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The track while scan (TWS) is a mode of
radar Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...
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 Situational awareness or situation awareness (SA) is the perception of environmental elements and events with respect to time or space, the comprehension of their meaning, and the projection of their future status. An alternative definition is tha ...
.


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 The SCR-584 (short for '' Set, Complete, Radio # 584'') was an automatic-tracking microwave radar developed by the MIT Radiation Laboratory during World War II. It was one of the most advanced ground-based radars of its era, and became one of the ...
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 A plan position indicator (PPI) is a type of radar display that represents the radar antenna in the center of the display, with the distance from it and height above ground drawn as concentric circles. As the radar antenna rotates, a radial tra ...
(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 A radar display is an electronic device to present radar data to the operator. The radar system transmits pulses or continuous waves of electromagnetic radiation, a small portion of which backscatter off targets (intended or otherwise) and retur ...
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 (SARH) is a common type of missile guidance system, perhaps the most common type for longer-range Air-to-air missile, air-to-air and surface-to-air missile systems. The name refers to the fact that the missile itself is ...
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 An oscilloscope (informally a scope) is a type of electronic test instrument that graphically displays varying electrical voltages as a two-dimensional plot of one or more signals as a function of time. The main purposes are to display repetiti ...
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 In antenna theory, a phased array usually means an electronically scanned array, a computer-controlled array of antennas which creates a beam of radio waves that can be electronically steered to point in different directions without moving th ...
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. However, the first operational track-while-scan radar in history was neither passive electronically scanned array nor
active electronically scanned array An active electronically scanned array (AESA) is a type of phased array antenna, which is a computer-controlled array antenna in which the beam of radio waves can be electronically steered to point in different directions without moving the an ...
radar. It was actually the
Soviet The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, ...
-made missile guidance, target detection and tracking radar known as B-200, firstly designed in 1953 by KB-1 (today known as
NPO Almaz JSC NPO Almaz named after A.A. Raspletin (russian: link=no, ГСКБ Концерна ПВО "Алмаз-Антей", former SB-1, 1947–50; KB-1, 1950–66; MKB Strela, 1966–71; TsKB Almaz, 1971–88; NPO Almaz, 1988–2008; GSKB Almaz-Ante ...
), as a part of multi-channel, stationary anti-aircraft missile system designated as S-25 (''Sistema-25'', initial name ''Berkut'' -
Golden eagle The golden eagle (''Aquila chrysaetos'') is a bird of prey living in the Northern Hemisphere. It is the most widely distributed species of eagle. Like all eagles, it belongs to the family Accipitridae. They are one of the best-known bird of p ...
, in English) or SA-1 ''Guild'' (by NATO designation), which was intended exclusively for defense against possible massive air raid on
Moscow Moscow ( , US chiefly ; rus, links=no, Москва, r=Moskva, p=mɐskˈva, a=Москва.ogg) is the capital and largest city of Russia. The city stands on the Moskva River in Central Russia, with a population estimated at 13.0 million ...
and especially
Kremlin The Kremlin ( rus, Московский Кремль, r=Moskovskiy Kreml', p=ˈmɐˈskofskʲɪj krʲemlʲ, t=Moscow Kremlin) is a fortified complex in the center of Moscow founded by the Rurik dynasty, Rurik dynasty. It is the best known of th ...
from long-range strategic
USAF The United States Air Force (USAF) is the air service branch of the United States Armed Forces, and is one of the eight uniformed services of the United States. Originally created on 1 August 1907, as a part of the United States Army Sign ...
bombers (especially the ones like
B-47 The Boeing B-47 Stratojet (Boeing company designation Model 450) is a retired American long- range, six-engined, turbojet-powered strategic bomber designed to fly at high subsonic speed and at high altitude to avoid enemy interceptor aircr ...
and later
B-52 The Boeing B-52 Stratofortress is an American long-range, subsonic, jet-powered strategic bomber. The B-52 was designed and built by Boeing, which has continued to provide support and upgrades. It has been operated by the United States Air ...
