For many years radar has been used to provide information about the intensity and extent of rain and other forms of precipitation. This application of radar is well known in the United States from the familiar television weather reports of precipitation observed by the radars of the National Weather Service. A major improvement in the capability of weather radar came about when engineers developed new radars that could measure the Doppler frequency shift in addition to the magnitude of the echo signal reflected from precipitation. The Doppler frequency shift is important because it is related to the radial velocity of the precipitation blown by wind (the component of the wind moving either toward or away from the radar installation). Since tornadoes, mesocyclones (which spawn tornadoes), hurricanes, and other hazardous weather phenomena tend to rotate, measurement of the radial wind speed as a function of viewing angle will identify rotating weather patterns. (Rotation is indicated when the measurement of the Doppler frequency shift shows that the wind is coming toward the radar at one angle and away from it at a nearby angle.)

The pulse Doppler weather radars employed by the National Weather Service, which are known as Nexrad, make quantitative measurements of precipitation, warn of potential flooding or dangerous hail, provide wind speed and direction, indicate the presence of wind shear and gust fronts, track storms, predict thunderstorms, and provide other meteorological information. In addition to measuring precipitation (from the intensity of the echo signal) and radial speed (from the Doppler frequency shift), Nexrad also measures the spread in radial speed (difference between the maximum and the minimum speeds) of the precipitation particles within each radar resolution cell. The spread in radial speed is an indication of wind turbulence.

Another improvement in the weather information provided by Nexrad is the digital processing of radar data, a procedure that renders the information in a form that can be interpreted by an observer who is not necessarily a meteorologist. The computer automatically identifies severe weather effects and indicates their nature on a display viewed by the observer. High-speed communication lines integrated with the Nexrad system allow timely weather information to be transmitted for display to various users.

The Nexrad radar operates at S-band frequencies (2.7 to 3 GHz) and is equipped with a 28-foot- (8.5-metre-) diameter antenna. It takes five minutes to scan its 1 degree beamwidth through 360 degrees in azimuth and from 0 to 20 degrees in elevation. The Nexrad system can measure rainfall up to a distance of 460 km and determine its radial velocity as far as 230 km.

A serious weather hazard to aircraft in the process of landing or taking off from an airport is the downburst, or microburst. This strong downdraft causes wind shear capable of forcing aircraft to the ground. Terminal Doppler weather radar (TDWR) is the name of the type of system at or near airports that is specially designed to detect dangerous microbursts. It is similar in principle to Nexrad but is a shorter-range system since it has to observe dangerous weather phenomena only in the vicinity of an airport. It operates from 5.60 to 5.65 GHz (C band) to avoid interference with the lower frequencies of Nexrad and ASR systems.

Airborne combat radar

A modern combat aircraft is generally required not only to intercept hostile aircraft but also to attack surface targets on the ground or sea. The radar that serves such an aircraft must have the capabilities to perform these distinct military missions. This is not easy because each mission has different requirements. The different ranges, accuracies, and rates at which the radar data is required, the effect of the environment (land or sea clutter), and the type of target (land features or moving aircraft) call for different kinds of radar waveforms (different pulse widths and pulse repetition frequencies). In addition, an appropriate form of signal processing is required to extract the particular information needed for each military function. Radar for combat aircraft must therefore be multimode—i.e., operate with different waveforms, signal processing, and antenna scanning. It would not be unusual for an airborne combat radar to have from 8 to 10 air-to-air modes and 6 to 10 air-to-surface modes. Furthermore, the radar system might be required to assist in rendezvous with a companion combat craft or with a refueling aircraft, provide guidance of air-to-air missiles, and counter hostile electronic jamming. The problem of achieving effectiveness with these many modes is a challenge for radar designers and is made more difficult by the size and weight constraints on combat aircraft.

The AN/APG-68(V)XM radar built for the U.S. F-16 (C/D) fighter is shown in the photograph. This is a pulse Doppler radar system that operates in a portion of the X-band (8- to 12-GHz) region of the spectrum. It occupies a volume of less than 0.13 cubic metre (4.6 cubic feet), weighs less than 164 kg (362 pounds), and requires an input power of 5.6 kilowatts. It can search 120 degrees in azimuth and elevation and is supposed to have a range of 35 nautical miles (65 km) in the “look-up” mode and 27.5 nautical miles (50 km) in the “look-down” mode. The look-up mode is a more or less conventional radar mode with a low pulse-repetition-frequency (prf) that is used when the target is at medium or high altitude and no ground-clutter echoes are present to mask target detection. The look-down mode uses a medium-prf Doppler waveform and signal processing that provide target detection in the presence of heavy clutter. (A low prf for an X-band combat radar might be from 250 hertz to 5 kHz, a medium prf from 5 to 20 kHz, and a high prf from 100 to 300 kHz.) Radars for larger combat aircraft can have greater capability but are, accordingly, bigger and heavier than the system just described.

