Page 89 - Fundamentals of Radar Signal Processing

CHAPTER 2

Signal Models

2.1 Components of a Radar Signal
While a radar transmits a controlled, well-defined signal, the signal measured at
the receiver output in response is the superposition of several major

components, none of them entirely under the control of the designer. The major
components are the target, clutter, noise , and, in some cases, jamming. These
signals are sometimes subdivided further. For instance, clutter can be separated
into ground clutter and weather clutter (such as rain), while jamming can be
separated into active jamming (noise transmitters) and passive jamming (such as
chaff clouds). Signal processing is applied to this composite signal; the goal is
to extract useful information regarding the presence of targets and their

characteristics, or to form a radar image. Noise and jamming are interference
signals; they degrade the ability to detect targets and measure their position and
velocity. Clutter may be interference in some cases, such as when detecting
aircraft, or may be the desired signal itself, as with a ground imaging radar. The
effectiveness of the signal processing is measured by the improvement it
provides in the various figures of merit, such as detection probability, signal-

to-interference ratio (SIR), or angle accuracy.
It was shown in Chap. 1 that conventional pulsed radars transmit
narrowband, bandpass signals. Transmitted energy is maximized by restricting
amplitude modulation to on-off pulsing; phase modulation is used to expand the
instantaneous bandwidth when needed to improve resolution. Thus, an
individual transmitted radar pulse can be written as

(2.1)

where a(t) is the constant amplitude pulse envelope, F is the radar carrier
t
frequency, and θ(t) may be a constant or may represent phase modulation of the
pulse. It will usually be assumed that a(t) is an ideal, square pulse envelope of

amplitude A and duration τ seconds. The instantaneous power of this signal is
2
just P = A /2. The signal at the receiver output will be a combination of echoes
s
of from targets and clutter, noise, and possibly jamming.
Because the target and clutter components are delayed echoes of the
transmitted pulse, they are also narrowband signals, although their amplitude

and phase modulation will in general be altered, e.g., by propagation loss and
Doppler shift. Receiver noise appears as an additive random signal. Thus, the