Page 181 - Fundamentals of Radar Signal Processing

FIGURE 3.1 Signal level versus range, and range swath or window R .
w

In a monostatic radar, the radar receiver must be isolated from the antenna
during pulse transmission so as to avoid damaging the sensitive receiver circuits
with leakage of the high-power transmitted signal. Consequently, the receiver is

off for the first τ seconds after pulse transmission begins, so the minimum range
from which a full echo can be received is R = cτ/2 m. In practice, R may be
min
min
somewhat larger to allow for the finite switching time needed to reconnect the
receiver to the antenna after pulse transmission, and in some environments to
protect against particularly strong near-in clutter. Any clutter or target scatterer

closer to the radar than R will produce an echo that arrives in part during the
min
initial τ seconds after transmission. That portion of the pulse echo will therefore
not be seen at the receiver. A pulse that is not received in whole or part because
it arrives during the time the receiver is isolated is said to be eclipsed.
The received signal is demodulated using a coherent receiver as described
in Chap. 1. The resulting complex-valued baseband signal is sampled at a high
rate, typically in the range of hundreds of kilohertz to a few tens of megahertz

and sometimes higher. To implement the desired range swath, sampling begins
at time t = 2R /c after pulse transmission and ends at time t + τ = 2R /c + τ.
1
1
2
2
The additional τ seconds at the end of the sampling period are needed to capture
the end of the echo of the pulse from the far edge of the swath. The resulting
samples are stored in a digital memory as shown in Fig. 3.2a, where each cube
represents a single baseband sample. A set of L samples from a single

transmitted pulse are referred to as range bins, range gates, range cells, or
fast-time samples. The phases of these complex samples are the fast-time
1
phase history of the pulse echo data.