 5.3 gigahertz that provided imagery of northern ice pack with 8-meter (26-foot) resolution. It was followed in 2007 by RadarSat 2, which featured an improved radar system with a best resolution of 3 meters (10 feet) and an antenna that could mechanically scan along the spacecraft flight track, increasing the swathe width.
More recently, commercial firms such as Iceye of Finland have been launching constellations of small SAR satellites, to sell observational data with global coverage. These constellations support "interferometric SAR (InSAR)" -- not to be confused with inverse SAR, ISAR -- in which phase comparisons are performed on different SAR returns from the same target to provide measurements to centimeter level, being able to track movements in earth masses or large human-made structures. InSAR requires precise knowledge of satellite orbits and lots of computing power to work, and is not very useful for more dynamic terrestrial applications of radar.
* Planetary probes have carried radars as well. The planet Venus is covered by heavy clouds and so camera systems can't be used to map the world. Radars, however, can cut through the cloud cover easily. A number of Venus orbiters, including the Soviet Venera 15 and 16 spacecraft and the US Magellan probe, were launched in the 1980s to use radars to map Venus. The Magellan probe was the most sophisticated of the lot, carrying a SAR that mapped the entire surface of the planet at a resolution of about a kilometer. The SAR used the probe's dish antenna, a spare from the Voyager deep-space probes, with the radar taking observations of a strip of the planet through the antenna, storing them on tape, and then using the antenna to transmit the contents of the tape back to Earth for interpretation.
The NASA Cassini Saturn orbiter, launched in 1997, carried a SAR system along the same lines as that of Magellan, using the probe's high-gain antenna to make observations of Saturn's moon Titan, which is also heavily shrouded by clouds. The Cassini radar was a multimode system, capable of performing imaging, altimetry, and even acting as a passive radiometer.
The ESA Mars Express orbiter, launched in 2003, carried a "sounding radar" designated the "Mars Advanced Radar for Subsurface & Ionospheric Sounding (MARSIS)", conceptually along the lines of a radar altimeter operating in the L through C bands from 1.3 to 5.5 gigahertz. It used twin whip antennas to perform atmospheric studies and probe into the planet's surface layer. The US Mars Reconnaissance Orbiter, launched in 2005, carried a similar "Shallow Subsurface Radar (SHARAD)" operating in the longwave HF band from 15 to 25 megahertz for probing under the surface.
* Crewed space capsules have long used rendezvous radar systems, generally linked to an automated flight control system to allow them to dock with another spacecraft.
The NASA space shuttle also carried Earth mapping radars. An Earth mapping radar designated the "Shuttle Imaging Radar A (SIR-A)" was flown in 1981, being used for orbital surveys. A space shuttle was flown in 2000 with a much more sophisticated radar system as the "Shuttle Radar Tomography Mission (SRTM)". It involved mapping the entire land surface of the Earth between latitudes 60 degrees North and 54 degrees south using an imaging radar that gave unprecedented altitude accuracy. The data had clear military applications, and was partly classified.
* Space-based radars have also been used for strictly military purposes, though not surprisingly the details are often classified and generally obscure. American military space-based radar efforts have mostly been focused on strategic intelligence. An experimental space radar satellite codenamed QUILL was put into orbit by the US in December 1964. It was secret for a long time and even now details are obscure, but it seems to have been a SLAR that recorded its radar imagery on a film strip, and returned the film to Earth in a re-entry capsule. The resolution of the imagery was too poor to be useful -- it seems to have been a proof-of-concept demonstrator -- and the US didn't return to the concept for over two decades.
Since 1988, the US has orbited five space-based SAR satellites, known originally by the name INDIGO and then LACROSSE, now referred to as ONYX, though the program is cloaked in secrecy and even the codename is something of a guess. It is clear that the ONYX series does provide an all-weather, day-night surveillance capability, with resolution of possibly a meter or less. It is to be replaced by a new generation of SAR satellites, though that is secret as well.
A follow-on system has been in the works. The US has also been interested in a new "space radar (SR)" network that will provide high-resolution SAR-MTI capabilities, allowing it to be used for tactical operations. However, the SR effort has encountered repeated delays and cost obstacles, resulting in a series of program holds and changes in direction. The status of the effort is unclear.
The US Navy has long operated "electronic ocean reconnaissance satellites (EORSAT)", orbiting ESM / SIGINT systems to track and observe naval vessels from their signal emissions. These spacecraft are designated as WHITE CLOUD or more formally the "Naval Ocean Satellite System (NOSS)". NOSS is secret, but it is known that early spacecraft were based on three modules that flew together in a triangular formation, linked by wires. The appearance of such a neat triangle in the evening or morning sky apparently led to stories about "flying triangle" unidentified flying objects (UFOs). Current NOSS systems only use two modules. The Chinese have continued to fly "flying triangle" EORSATs.
