Closed loop countermeasure to passive direction finding and location of radar transmitters using Doppler techniques

DETAILED DESCRIPTION In meeting this and other objects the invention makes use of the fact that the change in the Doppler shift the observer is attempting to measure is only, at the most, several hundred Hertz if RF carrier Doppler is being used, and fractions of a Hertz if time, or equivalently, PRF Doppler techniques are employed.
Therefore the frequency changes in the radar required to prevent the passive extraction of correct Doppler changes are small compared to the RF and PRF frequencies the radar employs.
Thus modulating these frequencies represents only a small perturbation to the normal operating frequencies used by the radar, and need not alter the operational use of the radar, or its performance if the perturbation is correctly done.
To create these perturbations, this invention varies a reference oscillator frequency in a manner that creates false passive Doppler information at the intercept receiver.
The properties of the perturbing signal, and its effect, are derived as follows.
The signal parameter measured in passive frequency Doppler location is ##EQU1## while for TOA passive Doppler the signal measured ultimately is ##EQU2## Hence time and frequency Doppler changes involve an essentially reciprocal relation, and so only the effect on Equation (1a) arising from the proposed perturbations needs to be discussed.
But although the following description focuses on f.
sub.
o, it is to be understood that the same perturbations are applied to f.
sub.
prf in the most desirable implementation of this invention.
It is important to note the radar requires reference oscillator coherence only over milliseconds, whereas Doppler passive location methods need signal coherence over seconds.
Therefore the RF or PRF reference oscillator frequency can be changed several times in the interval needed to direction find (DF) or locate the emitter in range without violating the radar coherence requirement.
The passive Doppler DF or location algorithm expects to see a signal of the form of Equation (1) for all its measurements