Page 180 - Fundamentals of Radar Signal Processing

CHAPTER 3

Pulsed Radar Data Acquisition

As has been seen, radar measures the spatial distribution of reflectivity in the
three-dimensional spherical coordinate system of range, azimuth angle, and
elevation angle. Pulsed radars do this by emitting a series of individual pulses
and recording the received voltage as a function of time, equivalent to range.
Modern pulsed radars use coherent receivers so that the measured voltage is
complex valued. They also record and process the data digitally. As with any

digital data acquisition system, the selection of sampling rates and quantization
strategies are crucial design decisions, affecting signal fidelity, resolution,
aliasing, and noise properties, as well as processor memory and computational
requirements.

3.1 Acquiring and Organizing Pulsed Radar Data

3.1.1 One Pulse: Fast Time
Suppose a radar transmits a single pulse of length τ seconds. The leading edge

of the pulse is emitted at time t = 0. As discussed in Chap. 2, the echo power at
the receiver due to clutter and targets will decay with range or time, typically at
–4
–1
rates between R and R , while the noise power generated within the receiver
will be constant. Figure 3.1 is a notional illustration of this behavior. Depending
on the goals of the particular radar mode of operation, the radar will measure

the received power over some interval in range, say from R to R . The interval
2
1
R – R is called the range swath or the range window R . The range to the
2
1
w
beginning of the range swath, R , may be influenced by a number of factors. For
1
example, for an airborne downlooking radar it might equal the altitude of the
radar, since no clutter echoes could occur at a shorter range. In a ground
imaging mode, it would likely be determined by the range to the nearest edge of
the antenna mainbeam. Similarly, the end of the range swath, R , might be set in
2
different radars by the far edge of the mainbeam on the ground, or by the

maximum expected detection range for targets of interest. Another constraint on
R is the unambiguous range, to be discussed shortly.
2