Application of Transversal Equalizer to Radar Receiver with Emphasis on Doppler Sensitivity

There is a basic constraint imposed on radar measurements when the transmitted signal is a simple, CW pulse. As a result, the radardesigner may wish to use a more complex signal and thereby extract more information from an echoing object. Unfortunately, the processed returns of the sophisticated waveforms usually consist of a peak response and small adjacent responses (called "time sidelobes") which degrade the resolving capability of the radar. One means of reducing the sidelobes is to use a transversal equalizer in the radar receiver. A mathematical model of the equalizer is presented in this article and the sensitivities of an equalized system to uncompensated Doppler frequency shifts are computed. It is shown that the Doppler frequency band over which the equalizer is effective decreases as (1) the signal bandwidth decreases, (2) the distances of the large sidelobes from the main pulse increase, and (3) the magnitudes of the uncompensated sidelobes increase. Testing the equalizer in a highly sensitive, tracking radar indicates that sidelobes 40db below the main response can be obtained for a fixed Doppler frequency shift. Experimental data illustrating the stability of an equalized system in the presence of a Doppler shift show that, for a typical pulse-compression system, the equalizer utility is generally confined to situations where the frequency shift is within ± 2 percent of the signal bandwidth. Accepted for the Air Force Franklin C. Hudson Chief, Lincoln Laboratory Office