15.1 INTRODUCTION [1 6]

As mentioned in Chapter 2, the angle of arrival (AOA) is the most valuable information that can be obtained from an enemy radar because the radar cannot change its position drastically in a very short time frame (i.e., a few milliseconds). Unfortunately, the AOA information is also the most difficult information to obtain. It requires several antennas with receivers. The two common approaches to measure AOA are based on amplitude and phase comparisons. Another approach is to use the Doppler frequency shift generated by the aircraft movement. However, this approach is closely related to the phase measurement system. If the requirement is to measure AOA on simultaneous signals, the problem becomes even more complicated because receivers with multiple signal capability are needed.

In an amplitude comparison system, the amplitude of all the receivers must be matched from the antennas through the outputs of the receivers. This approach usually generates an AOA resolution of 15 deg, which is less than the desired value of 1 deg required in modern electronic warfare (EW) applications. In a phase measurement system, the phase of all the receivers must be matched from the antennas through the outputs of the receivers, which is a very difficult task in receiver design. The phase comparison system usually can generate AOA resolution of 1 deg, which satisfies the modern EW requirement. The accuracy obtained from an amplitude comparison system is not theoretically limited. In most of the amplitude comparison systems, the AOA coverage is rather wide;