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Experimental passive range and AOA system shows promise.

Tests conducted by the Hughes Radar Systems Group have produced promising results for a system which uses passive sensors to rapidly determine both the range and bearing to radar emitters. This technique is based upon prior Wright Laboratory projects and is being developed at Hughes on IR&D funding.

Accurate bearing (angle of arrival, or AOA) has long been possible using interferometry techniques wherein the wavefront phase difference seen by the various interferometry antennas located on the receiving platform is used to measure the AOA to the emitter, typically with 1|degree~ or better accuracy.

Accurate range could only be determined quickly using active systems such as radar or laser ranging, however. Passive systems to determine range to the emitter from a moving interferometry receiving platform were usually based on a triangulation principle. If one assumed that the emitter was fixed, range could be calculated if the precise location of the receiver (AOA) was known over a period of time. This technique suffered from limited accuracy (AOA data must be particularly precise), the extended time necessary for the receiver to collect and process a significant number of change-of-AOA bearings and the fact that the technique is dependent on optimal path geometry (the receiver must be moving across, not toward or away from the emitter).

Quickly and accurately determining the range to an emitter is a requirement for both offensive and defensive EW systems. This information is required to avoid, degrade or destroy the threat system with optimum effectiveness.

The Hughes system employs a two-element antenna array to generate a signal proportional to the phase rate of change (PRC) and hence the direction of travel of a wavefront under observation. Within a 5-sec observational frame, the loci of ground ranges of all RF emitters that could possibly generate the observed PRC is computed. This brackets the minimum and maximum range of the emitter to be located. When combined with AOA information, the loci uniquely and passively define the bearing and range of the target emitter.

The direction-finding and location system (DFLS) is housed in a wing-tip antenna, a fuselage-mounted interferometry pod, a hardware "box" and a computer. RF switches are used to sample the signal received by these antennas. A stable YIG oscillator is used to down convert the RF signal to baseband for phase discrimination. Computational operations, as well as input and display functions, are under the control of an HP 386 processor. The system also processes accelerometer data, yaw rate information and GPS location data.

The recent tests used a ground-based Furino Marine Radar operating at 9.4 GHz with the DFLS system installed aboard the Hughes corporate Sabreliner aircraft. A racetrack pattern of about 1 x 5 nautical mi was flown at a 230-kt indicated airspeed and an altitude of 3,700 ft.

Antenna spacing of 3.5 in. for AOA and 25 ft for phase rate-of-change was used. Actual range to the emitter was 3 to 5 nautical mi and the range determined by the system correlated to the actual range within 12 sec of initial acquisition of the radar signal by the DFLS. The percentage range error dropped below 10% after 8 sec and improved to better than 2% in as little as 20 sec after acquisition.

Hughes plans to conduct a more ambitious test program at a 20,000-ft altitude on a 20-nautical-mi racetrack pattern against airborne emitters in 1993.
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Title Annotation:angle of arrival passive sensors
Author:Gershanoff, H.
Publication:Journal of Electronic Defense
Date:Dec 1, 1992
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