Principles of Modern Radar - Volume 2 1891121537

12.10 Data Processor-Based EP 579The radar interprets the velocity of the jammer’s Doppler offset asv V,FT =− c2 f cf D,FT = v FTf jf c(12.27)In other words, the apparent range rate offset of the false target as observed by the Dopplertracker is different by a factor of f j /f c from the jammer’s intention. An error in the absolutevelocity of a false target or VGPO is not necessarily a problem for the jammer if the radardoes not compare the range and Doppler filter estimates. However, if the radar does performthe Ṙ − v comparison, the error may be detected.In contrast, the jammer’s range-rate offset will appear as intended in the radar’srange tracker because it is based on a time variation of the jammer coherent delay, andthe conversion of range offset to coherent delay does not require any measurement orassumption of radar parameters: as per Equation 12.1, t delay = 2R offset /c. The differencebetween the range and Doppler tracker velocity estimates now includes the discrepancybetween the jammer’s coherent delay and Doppler programs:v FT = v FT − v FTf jf c= v FTf c − f jf c= v FTf jf c(12.28)where f j is the difference between actual and assumed RF. Thus, a 10% error in thejammer’s assumption of the radar RF will produce a disagreement between the two trackfilters of 10% of the intended range-rate offset relative to the jammer platform.As an example, suppose that the jammer intends to impart an apparent 300-m/s differencein radial velocity between its own platform and the range-velocity false target,and the jammer assumes the radar RF to be 9 GHz. The Doppler offset it applies is then18,000 Hz. Suppose, however, that the radar RF is actually 10 GHz, and the jammer issimply assuming a midband frequency within an 8–10 GHz channel. In this case there isa 10% error in the assumed RF, and there would be an inconsistency in radial velocity ofapproximately 10%, or 30 m/s, between the range and Doppler track filters. This correspondsto a difference in Doppler of 2000 Hz. One would expect this to far exceed anymeasurement errors in the radar, as typical Doppler filter widths of pulse-Doppler radarsare less than half this amount.A radar with frequency agility can attempt to force the above condition against jammerswith poor real-time frequency measurement capability [6]. Many jammers are capableof measuring frequency to an accuracy of better than a few MHz on a single pulse.The processing logic needed to incorporate real-time frequency updates in the Doppleroffset used for VGPO is very straightforward (a simple multiply) and therefore the VGPOprogram can be updated with each change in radar RF. A radar with the ability to transmitsimultaneously on two or more frequencies, as described in Section 12.7.8, makes the taskof the jammer much more difficult. To avoid being detected by range-Doppler comparisonagainst such a radar, the jammer must apply an independent Doppler modulation toeach radar frequency, accomplished through channelization in the receiver or techniquegenerator.12.10.5 Track Filter Acceleration LimitThe radar range, Doppler, azimuth, and elevation track filters can be designed with accelerationlimits to reject EA false targets that exhibit kinematically unrealistic behavior[6,9]. It is in the interest of the radar to keep the track filter gains from being unnecessarily