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Radar systems are essential elements in any military organization’s defense efforts, whether on land or at sea. They are simple in concept: transmit a pulse and then receive the reflections from a target to extrapolate information about that target. To civilians, radar technology is considered important but fairly standard among the different armed forces.

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Nevertheless, the number of different radar systems and technologies in use by the armed forces is quite staggering. These systems are constantly receiving improvements as RF/microwave components and subsystems evolve—for instance, the availability of higher-power gallium nitride (GaN) semiconductors in smaller packages.

Radar technology has evolved a great deal from its origins in World War II. Not only is it now integrated into systems for land, sea, and air defense-related applications, but it has been adapted for prediction weather, for medical use, and even at millimeter-wave frequencies for automotive use in collision-avoidance systems.

It its simplest form, a radar consists of a transmitter that broadcasts pulsed signals in search of a target, whether known or unknown. A radar receiver detects pulse reflections from a target and the amplitude of those signals—along with the differences in time of arrival and the time that the pulses were transmitted—to determine information about the target including size, location, and direction.

Military radar are used to detect a wide range of targets, including land vehicles, ships, aircraft, projectiles, missiles, and satellites. Radar is also used to guide weapons to a particular target, and helps navigate ships and aircraft. Pulse radar systems are the most essential forms of radar, using repetitive strings of short pulsed signals to identify a target. Some of these systems rely on simply receiving the reflected pulse echoes to determine information about the target from the nature of those returning signals.

Some rely on decoding the Doppler frequency shift of the returning signals in order to reject the radar returns from targets that are not moving. This simplifies the detection of, for example, a ground vehicle, from the surrounding environment (known as clutter). Some of these pulse Doppler radar systems are further designated moving-target-indicator (MTI) radar for their capabilities in detecting moving targets from the objects and environment around them.

MTI pulse Doppler radar systems tend to use low pulse repetition frequencies (PRFs) to achieve an unambiguous target range measurements, such as a PRF in the Hz range. Standard pulse Doppler radar systems are more likely to use a higher PRF—more in the kHz range. A low PRF enables accurate measurements of target range while a higher PRF is more capable of accurate measurements of target speed.

Because aircraft themselves are moving as they are attempting to make MTI radar measurements, they employ a variation of the MTI radar system known as the airborne moving-target-indicator (AMTI) radar. Since stationary clutter is moving relative to the moving radar system on the aircraft, these additional Doppler effects must be corrected in an AMTI to execute accurate moving target measurements.

Synthetic-aperture-radar (SAR) systems are typically moving systems, such as on satellites or aircraft, with an antenna beam oriented perpendicular to the direction of travel. An SAR system analyzes sequentially received signals and adds them for analysis, creating a high-resolution image of a target. An inverse SAR (ISAR) works with Doppler frequency shift to obtain movements of a target relative to the movement of the radar system, such as on an aircraft.

SAR and  ISAR systems also are known as imaging radar systems. The U.S. Defense Advanced Research Projects Agency (DARPA) currently is seeking a radar system that can provide the performance of an SAR system, but more cost effectively, through its recently announced Advanced Scanning Technology for Imaging Radar (ASTIR) program.

These are just a few examples of the many types of pulsed radar systems used on land, at sea, and in the air by military forces. Radar systems can also operate with continuous-wave (CW) signals. CW radar systems transmit and receive signals at the same time, and depend upon the Doppler frequency shift of a moving target to identify it from returned signals from nonmoving objects.

CW radar systems can measure the radial velocity of targets from their Doppler frequency shift and determine the direction of arrival of a received signal—but not the range of the target. This basic radar format is typically used for such applications as target tracking and vehicle speed detection. (Doppler techniques also are widely used in commercial weather radar systems.)

By using a variation of this radar type, frequency-modulated CW radar (FM-CW radar), the transmitted frequency is continually changing with time. With such a system, the frequency of the reflected radar signals will differ from the frequency of the transmitted signals according to the changes with time, and it is possible to determine the range of a target according to the proportional changes in modulated frequency with time.

The range of a target also can be determined by using CW signals and changing the phase of the transmitted signals, with the received changes in phase corresponding to the range of the target. 

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