Track while scan
Encyclopedia
The track while scan is a mode of radar
operation in which the radar allocates part of its power to tracking the target or targets while part of its power is allocated to scanning, unlike the straight tracking mode, when the radar directs all its power to tracking the acquired targets. In the TWS mode the radar has a possibility to acquire additional targets as well as providing an overall view of the airspace and helping maintain better situational awareness.
Ground-based radars like the SCR-584 automated this process early in their evolution. In search mode the SCR-584 rotated its antenna through 360 degrees and any returns were plotted on a plan position indicator
(PPI). This gave the operators an indication of any targets within its ~25 mile detection range and their direction relative to the radar van. When one of the returns was considered interesting, the radar was flipped to tracking mode and "locked-on". From then on it would automatically keep its antenna pointed at the target, feeding out accurate direction, altitude and range information on a B-Scope display. Operator workload was greatly reduced.
Advances in electronics meant it was only a matter of time before automated radars like the SCR-584 could be reduced in size and weight enough to fit into an aircraft. These started appearing in the late 1950s and remained common until the 1980s.
The introduction of semi-active radar homing
missiles made the lock-on concept especially important. These missiles use the launching aircraft's own radar to "paint" the target with a radar signal, the missile listens for the signal being reflected off the target to home in on. This requires the radar to be locked-on in order to provide a steady guidance signal. The drawback is that once the radar is set to tracking a single target, the operator loses information about any other targets. This is the problem that track while scan is meant to address.
In traditional radar systems, the display is purely electrical; signals from the radar dish are amplified and sent directly to an oscilloscope
for display. There is a one-to-one correspondence between "blips" on the display and a radio signal being received from the antenna. When the antenna is not pointed in a particular direction, the signal from any targets in that direction simply disappear. To improve the operator's ability to read the display, the oscilloscopes typically used a slowly-fading phosphor as a crude form of "memory".
TWS radars disconnect the display from the antenna, sending the signals to a computer instead of the display. The computer interprets the signal and develops a "track file" for anything that would have normally caused a blip. The next time the radar returns to that area, any returns are correlated with the original recording, and the track file is updated or discarded as appropriate. A second system continuously reads the data in the track files from memory, and displays this on the radar as a series of annotated icons. Unlike the straight tracking mode, TWS radars have to solve an additional problem of recognizing whether each target discrimination/detection defines a new target or belongs to already tracked targets.
With the location of targets known even when the radar antenna is not pointed at them, TWS radars can return to the same area of sky on their next scan and beam additional energy toward the target. So in spite of the radar not constantly painting the target as it would in a traditional lock-on, enough energy is sent in that direction to allow a missile to track. A phased array antenna helps here, by allowing the signal to be focused on the target when the antenna is in that direction, without it having to be pointed directly at the target. This means that the target can be painted for a longer period of time, whenever the antenna is in the same general direction. Advanced phased array radars make this even easier, allowing a signal to be continually directed at the target.
The original tracking radar system was the Semi-Automatic Ground Environment (SAGE) system developed for the US Air Force. SAGE required enormous computers to develop and maintain tracks for up to dozens of aircraft. Early airborne TWS radar typically only tracked a single target while scanning. The original TWS airborne set was the Hughes Aircraft
AN/ASG-18
of the XF-108 Rapier
, which could track a single target. The Westinghouse AN/APQ-81 for the F6D Missileer
was more advanced, tracking up to eight targets, but required its own operator.
It was not until the introduction of digital computers, and especially microprocessors, that TWS in airborne applications became practical. Development of TWS generally followed the development of the microprocessors that eventually powered them; the AN/AWG-9 of the F-14 Tomcat
used an Intel 8080
and could track 24 targets.
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
operation in which the radar allocates part of its power to tracking the target or targets while part of its power is allocated to scanning, unlike the straight tracking mode, when the radar directs all its power to tracking the acquired targets. In the TWS mode the radar has a possibility to acquire additional targets as well as providing an overall view of the airspace and helping maintain better situational awareness.
Background
Early airborne radar systems generally operated purely as tracking systems, with a dedicated radar operator manually "tuning" the system to locate targets in a relatively narrow field-of-view in front of the aircraft. The searching area could be moved using a variety of methods, typically phase-shifting or lobe switching on lower frequency systems that required large antennas, or by moving the radar dish on microwave frequency radars. Engagements would start with ground controllers guiding the aircraft into the general area of the target via voice commands to the pilot, and once the aircraft got into range its own radar would pick up the target for the final approach when the radar operator would provide voice commands to the pilot. There was no real distinction between seeking out a target and tracking it.Ground-based radars like the SCR-584 automated this process early in their evolution. In search mode the SCR-584 rotated its antenna through 360 degrees and any returns were plotted on a plan position indicator
Plan position indicator
The plan position indicator , is the most common type of radar display. The radar antenna is usually represented in the center of the display, so the distance from it and height above ground can be drawn as concentric circles...
(PPI). This gave the operators an indication of any targets within its ~25 mile detection range and their direction relative to the radar van. When one of the returns was considered interesting, the radar was flipped to tracking mode and "locked-on". From then on it would automatically keep its antenna pointed at the target, feeding out accurate direction, altitude and range information on a B-Scope display. Operator workload was greatly reduced.