* The USSR was actually a pioneer in military space radar. The Soviet US-A series "radar ocean reconnaissance satellites (RORSAT)" was developed from the early 1960s, with introduction to operation in the early 1970s. The US-A RORSATs were intended to keep an eye on Western fleet movements, particularly aircraft carrier task forces.
Picking out large naval targets on the surface of the ocean didn't require a lot of sophistication, but since the RORSATs were operating from Earth orbit, they did require range. That meant a simple but powerful radar, which led to the problem of providing the needed electrical power. Since radar sensitivity, all other things being equal, falls off with the fourth power of range, that meant there was a need to make the orbit as low as possible. However, if large solar arrays were used to power the spacecraft, the traces of the upper atmosphere at relatively low orbits would quickly drag the spacecraft out of the sky. The Soviets took the drastic option of fitting small nuclear reactors to the RORSATs. The system was designed so that the reactor would be ejected before the spacecraft finally fell to Earth, with the reactor kicked up into a high orbit where it could remain for centuries until its radioactive material decayed into harmless isotopes.
The US-A RORSATs remained in service into the late 1980s. They not only tracked the movements of Western fleets, they also provided targeting for air-launched and ship / submarine-launched long-range antiship missiles. Ultimately, the Soviets developed a "Legenda" system in which the missiles received their guidance directly from the satellite.
There were two accidents with RORSATs in which the fission reactors fell to Earth -- doing no major outright harm, but making a radioactive mess and giving the spacecraft a very bad name. In 1988, during the period of "glasnost" just before the fall of the Soviet Union, the RORSATs were abandoned. The Soviets did and the successor Russian state still does retain a space-based targeting capability, but through their US-P and US-PM EORSAT series, comparable to the US Navy's NOSS spacecraft.
The Soviets flew two "Almaz" manned military space stations -- not counting failures -- under the "Salyut" program in the 1970s, and were planning to add radar reconnaissance capabilities to later flights. However, the conclusion of the manned Almaz stations was that maintaining a crew was far more trouble than it was worth, and so the decision was made to develop an unmanned "Almaz-T" imaging radar satellite to provide the USSR's answer to ONYX. Following a launch failure in 1986, an Almaz-T was put into orbit in 1987, with a second following in 1991. They were actually only used for civilian observations -- possibly because their SAR resolution was not good enough for military purposes -- and their orbital lifetimes were short. Other nations, including China, Japan, Germany, Italy, India, and Israel have flown radar spysats as well.
* As something of a footnote to space radars, ground-based radio telescopes have been used to conduct radar studies of other bodies in the solar system. Radar pulses were sent to the Moon as far back as the late 1940s, though this was really not much more than a stunt. In the late 1950s and early 1960s, techniques were developed that allowed large aperture radio telescopes to perform radar scans of other worlds, in specific cloud-covered Venus. Radar measurements were able to determine the rotation of Venus and make out some gross surface details.
Interplanetary radar of course requires a good deal of transmitter power and receiver sensitivity since the distances are so great. A subtler problem is that the pulses take so long to return, making the timing of the echoes difficult. This problem was addressed by sending out a pulse train with a "pseudorandom" pattern that did not repeat for the duration of the observation session, allowing the return echo to be interpreted to yield precise timing. Such long-range radar observations are still used, mostly to provide images of asteroids, not only those passing in the vicinity of the Earth, but even for asteroids inside the asteroid belt between the planets Mars and Jupiter.
BACK_TO_TOP* This discussion has somewhat neglected radars used by ground forces. Armies on the move of course haul surveillance and targeting radars along with them for air defense, but ground forces also have radars for their own purposes.
One interesting example is the "Longbow" millimeter-wave radar used on the Boeing AH-64 Apache battlefield helicopter. The antenna is in a drum on top of the main rotor system, allowing the helicopter to stay behind cover and get a snapshot of the battle area, obtaining a list of potential targets and prioritizing them for the crew. It can then target Hellfire missiles for offensive action. Millimeter wave radars suffer from atmospheric attenuation, particularly in damp weather, and so are limited in range, but for a battlefield targeting system like the Longbow that's not a particular problem. Millimeter wave radars of course have very high resolution. The Longbow radar also provides obstacle warnings to the aircrew.
Infantry have also used small man-portable radars for perimeter defense, the best known being the AN/TPS-25 unit used by the US in Vietnam. The AN/TPS-25 could provide surveillance out to several kilometers, but apparently it was mainly designed to spot large vehicles in open terrain in the dark and poor weather, and was not so good at picking up sappers trying to crawl through a base perimeter. However, it appears that perimeter defense radars have been considerably refined since that time, though details are scarce.
