Additional Topics. EW and Navigation Systems

EW and Navigation Systems. Before WW2, specialized direction-finding systems were developed for navigation purposes. From measurements of the angles to radio stations or beacons at known locations, position estimates could be computed. Although there were limitations on the achievable accuracy, this capability was extremely important, particularly at night and in bad weather. During WW2, more sophisticated systems were developed and deployed. Examples include Knickebein, X-Gerat, Y-Gerat, Decca Navigator, GEE, G-H, and Oboe.

Various efforts were made to jam the signals associated with these systems, particularly those used for bombing navigation. Luftwaffe attempts to use the Knickebein, X- Great, and Y-Gerat navigation systems to guide bombers to targets in the U.K. were successfully countered by jamming, although a series of damaging raids was conducted using the X-Gerat system before effective jamming techniques were devised (5). German attempts to jam allied systems, such as GEE and Oboe, were generally less successful.

For example, by the time successful jamming was initiated against Oboe signals at 200 MHz, the Mark III version had moved to 3 GHz. At this frequency, the technical capabilities of the Germans were inadequate for the implementation of effective countermeasures.

In addition, both sides made efforts to interfere with enemy radio beacons, sometimes with the result that aircraft got lost or were even captured after landing in unfriendly territory.

After WW2, various navigation systems were developed and deployed. More recently, the global positioning system (GPS) has become very important, particularly in Western countries, because of the availability of worldwide coverage and the high accuracy that can be achieved.

This availability has led to the widespread use of GPS for guiding precision weapons and defining target locations. The military importance of GPS has motivated the development and marketing of GPS jammers. At the same time, recognition of the potential impact of GPS jamming has resulted in serious efforts to develop and implement anti-jam features in military GPSs (31).

EW and IFF Systems. Identification friend foe (IFF) systems are used to provide a means of quickly and positively identifying friendly aircraft. When an unknown aircraft is observed, the IFF system transmits a specially coded signal and looks for the transmission of an appropriate signal in response from the IFF system in the unknown aircraft.

After early IFF systems were deployed in British bombers during WW2, the Germans discovered that the bombers could be tracked by transmitting signals to trigger their IFF systems and observing the IFF signals transmitted in response. Significant losses of aircraft resulted until it was realized that the IFF signals were being exploited, and the systems were removed from the aircraft (5). Since then, significant efforts have been made to reduce the vulnerability of modern IFF systems to EW.

Countermeasures Against IR Sensors. Passive infrared (IR) sensors have important military applications (32). Antiaircraft missiles using IR guidance systems have proven to be very effective in the absence of effective countermeasures, particularly for low-altitude air defense. Other important applications include ground-to- air and air-to-ground target acquisition, fire control, and night vision. In ground combat, the use of IR sensor technology has greatly increased the effectiveness of operations at night and under conditions of bad weather and haze.

The usefulness of IR sensors has been enhanced progressively by technical advances in IR detectors and the processing of their outputs. IR sensors have been evolved to operate in both the long-wave infrared and mid-wave infrared bands. These dual-band sensors can provide robust performance over a wide range of environmental conditions.

The importance of IR sensors has motivated the expenditure of considerable effort on the development of technology and techniques designed to reduce the effectiveness of IR sensors and their associated weapon systems. This work is very comprehensive and includes modeling and experimental measurements of the IR radiation emitted by platforms, such as ships and aircraft, and the behavior of threat IR sensors.

Flares have been widely used as decoys to distract the IR sensor-based missile guidance systems for the protection of aircraft. The use of flares is often combined with evasive action to ensure that the missile-guidance system continues to track the flare and that the missile’s path toward the flare does not take it near the aircraft. Infrared counter measure (IRCM) systems generate an IR signature whose power is modulated in a way that is intended to confuse the tracking system associated with typical IR sensor-based guidance systems. Directional infrared counter measures systems extend the IRCM concept by directing the modulated IR energy toward the threat sensor. Another idea is to use a laser to blind the IR sensor.

IR deception techniques for aircraft have achieved significant successes against more basic IR sensors. However, the development of increasingly sophisticated IR sensors has necessitated continued work on the development of IR countermeasures.

Improvised IR deception measures have been used with some success to simulate ground targets.

The reduction of IR signatures associated with platforms, such as surface ships and aircraft, can significantly improve their survivability. Various measures have been used:

- Cooling visible exhaust duct metal surfaces with air or water;
- Shrouding visible exhaust duct metal surfaces;

- Cooling engine exhaust plumes by mixing them with cool ambient air;
- Cooling exposed surfaces heated by the sun with water;

- Coating exposed surfaces with low-emittance materials;
- Covering ground-based assets with IR camouflage netting.