Pulsed Doppler Radar

A pulsed Doppler radar obtains target range and velocity information by coherently processing echoes from a sequence of transmitted pulses. Because of the finite duration and bandwidth of the transmitted signal, it is difficult to resolve targets from clutter having similar velocity and range. The difficulty arises both because of the finite width of the receiver mainlobe and because of the presence of sidelobes. Windowing is the standard technique for reducing sidelobes, but windowing always increases the width of the mainlobe. In the following results of computer simulations are presented which demonstrates that our technique improves velocity resolution at the same time that it reduces mainlobe width.

Figures 1 through 6 on the following pages show ambiguity surfaces for a pulse Doppler radar transmitting an equispaced sequence of continuous wave (CW) pulses having the same duration and frequency. In particular, the figures compare the ambiguity surfaces obtained by velocity processing with the standard Discrete Fourier Transform (DFT), with the windowed DFT algorithm, and with our algorithm.

The ambiguity surface shown is the system response to a point scatterer of zero velocity at 90 km range in the absence of any noise or clutter. For several different processing systems we will present both the 3-dimensional ambiguity surface, which shows system response versus estimated range and estimated velocity, and the 2-dimensional cut, which shows system response versus estimated velocity at the actual scatterer range (i.e. at the range of maximum response).

In the 3-dimensional ambiguity surface plots, the x-axis represents estimated velocity and the y-axis (into the page) represents estimated range. Minimum range is at the front of the plot. Zero velocity is at the center of the x-axis. The z-axis represents system response plotted in dB.

The target range is at the mid-point of the y-axis. For ranges smaller or larger than those plotted the system response is zero (minus infinity in dB), because the echo has either not yet reached the receiver or has completed its return.

Each 2-dimensional plot is a cut through the ambiguity surface at the range of the target. That is, the cuts lie in the (x-z) plane perpendicular to the y-axis at the range of maximum system response, thus showing the velocity response at the target range.

The z-axis in all plots extends from 0dB at the top to -100dB at the bottom of the plot.