Air Surveillance Radars
Air Surveillance Radars are designed for early warning, land and maritime surveillance, whether for fixed-wing aircraft, helicopters, or remotely piloted vehicles (RPV's). 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.
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.
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 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.
Tracking Radars 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.
There are two different Track-While-Scan (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.
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.
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