Another class of radar more or less unique to ground forces are radars designed to track incoming artillery rounds to direct counter-battery fire. The contemporary Hughes AN/TPQ-37 "Firefinder" radar is a three-dimensional phased-array radar operating in the X band (around 10 GHz / 3 cm) that can automatically backtrack trajectories of multiple adversary artillery sites simultaneously.
* One particularly interesting new radar is a millimeter-wave system created by Britain's QinetiQ advanced-technology research organization, and installed at the Vancouver International Airport in Canada to spot small objects on the airport runways that could potentially damage aircraft. The "Tarsier" radar system features four towers covering the runways, able to spot objects as small as 5 centimeters (2 inches) wide to an accuracy of 3 meters (10 feet). The radars scan once every two minutes and send a report to a display in the airport control center, with the reports logged in a database. Presumably the Tarsier discriminates against objects that move between scans so it doesn't flag every bird that lands on the runway. QinetiQ is working on similar systems for military perimeter defense.
* The one radar technology that almost everyone is familiar with is the radar gun used by traffic policy to nab speeders. These are hand-held continuous-wave Doppler radars, of course intended to give speeds but not ranges. A radar gun both indicates the speed of the target and gives an audio tone when the gun is tracking a target. The tone is broken if the gun isn't tracking and continuous when it is, with the tone increasing in pitch with the speed of the target.
Radar guns can be confused by electromagnetic interference from various sources, and officers need to be trained in their use to ensure that they understand how to properly use the gun. There is also the fact that the gun only measures the speed of a target along the straight line to the gun. This is no great factor if a police car is sitting alongside the road and the target is some distance away, but as the target approaches, the angle of the target's direction of motion relative to the radar beam increases, making the target appear slower than it really is. This is known as "cosine error".
Radar detectors are available that give an alarm when the car carrying it is illuminated by a radar gun. They are legal in most places. Units to jam radar guns are also available -- just not in general legally.
Cars are also increasingly featuring radars for foul-weather service and particularly collision detection and avoidance. These systems are generally FM-CW radars operating in the millimeter-wave band at 77 GHz. In the past, they've generally been based on gallium arsenide solid state technology, which is expensive, but they've been moving towards a new silicon-germanium solid-state technology, which is not only cheaper but has better performance. The radars are linked to "smart" collision-avoidance systems that will warn the driver if a collision is possible, or in an obvious emergency take braking action on their own.
* Another fascinating new application of radar is the personal radar set that can allow soldiers, police, and rescue workers to see through walls and determine who is inside of a room. A number of "ultrawideband micropower" radars have been developed, typically with a PRF of 10 MHz, with pulse widths less than 500 picoseconds long, with a pulse power of less than 100 microwatts. The pulse period is dithered by a pseudorandom interval to spread signal power over a wide band, ensuring that the radar doesn't violate RF interference regulations. In fact, between the low pulse power and dithered pulse period, it's hard for targets to detect the signal.
The return echo from the pulses is picked up by an array of receive antennas arranged around the chassis, with the time delay of reception between antennas used to get a fix on the position of a target. They are MTI systems, only picking up moving targets, but sensitive enough to even pick up the slight movements of breathing. The display can be switched from a B-scope format, giving the location of targets on the floor of the room, to a C-scope format, giving their heights.
* One final note on unusual radars is that there has been research into "quantum radar", based on a phenomenon known as "quantum entanglement". Entanglement concerns the generation of two quantum entities that share complementary properties and are then separated; measuring the properties of one of the quantum entities at the local end reveals the properties of the other quantum entity at the remote end.
Experiments have been performed in the lab using microwaves as the quantum entities, but nobody has built a workable quantum radar system, and there is skepticism that anybody ever will. For the time being, the idea is being tinkered with by physicists as a fundamental research effort.
BACK_TO_TOP* The highly focused, short wavelength laser beam has obvious potential in radar technology. Laser rangefinders have been around for decades and are a common piece of gear on strike aircraft. The principle is simple in concept, exactly the same as a radar: a short laser pulse is sent out and the time interval to reception of the echo return is measured to give the range. Modern military laser rangefinders used by ground forces also have Global Positioning System (GPS) capabilities, allowing the GPS coordinates of the spotted target to be relayed to a strike aircraft or other attack platform for download into a GPS-guided munition.
