Radar warning receiver

Typically fitted to military aircraft, radar warning receivers (RWR) detect the radio emissions of radar systems, whether ground-based or on-board other aircraft. The system is often capable of classifying the source of the radar by type (Pulsed power or continuous wave radar, for example) and strength. It may be further capable of using this data to identify the type of threat the detected radar poses, for example an incoming radar-guided surface-to-air missile (SAM).

The RWR usually has a visual display somewhere prominent in the cockpit (in some modern aircraft, in multiple locations in the cockpit) and also generates audible tones which feed into the pilot's (and perhaps RIO/co-pilot/GIB's in a multi-seat aircraft) headset. The visual display often takes the form of a circle, with symbols displaying the detected radars according to their direction relative to the current aircraft heading (i.e. a radar straight ahead displayed at the top of the circle, directly behind at the bottom, etc.). The distance from the center of the circle, depending on the type of unit, can represent the estimated distance from the generating radar, or to categorize the severity of threats to the aircraft, with tracking radars placed closer to the center than search radars. The symbol itself is related to the type of radar or the type of vehicle that carries it, often with a distinction made between ground-based radars and airborne radars. Audible tones are usually assigned to each type of threat or type of radar and are fairly distinctive. The more serious the threat, the more shrill the tone in order to get the pilot's attentionfact|date=September 2007. For example, an active missile seeker might be a high pitched, almost continuous trill whereas the radar of an obsolete fighter type or SAM system might be a low pitched, intermittent buzz.

The RWR is used for identifying, avoiding, evading or engaging threats. For example, a fighter aircraft on a combat air patrol (CAP) might notice enemy fighters on the RWR and subsequently use its own radar set to find and eventually engage the bandit. In addition, the RWR helps identify and classify threats - it's hard to tellfact|date=September 2007 which blips on a radar are dangerous, but since different fighter aircraft typically have different types of radar sets, once they turn them on and point them near the aircraft in question it may be able to tell, by the direction and strength of the signal, which of the blips is which type of fighter.

A non-combat aircraft, or one attempting to avoid engagements, might turn its own radar off and attempt to steer around threats detected on the RWR. Especially at high altitude (more than 30,000 feet AGL), very fewfact|date=September 2007 threats exist which will not be giving out radiation. As long as the pilot is careful to check for aircraft that might try to sneak up without radar, say with the assistance of AWACS or GCI, it should be able to steer clear of SAMs, fighter aircraft and high altitude, radar-directed AAA.

SEAD and ELINT aircraft often have very powerful RWR equipment like the U.S. HTS (HARM targeting system) pod which is able to find and classify threats which are much further away than those detected by a typical RWR, and may be able to overlay threat circles on a map in the aircraft's multi-function display (MFD), providing much better [www.dtic.mil/descriptivesum/Y2000/Navy/0603270N.pdf] information for avoiding or engaging threats, and may even store information to be analyzed later or transmitted to the ground to help the commanders plan future missions.

The RWR can be an important tool for evading threats if avoidance has failed. For example, if a SAM system or enemy fighter aircraft has fired a missile (for example, a SARH-guided missile) at the aircraft, the RWR may be able to detect the change in mode that the radar must use to guide the missile and notify the pilot with much more insistent warning tones and flashing, bracketed symbols on the RWR display. The pilot will then knowfact|date=September 2007 to take evasive action, and what type of evasive action to take, in order to break the missile lock or dodge the missile. The pilot may even be able to visually acquire the missile after being alerted to the possible launch. What's more, if an actively guided missile is tracking the aircraft, the pilot can use the direction and distance display of the RWR in order to work out which evasive maneuvers to perform in order to outrun or dodge the missile. For example, the rate of closure and aspect of the incoming missile may allow the pilot to determine that if s/he dives away from the missile, it will be unlikely to catch up, or if it is closing fast, that it is time to jettison external supplies and turn toward the missile in an attempt to out-turn it.

RWR types in service

* AN/ALR-46 (F-4 Phantom II, RF-4 Phantom II, F-5)
* AN/ALR-56 (USA; F-15 Eagle, Canada; CC-130 Hercules)
* AN/ALR-66 (USA; P-3C Orion)
* AN/ALR-67 (USA; AV-8B Harrier II, F-14 Tomcat, F/A-18E/F Super Hornet, EA-6B Prowler, Canada; CF-188 Hornet)
* AN/ALR-69 (USA; B-52H Stratofortress, lacks Frequency Selective Receiver Capabilities (FSRS) capabilities however, A-10 Thunderbolt II, AC-130 Spectre, F-16 Fighting Falcon, HH-53, MC-130, F-4E Phantom II).
* AN/ALR-76 (USA; S-3, EP-3)
* AN/ALR-94 (USA; F-22)
* AN/APR-39 (USA, AH-1, AH-64 Apache, CH-46 Sea Knight, CH-47 Chinook, CH-53, EH-60 Black Hawk, KC-130 Hercules, MH-47 Chinook, MH-60 Black Hawk, OH-58, OV-1 Mohawk, RC-12, RV-1, UH-1 Iroquois, UH-60 Black Hawk, V-22 Osprey)
* ALR-2002 (Australia; developmental, project being cancelled)
* ALR-400 (Spain; F-1M , EF-18M)
* SPO-15 Beryoza (India; MiG-29 Fulcrum, Russia; Su-27SK Flanker-B, China; J-11)
* Tarang (India; MiG-27 Flogger, LCA Tejas, Jaguar, Su-30MKI Flanker)

Example of an RWR display

This is what an RWR display might look like and is particularly representative of the AN/ALR-69. It shows a ground-based search radar nearly straight ahead, an SA-6 SAM system about 30 degrees to the right of straight ahead (currently the closest and most dangerous threat as indicated by the diamond and hat) and an F-15 fighter behind and slightly to the left. The distance of the threats from the centre is not necessarily indicative of the threat's absolute range, but rather the proximity to the lethal envelope of the threat. For example, if an SA-2 and a ZSU-23-4 are both dead ahead of the aircraft and at the same range, the SA-2 will actually appear nearer to the centre of the display because the SA-2 has a much greater lethal envelope than the ZSU-23-4. The concentric rings on the display signify different 'lethality zones'. The outer ring signifies that the threat is beyond engagement range (ie the range it can engage you, not vice versa), the middle ring signifies that the threat is within engagement range, the inner ring equates to a 'no-escape' zone. In the example below, the SA-6 and F-15 are both well within the engagement zone and on the borderline of the 'no-escape' zone. This ordering of the rings applies to the AN/ALR-69 found in, for example, the F-16. However, some sources claim the order is reversed on the AN/ALR-67, with the 'critical' zone being the outer ring. In many ways this would make more sense, because it allows greater spacial separation of the highest priority threats on the display, allowing the pilot a greater degree of angular resolution when determining the direction of the threat.

References

* [http://www.fas.org/man/dod-101/sys/ac/equip/an-alr-46.htm AN/ALR-46 at FAS]
* [http://www.fas.org/man/dod-101/sys/ac/equip/an-alr-56.htm AN/ALR-56 at FAS]
* [http://www.fas.org/man/dod-101/sys/ac/equip/an-alr-67.htm AN/ALR-67 at FAS]
* [http://www.fas.org/man/dod-101/sys/ac/equip/an-alr-69.htm AN/ALR-69 at FAS]
* [http://www.fas.org/man/dod-101/sys/ac/equip/an-apr-39.htm AN/APR-39 at FAS]
* [http://www.qsl.net/n9zia/ew-programs.html]