Advances in electronics meant it was only a matter of time before automated radars like the SCR-584 could be reduced in size and weight enough to fit into an aircraft. These started appearing in the late 1950s and remained common until the 1980s.
The introduction of semi-active radar homing
Semi-active radar homing
Semi-active radar homing, or SARH, is a common type of missile guidance system, perhaps the most common type for longer-range air-to-air and surface-to-air missile systems. The name refers to the fact that the missile itself is only a passive detector of a radar signal – provided by an external ...
missiles made the lock-on concept especially important. These missiles use the launching aircraft's own radar to "paint" the target with a radar signal, the missile listens for the signal being reflected off the target to home in on. This requires the radar to be locked-on in order to provide a steady guidance signal. The drawback is that once the radar is set to tracking a single target, the operator loses information about any other targets. This is the problem that track while scan is meant to address.
In traditional radar systems, the display is purely electrical; signals from the radar dish are amplified and sent directly to an oscilloscope
Oscilloscope
An oscilloscope is a type of electronic test instrument that allows observation of constantly varying signal voltages, usually as a two-dimensional graph of one or more electrical potential differences using the vertical or 'Y' axis, plotted as a function of time,...
for display. There is a one-to-one correspondence between "blips" on the display and a radio signal being received from the antenna. When the antenna is not pointed in a particular direction, the signal from any targets in that direction simply disappear. To improve the operator's ability to read the display, the oscilloscopes typically used a slowly-fading phosphor as a crude form of "memory".
Track while scan
Track while scan radars became possible with the introduction of two new technologies; phased-array radars and computer memory devices. Phased-array antennas became practical with the introduction of tunable high-power coherent radio frequency oscillators in the 1960s. By shifting the phase slightly between a series of antennas, the resulting additive signal can be steered and focused electronically. Much more important to the development of TWS was the development of digital computers and their associated memories, which allowed the radar data to be remembered from scan to scan.TWS radars disconnect the display from the antenna, sending the signals to a computer instead of the display. The computer interprets the signal and develops a "track file" for anything that would have normally caused a blip. The next time the radar returns to that area, any returns are correlated with the original recording, and the track file is updated or discarded as appropriate. A second system continuously reads the data in the track files from memory, and displays this on the radar as a series of annotated icons. Unlike the straight tracking mode, TWS radars have to solve an additional problem of recognizing whether each target discrimination/detection defines a new target or belongs to already tracked targets.
With the location of targets known even when the radar antenna is not pointed at them, TWS radars can return to the same area of sky on their next scan and beam additional energy toward the target. So in spite of the radar not constantly painting the target as it would in a traditional lock-on, enough energy is sent in that direction to allow a missile to track. A phased array antenna helps here, by allowing the signal to be focused on the target when the antenna is in that direction, without it having to be pointed directly at the target. This means that the target can be painted for a longer period of time, whenever the antenna is in the same general direction. Advanced phased array radars make this even easier, allowing a signal to be continually directed at the target.
The original tracking radar system was the Semi-Automatic Ground Environment (SAGE) system developed for the US Air Force. SAGE required enormous computers to develop and maintain tracks for up to dozens of aircraft. Early airborne TWS radar typically only tracked a single target while scanning. The original TWS airborne set was the Hughes Aircraft
Hughes Aircraft
Hughes Aircraft Company was a major American aerospace and defense contractor founded in 1932 by Howard Hughes in Culver City, California as a division of Hughes Tool Company...
AN/ASG-18
Hughes AN/ASG-18
The Hughes AN/ASG-18 was a prototype fire control system and radar, originally intended for the XF-108 Rapier interceptor aircraft, for the United States Air Force....
of the XF-108 Rapier
XF-108 Rapier
The North American XF-108 Rapier was a proposed long-range, high-speed interceptor aircraft designed by North American Aviation. Initiated to defend the United States from supersonic Soviet bombers, the program, initially called NA-257, was cancelled in 1959 due to a shortage of funds and the...
, which could track a single target. The Westinghouse AN/APQ-81 for the F6D Missileer
F6D Missileer
The Douglas F6D Missileer was a proposed carrier-based US Navy fleet defense fighter aircraft, designed by Douglas Aircraft Company in response to a 1959 US Navy requirement...
was more advanced, tracking up to eight targets, but required its own operator.
It was not until the introduction of digital computers, and especially microprocessors, that TWS in airborne applications became practical. Development of TWS generally followed the development of the microprocessors that eventually powered them; the AN/AWG-9 of the F-14 Tomcat
F-14 Tomcat
The Grumman F-14 Tomcat is a supersonic, twin-engine, two-seat, variable-sweep wing fighter aircraft. The Tomcat was developed for the United States Navy's Naval Fighter Experimental program following the collapse of the F-111B project...
used an Intel 8080
Intel 8080
The Intel 8080 was the second 8-bit microprocessor designed and manufactured by Intel and was released in April 1974. It was an extended and enhanced variant of the earlier 8008 design, although without binary compatibility...
and could track 24 targets.