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International contractor BAE Systems has revealed that it is proceeding to full-scale development of a collision warning system (CWS) for use on military fast jet aircraft. Designed to reduce the incidence of near misses and mid-air accidents, the new system is described as making use of secondary surveillance radar techniques, with the host aircraft carrying an interrogator that transmits Mode A and C interrogation signals.
Functionally, a received response is used to generate range and bearing data, with that for range being determined via the delay time between transmission and reception. Bearing is established using the monopulse technique and is derived by measuring either the phase or amplitude differential between the transmitted and received signal at spatially separated receiver antenna elements. Taking these values together with a calculated range rate, the CWS uses the Tau criteria (range divided by range rate) to assess the likelihood of an impending collision. A positive calculation activates a voice warning message that is outputted to the host aircraft's crew headsets.
An associated situational awareness unit can also be used to visually display target tracks together with their interrogator and height codes. BAE Systems further notes that the described equipment specifically addresses the problems associated with low level flight and utilises specially developed algorithms to minimise multipath effects, thereby reducing the number of false alarms generated. In more detail, the described CWS transmits on a spot frequency of 1030 MHz (with a 10 W peak output) and receives on a spot frequency of 1090 MHz. Range is given as being eight nautical miles, with the ability to handle closing speeds of better than 1000 knots. Azimuth and elevation coverages are given as ±60° and better than -10° to +20°. The system's bearing accuracy is noted as being ±15°.