, capable of stratospheric flights, which made them completely immune to ordinary anti-aircraft guns). Since the S-25 was also designed as the first multi-channel missile system in history (the first one which had the ability to engage multiple targets completely simultaneously - as much as twenty targets by a single battery, each with up to three missiles), it thus required a proper radar capable of fulfilling such a demanding task, what ultimately resulted in creation of the B-200, as the very first
fire-control radar A fire-control radar (FCR) is a radar that is designed specifically to provide information (mainly target azimuth, elevation, range and range rate) to a fire-control system in order to direct weapons such that they hit a target. They are sometim ...
intended for multiple guidance of missiles on multiple different air targets, what was ensured exactly by its TWS ability. Rather than using later phased-array antennas and multiprocessor digital computers (both which did not exist yet back in the time) the TWS ability in B-200 was actually achieved by an alternative method, that is, by the so-called "brute use of force" approach (B-200 featured massive and very bulky electronics featuring many
analog computers An analog computer or analogue computer is a type of computer that uses the continuous variation aspect of physical phenomena such as electrical, mechanical, or hydraulic quantities (''analog signals'') to model the problem being solved. In ...
together with its own power supply in shape of heavy generators, regulators, stabilizers and ventilators, all which were placed inside a relatively large concrete bunker). The USSR created 56 of those radar sites between 1954 and 1956 (as many as there were S-25 missile sites) in two large concentric rings around Moscow which represented two lines of anti-aircraft defense, each of them featuring multiple S-25 sites (34 of those were located on the outer and remaining 22 on the inner ring). The B-200 was a 3D, UHF, S/E-band radar, which had instrumented detection range of 150km and ability to track as much as 30 different targets simultaneously (on 20 of those it could also launch S-25 missiles), while still scanning for new targets. It was the first radar in the world capable of such features, which will be surpassed for the first time yet half a century later, by modern Russian
S-400 The S-400 Triumf (russian: link=no, C-400 Триумф – Triumf; translation: Triumph; NATO reporting name: SA-21 Growler), previously known as the S-300 PMU-3, is a mobile, surface-to-air missile (SAM) system developed in the 1990s by Russ ...
system (whose 92N2 fire-control radar can engage as much as 80 different targets simultaneously, each with two missiles). The B-200 also featured very unique and advanced design for its time as well as an unusual operating mode; consisting of two symmetrical antennas (one intended for azimuth and the other one for elevation surveillance), each featuring two hexagonal, diamond-shaped ''discs'' (each as much as 10 meters high), both which were rotating around its own axles (like a
propeller A propeller (colloquially often called a screw if on a ship or an airscrew if on an aircraft) is a device with a rotating hub and radiating blades that are set at a pitch to form a helical spiral which, when rotated, exerts linear thrust upon ...
or
windmill A windmill is a structure that converts wind power into rotational energy using vanes called windmill sail, sails or blades, specifically to mill (grinding), mill grain (gristmills), but the term is also extended to windpumps, wind turbines, and ...
) in mutually opposite directions and as fast as 50 turns per minute, which enabled them as much as target scans. The B-200 together with S-25 were serving as Moscow's main line of defense against possible air raid for almost 30 years (1955-1982), until later being surpassed by self-propelled, long-range missile system S-300 (nowadays S-400), mainly due to complete immobility of the whole S-25 system. Despite not falling in category of modern phased-array radars, the B-200 is nevertheless also considered to be the first modern fire-control radar in history (the one intended for missile guidances), since most of the today's radars of the type share the TWS ability. From the
Western Western may refer to: Places *Western, Nebraska, a village in the US *Western, New York, a town in the US *Western Creek, Tasmania, a locality in Australia *Western Junction, Tasmania, a locality in Australia *Western world, countries that id ...
side, the first operational TWS radar was the Royal Navy's (RN) Type 984 naval radar, which firstly appeared three years later than Soviet B-200 ground-based radar (in 1956), with only three such radars manufactured (for three Royal Navy
aircraft carriers An aircraft carrier is a warship that serves as a seagoing airbase, equipped with a full-length flight deck and facilities for carrying, arming, deploying, and recovering aircraft. Typically, it is the capital ship of a fleet, as it allows a n ...
- the HMS
