JED - October 2010 - (Page 96)
EW Against Modern Radars – Part 11
Pulse Doppler Radar
By Dave Adamy
Electronic Protection Features of Pulse Doppler Radars
A Pulse Doppler (PD) radar has inherent Electronic Protection (EP) features, including: • It expects its return in a narrow frequency range, so it can discriminate against non-coherent jamming. • It can see spurious outputs from jammers. • It can see frequency spreading from Chaff. • It can see separating targets. • It can correlate range rate and Doppler shift. PD radars are coherent, because each pulse is a sample of the same RF signal, as shown in Figure 1. Thus, both the time of arrival and Doppler shift of received signals can be measured. The time of arrival allows determination of range to the target and the Doppler shift is caused by the radial velocity of the target relative to the radar. As will be discussed later, there are some significant ambiguity issues that must be overcome by PD radar processing. The processor in a PD radar forms a matrix of range vs. velocity as shown in Figure 2. The range cells show the time of arrival of received pulses relative to the transmitted pulse, and each cell is one “range resolution” deep. The time resolution (or the depth of a range cell) is half of the pulse width. This gives the PD radar a range resolution of: Range cell depth = (pulse width/2) x speed of light These range cells are contiguous during the whole time between pulses. The velocity cells are fed by a bank of channelized filters, or channelization by fast Fourier transform processing. The width of the velocity (i.e. Doppler frequency) channels is the
The Journal of Electronic Defense | October 2010
Figure 2: Pulse-Doppler radar processing allows generation of a range vs. return frequency matrix.
bandwidth of each filter. The inverse of the filter bandwidth is the coherent processing interval (CPI), which is the time over which the radar processes the signal. Note that in a search radar, the CPI can be as long as the time the radar’s antenna is illuminating the target. Thus, the frequency channels can be very narrow. For example, if the radar beam illuminates the target for 20 msec, the filters could be 50 Hz wide. The number of pulses that are integrated by the radar determines its processing gain (above the noise level). The processing gain is: Processing Gain (in dB) is 10 log (CPI x PRF) or 10 log (PRF / filter BW)
Consider the use of range-gate-pulloff (RGPO) deceptive jamming (discussed in the January 2010 “EW 101”). Figure 3 shows the true return pulse and the false pulse generated by the jammer. In a conventional radar, the processor has an early and a late gate (rather than the contiguous range cells of the PD radar).
Figure 1: A Pulse-Doppler radar is coherent and uses complex processing to deal with ambiguities.
Table of Contents for the Digital Edition of JED - October 2010
JED - October 2010
The View From Here
From the Presidents
What’s Next in IED Jammers?
Electronic Warfare in Today’s Surface Navy
ELINT Receivers Tackle Dense Signal Environments
A Structural View of EM Spectrum Warfare
AOC 2010 Award Winners
JED Sales Offices
Index of Advertisers
JED Quick Look
JED - October 2010