The AN/APG-77 radar for the U.S. Air Force F-22 stealth dual-role fighter employs what is called an active-aperture phased-array radar rather than a mechanically scanned planar-array antenna. At each radiating element of the active-aperture phased-array is an individual transmitter, receiver, phase shifter, duplexer, and control.

Ballistic missile defense and satellite-surveillance radars

The systems for detecting and tracking ballistic missiles and orbiting satellites are much larger than those for aircraft detection because the ranges are longer and the radar echoes from space targets can be smaller than echoes from aircraft. Such radars might be required to have maximum ranges of 2,000 to 3,000 nautical miles (3,700 to 5,600 km), as compared with 200 nautical miles (370 km) for a typical long-range aircraft-detection system. The average power of the transmitter for a ballistic missile defense (BMD) radar can be from several hundred kilowatts to one megawatt or more, which is about 100 times greater than the average power of radars designed for aircraft detection. Antennas for this application have dimensions on the order of tens of metres to a hundred metres or more and are electronically scanned phased-array antennas capable of steering the radar beam without moving large mechanical structures. Radar systems for long-range ballistic missile detection and satellite surveillance are commonly found at the lower frequencies (typically at frequency bands of 420–450 MHz and 1,215–1,400 MHz).

The Pave Paws radar (AN/FPS-115) is an ultrahigh-frequency (UHF; 420–450 MHz) phased-array system for detecting submarine-launched ballistic missiles. It is supposed to detect targets with a radar cross section of 10 square metres at a range of 3,000 nautical miles (5,600 km). The array antenna contains 1,792 active elements within a diameter of 72.5 feet (22 metres). Each active element is a module with its own solid-state transmitter, receiver, duplexer, and phase shifter. The total average power per antenna is about 145 kilowatts. Two antennas make up a system, with each capable of covering a sector 120 degrees in azimuth. Vertical coverage is from 3 to 85 degrees. An upgraded variant of this type of radar is used in the Ballistic Missile Early Warning System (BMEWS) network, with installations in Alaska, Greenland, and England. BMEWS is designed to provide warning of intercontinental ballistic missiles (ICBMs). Each array antenna measures about 82 feet (25 metres) across and has 2,560 active elements identical to those of the Pave Paws system. Both the BMEWS and Pave Paws radars detect and track satellites and other space objects in addition to warning of the approach of ballistic missiles.

A BMD radar has to engage one or more relatively small reentry vehicles (RVs) that carry a warhead. Ballistic missile RVs can be made to have a very low echo (low radar cross section) when illuminated by radar. They were the original low-radar-cross-section targets and appeared more than 20 years before the more highly publicized stealth aircraft became a reality in the late 1980s. Ballistic missile defense requires battle-management radars that not only detect and track a relatively small target at sufficient range to engage effectively but also must reliably distinguish the reentry vehicles that carry warheads from the many confusion targets that can be present. Confusion targets include decoys, chaff (strips of metallic foil that produce an echo similar in size to that of the reentry vehicle), exploded tank fragments, and other objects released by the attacking missile. The BMD radar must also be able to fulfill its mission in spite of hostile countermeasures and defend against ballistic missiles that can reenter at low angles (depressed trajectories). In addition, the radar must be located in a defended region and be hardened to survive either a conventional or a nuclear attack.

There are at least two basic approaches to ballistic missile defense depending on whether the RV is engaged outside the atmosphere (exoatmospheric) or within the atmosphere (endoatmospheric). Exoatmospheric engagement is attractive, since it occurs at long range and a single system can defend a large area, but it requires some reliable method to select the warhead from the many extraneous objects that can accompany the warhead. An endoatmospheric ballistic missile defense system takes advantage of the slowing down of the lighter objects (decoys, chaff, and fragments) when they reenter the atmosphere and encounter air resistance. After reentry, the heavy warhead will be separated from the accompanying lighter “junk” and thus can be engaged. A significant limitation, however, is that endoatmospheric ballistic missile defense results in a much smaller defended area.

In the 1960s there were several different systems considered for defense against ICBMs. Both the United States and the Soviet Union devised defenses, but only the Soviet Union deployed such a system, and the antiballistic missile treaty of 1972 limited it to defense of a single region (Moscow). With the increased threat from tactical ballistic missiles in the 1990s, new radar concepts were explored. One was the U.S. Army’s Theater High Altitude Area Defense Ground Based Radar (THAAD GBR). This is a mobile solid-state active-aperture phased-array radar that operates within the X-band of the spectrum. A different approach to ballistic missile defense is the Israeli tactical system known as Arrow, which employs an L-band (1- to 2-GHz) active-aperture phased-array radar.