Page 368 - Fundamentals of Radar Signal Processing

FIGURE 5.15 Conversion of the fast-time/slow-time data matrix to a range-
Doppler matrix.

Consider the notional pulse Doppler spectrum for one range bin shown in

Fig. 5.16. The DTFT of the data is shown in a fashion similar to earlier figures.
The white dots represent the samples of the DTFT computed by the DFT; these
are the only actually available data. Assuming that the clutter has been centered
at zero Doppler, spectral samples at or near zero frequency will be dominated
by the strong clutter signal even though noise is also present. Spectral samples
in the clear region have only thermal noise to interfere with signal detection.

Each clear-region spectrum sample can be individually compared to a noise-
based threshold to determine whether the signal at that range bin and Doppler
frequency appears to be noise only or noise plus a target. If the sample crosses
the threshold it not only indicates the presence of a target in that range bin but
also its approximate velocity, since the Doppler frequency bin is known. The
samples that are clutter-dominated are often simply discarded on the grounds
that the SIR will be too low for successful detection. However, other systems

use a technique called clutter mapping, discussed in Sec. 5.6.1, to attempt
detection of strong targets in the clutter region using a clutter-based threshold.