Eagle Eagle is the common name for many large birds of prey of the family Accipitridae. Eagles belong to several groups of genera, some of which are closely related. Most of the 68 species of eagle are from Eurasia and Africa. Outside this area, just ...
,
Hermes Hermes (; grc-gre, Ἑρμῆς) is an Olympian deity in ancient Greek religion and mythology. Hermes is considered the herald of the gods. He is also considered the protector of human heralds, travellers, thieves, merchants, and orato ...
and
Victorious ''Victorious'' (stylized as ''VICTORiOUS'') is an American sitcom created by Dan Schneider (TV producer), Dan Schneider that originally aired on Nickelodeon, debuting on March 27, 2010, and concluding on February 2, 2013 after four seasons. Th ...
). Also, the Type 984 was not a fire-control radar and was thus not intended for guidance of missiles, but a radar intended for
ground-controlled interception Ground-controlled interception (GCI) is an air defence tactic whereby one or more radar stations or other observational stations are linked to a command communications centre which guides interceptor aircraft to an airborne target. This tactic was p ...
as well as an
early warning An early warning system is a warning system that can be implemented as a Poset, chain of information communication systems and comprises sensors, Detection theory, event detection and decision support system, decision subsystems for early identi ...
radar, due to which also required TWS ability. It was the first Western radar which featured ability to engage aircraft on multiple airborne targets while still simultaneously scanning for new ones. The Type 984 was also the first naval TWS radar in history. In the United States, the original tracking radar system was the
Semi-Automatic Ground Environment The Semi-Automatic Ground Environment (SAGE) was a system of mainframe computer, large computers and associated computer network, networking equipment that coordinated data from many radar sites and processed it to produce a single unified image ...
(SAGE) system developed for the
US Air Force The United States Air Force (USAF) is the air service branch of the United States Armed Forces, and is one of the eight uniformed services of the United States. Originally created on 1 August 1907, as a part of the United States Army Signal ...
in 1958. 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 The Hughes Aircraft Company was a major American aerospace and defense contractor founded on February 14, 1934 by Howard Hughes Howard Robard Hughes Jr. (December 24, 1905 – April 5, 1976) was an American business magnate, record-setting p ...
AN/ASG-18 The Hughes AN/ASG-18 Fire Control System was a prototype airborne fire control radar system for the planned North American XF-108 Rapier interceptor aircraft, and the Lockheed YF-12 for the United States Air Force. It was the US's first Pulse-Doppl ...
of the
XF-108 Rapier The North American XF-108 Rapier was a proposed long-range, high-speed interceptor aircraft designed by North American Aviation intended to defend the United States from supersonic Soviet strategic bombers. The aircraft would have cruised at ...
, which could track a single target. The Westinghouse AN/APQ-81 for the
F6D Missileer The Douglas F6D Missileer was a proposed carrier-based fleet defense fighter designed by Douglas Aircraft Company in response to a 1959 United States Navy requirement. It was designed to be able to loiter for extended periods at a relatively ...
was more advanced, tracking up to eight targets, but required its own operator. It was not until the introduction of
digital computers A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations (computation) automatically. Modern digital electronic computers can perform generic sets of operations known as programs. These programs ...
, and especially
microprocessors A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circu ...
, 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 The AN/AWG-9 and AN/APG-71 radars are all-weather, multi-mode X band pulse-Doppler radar systems used in the F-14 Tomcat, and also tested on TA-3B. It is a very long-range air-to-air system with the capability of guiding several AIM-54 Phoeni ...
of the
F-14 Tomcat The Grumman F-14 Tomcat is an American carrier-capable supersonic aircraft, supersonic, twinjet, twin-engine, two-seat, twin-tail, variable-sweep wing fighter aircraft. The Tomcat was developed for the United States Navy's Naval Fighter Experi ...
used an
Intel 8080 The Intel 8080 (''"eighty-eighty"'') is the second 8-bit microprocessor designed and manufactured by Intel. It first appeared in April 1974 and is an extended and enhanced variant of the earlier 8008 design, although without binary compatibil ...
and could track 24 targets.


See also

* Track before detect *
Type 984 radar Type 984 was a Royal Navy radar system introduced in the mid-1950s, designed by the Admiralty Signals and Radar Establishment. Type 984 was a 3D S band system used for both ground controlled interception (GCI) and as a secondary early warning ...


References

{{reflist Radar