Laser rangefinders have been used on spacecraft as well. The NASA Mars Global Surveyor probe used a "Mars Orbiting Laser Altimeter (MOLA)" instrument to obtain a precision altimetry map of Mars, since the thin Martian atmosphere does not block a laser beam anywhere near as much as does the Earth's. To obtain a range reading, MOLA fired a laser pulse with a duration of about 8 picoseconds, and then timed the interval required for the pulse to be reflected off the Martian surface, using a telescope with an aperture of 50 centimeters (20 inches) to observe the pulse. At the spacecraft's orbital altitude of 400 kilometers, the round-trip time was about two milliseconds. MOLA could obtain elevation measurements precise to one meter (3.3 feet). This implied that the position of the spacecraft was well-known, and the position of the spacecraft was established through tracking schemes implemented by the NASA Deep Space Network.
True laser radars or "lidars" can in potential provide high-resolution targeting and imaging. Lidars are generally experimental at this time, with development work focusing on use in smart munitions and targeting systems, though work has been done on SAR lidar systems as well. They can provide a precise image of a target but have problems with haze conditions.
A particularly interesting aircraft-mounted lidar instrument, known as the "Laser Vegetation Imaging Sensor (LVIS)", has been developed to allow mapping the depth of forest canopies. LVIS fires 300 10-nanosecond pulses a second, with a receiving system that acquires the reflections from the pulses from the top of the forest canopy, the forest floor, and levels of the canopy in between. The difference in timing between the first reflections and the last gives the height of the canopy, with the pattern of reflections also providing data on vegetation density and condition.
The Clementine Moon orbiter, launched in 1996, carried an interesting combination rangefinder and lidar imager. From its orbital altitude, the instrument had an imaging resolution of about 100 meters (330 feet) in the visible wavelength ranges, and 150 meters (500 feet) in the near-infrared ranges. It could also take range measurements with a precision of plus or minus 40 meters.
BACK_TO_TOP* I must emphasize that this document was not written by an expert on radars. I was trained as an electronics engineer, but I have no formal background in radar technology. I was just curious about the subject, and so wrote this document as a learning exercise. I have no doubt there are some naive comments in it -- I'll fix them as I learn better.
One of the frustrating things about radar is that there are many books on the subject for people who want to design radars, but a real scarcity of radar books for casual readers who just want to know how they work. The formal texts are all very well and good in their place, but they are pointlessly difficult for casual readers, full of material of interest only to professionals that is quickly forgotten by others. For example, some formal texts have chapters of math on RCS -- when the whole concept can be explained in all necessary detail useful to a casual student in a few paragraphs, and with no reference to math at all. The texts are also expensive. Since I haven't found any radar book that I would consider useful for casual readers, I ended up having to write my own.
This document began life as an appendix to another document on the historical origins of radar, THE WIZARD WAR. I felt I needed to provide a technical introduction so that readers could make sense of the historical narrative, and so I threw together a very quick and dirty outline off the top of my head. As it emerged, it seemed to be detailed enough to deserve being spun off as a separate document, and since I was planning on writing a general document on radars eventually anyway, I released it as v1.0.0.
It was something of an embarrassment, certainly nothing that would win any prizes, but I finally found the time to get back to it and improve on it. I've been refining it since then.
* Sources:
The BBC's "Hitchhiker's Guide" online encyclopedia has an interesting mixed bag of documents on radar. I downloaded the lot of them, sifted through them for the v2.0.0 version of this document, and they had a lot of influence on it.
Those interested in going on to more advanced studies in radar will find Stimson's INTRODUCTION TO AIRBORNE RADAR probably the best bet. Stimson is very knowledgeable and clear, though I would have to add a condition to "clear" in that he doesn't really have the knack of getting the basic simple "light bulb going on" idea into the head of a novice. Once the reader gets the drift of what he's talking about, however, it all becomes very understandable.
I do not recommend Skolnick's INTRODUCTION TO RADAR SYSTEMS. It may very well be an excellent book for someone who wants to learn how to design radars; for someone who doesn't intend to have a career in the field and is simply curious about how radars work, it is gross overkill.
* Revision history:
v1.0.0 / 01 feb 03 v2.0.0 / 01 jan 05 / Major expansion from 2 to 5 chapters. v2.1.0 / 01 jan 07 / Comprehensive cleanup. v2.1.1 / 01 feb 07 / Follow-up polishing release. v2.1.2 / 01 feb 09 / Review & polish. v2.1.3 / 01 nov 09 / Review & polish. v2.1.4 / 01 nov 11 / Review & polish. v2.1.5 / 01 oct 13 / Review & polish. v2.2.0 / 01 jun 15 / Trimming, review & polish. v2.2.1 / 01 may 17 / Trimming, review & polish. v2.2.2 / 01 apr 19 / Review & polish. v2.2.3 / 01 mar 21 / Review & polish. v2.2.4 / 01 mar 21 / Review & polish. v2.2.5 / 01 dec 24 / Review & polish.BACK_TO_TOP
