Equipment working within the electromagnetic spectrum to make the pilot aware of the combat situation surrounding the aircraft are known as Electronic Support Measures (ESM). The pilot bases his Situational Awareness (SA) on the
ESM on-board the aircraft, and the better equipped the aircraft is, the better
SAthe pilot may obtain. In this section some of these measures are described.
2.4.1 Radar Warning Receiver
Different types of radar systems have different characteristics, and this is used by the Radar Warning Receiver (RWR) to determine from which type of radar system incoming radar waves originate. This is done by finding the properties of the wave in a lookup table. In this table the kind of missile often associated with the radar system may also be found. Based on the table a warning symbol
2.4 Electronic Support Measures 11
is shown in theazimuth indicator, and an audio warning is given to the pilot.
The symbol displayed on the azimuth indicator shows the type of the radar system and the direction towards it. If theRWRcan not show the type of radar system related to the detected radar signal, the azimuth indicator will indicate the radar system as being of an unknown type. Appendix A describes some of the RF-based threats detectable by a RWR. An azimuth indicator is shown in Figure 2.6.
Figure 2.6: An azimuth indicator as part of the Advanced Threat Display.
(Photo courtesy of Terma.)
The position of a symbol shown in the azimuth indicator indicates the angle towards the threat and the proximity to thelethal envelope of the threat. The lethal envelope is the range in which the threat can engage the aircraft, and if the aircraft is close to, or within, the range of a threat this is shown in the azimuth indicator. For some azimuth indicators the symbols closest to the centre will represent the most imminent threats, while others will have these farthest away from the centre. While the first of these may seem most intuitive, the latter has its advantages. It will allow greater spatial separation of the highest priority threats on the display, making it easier for the pilot to determine directions to threats.
Usually the aircraft will be detected by enemy search radar before it is being tracked or locked upon. Radar characteristics vary from search radar to tracking radar and the RWR on-board the aircraft is able to distinguish between these radar modes based on the characteristics of incoming radar radiation. It is worth noting that not all symbols shown in the azimuth indicator represent threats.
In any given scenario there may be numerous radars present, and possibly none or only a few of these represent a threat. Symbols representing search radars and acquisition and tracking radars may all be displayed simultaneously on the
12 Electronic Warfare
azimuth indicator. Most newerRWRsystems offer the possibility of prioritizing the threats and showing symbols for the threats with the highest priority only.
OlderRWRsystems will only show the symbols of tracking radars and launched missiles.
2.4.2 Missile Warning System
The Missile Warning System (MWS) (sometimes referred to as Missile Approach Warning System (MAWS)) informs the pilot when a missile is approaching the aircraft. In a passiveIRbasedMWSthis is detected by continuously analysingIR
images of the aircraft surroundings. These images are acquired using on-board
IRsensors orIRcameras. If the images contain a hot spot (possibly indicating the plume of an approaching missile) that increases in size over a relatively short time span and which seems to follow the aircraft, a missile warning is issued.
In a passive Ultraviolet (UV) based MWS the images analysed are showing in-formation from theUVpart of the electromagnetic spectrum. This type ofMWS
has some benefits compared to the IR-basedMWS since theUV characteristics of a missile plume may change during its flight. Information about the missile (time since launch, time to burn out, etc.) may then be extracted from theUV
images.
A RF based MWS is an active system working in the radar band. It can de-termine the range and velocity of an approaching missile, thus giving the pilot an estimate of the time left before the aircraft is hit, known as the Time-to-Go (TTG). This helps the pilot to find the best point in time for performing evasive actions. A drawback to this kind of MWSis that missiles may be very hard to detect due to smallRCSvalues. Another drawback is that missiles may be designed to follow the emitted radar radiation thus unfortunately converting theMWSinto a missile attraction system.
2.4.3 Identification Friend or Foe
In a complex battle scene with many military platforms, including aircraft, ships, and/or ground-based vehicles, it might be difficult for the pilot to tell friend from foe. To help this the vehicles may be equipped with transponders identifying themselves. An aircraft equipped with an Identification Friend or Foe System (IFF) can then detect the transponder signal and it will identify the transponding vehicle as ”friend” or ”foe”. Since not all aircraft are equipped with an IFF transponder, or a given transponder may not be turned on, the