Introduction to Radar Basics - MATLAB Simulations for Radar Systems Design - page 58

Pulsed Radar

Appendix 1A

1A.1. Introduction

Pulsed radars transmit and receive a train of modulated pulses. Range is
extracted from the two-way time delay between a transmitted and received
pulse. Doppler measurements can be made in two ways. If accurate range mea-
surements are available between consecutive pulses, then Doppler frequency
can be extracted from the range rate . This approach works fine as
long as the range is not changing drastically over the interval

R ß = ∆ R ∆ t ⁄

∆ t

. Otherwise,

pulsed radars utilize a Doppler filter bank.

Pulsed radar waveforms can be completely defined by the following: (1)
carrier frequency which may vary depending on the design requirements and
radar mission; (2) pulsewidth, which is closely related to the bandwidth and
defines the range resolution; (3) modulation; and finally (4) the pulse repeti-
tion frequency. Different modulation techniques are usually utilized to enhance
the radar performance, or to add more capabilities to the radar that otherwise
would not have been possible. The PRF must be chosen to avoid Doppler and
range ambiguities as well as maximize the average transmitted power.

Radar systems employ low, medium, and high PRF schemes. Low PRF
waveforms can provide accurate, long, unambiguous range measurements, but
exert severe Doppler ambiguities. Medium PRF waveforms must resolve both
range and Doppler ambiguities; however, they provide adequate average trans-
mitted power as compared to low PRFs. High PRF waveforms can provide
superior average transmitted power and excellent clutter rejection capabilities.
Alternatively, high PRF waveforms are extremely ambiguous in range. Radar
systems utilizing high PRFs are often called Pulsed Doppler Radars (PDR).
Range and Doppler ambiguities for different PRFs are in Table 1A.1 .