Radar Design Principles: Signal Processing and the Environment, Second Edition

Chapter 10: Environmental Limitations of CW Radars

Overview

The continuous wave (CW) radar is frequently used for detection and tracking of moving targets. [1] In its simplest form a single sinusoid is transmitted, and the received signals are mixed with the transmitted carrier frequency. The existence of moving targets is determined from the beat note or Doppler frequency shift f d.

(10.1)

where v is the radial velocity difference between the target and the radar (positive for closing geometries) and ? is the carrier wavelength. The advantage of this technique is its simplicity.

As with virtually all radar waveforms, the angle of arrival of the target echoes can be determined with multiple receive apertures or with monopulse receivers. On the other hand, there is no target-range determination or resolution except with special geometries that occur with lunar or planetary observation radars. If range resolution is required, the transmit waveform must be modulated. Frequency modulation is discussed in Sec. 10.2 and Chap. 13, and binary-phase modulation is discussed in Chap. 12.

Since the CW receiver responds to echoes from all ranges, it is generally necessary to separate the leakage or spillover from the transmitter and signals from close-in clutter. This is usually accomplished by filtering out received signals at the carrier frequency and at the Doppler frequency of the clutter. An alternate technique is to pulse the transmitter at a rate higher than twice the expected Doppler frequency. This technique is called interrupted CW or ICW to distinguish it from pulse...