Page 366 - Fundamentals of Radar Signal Processing

FIGURE 5.14 The Tikhonov PDF for phase.

Other sources of limitation due to radar system instabilities include
instability in transmitter or oscillator frequencies, transmitter phase drift,
coherent oscillator locking errors, PRI jitter, pulse width jitter, and quantization
noise. Simple formulas can be developed to bound the achievable clutter
attenuation due to each of these error sources, as well as some others not
mentioned. Still another source is PRI stagger. When used in a range-ambiguous
scenario, PRI stagger results in clutter from different distant ranges aliasing to

the same near ranges on different PRIs, so that the clutter becomes less
stationary and the clutter attenuation is degraded. Additional information and
analysis on these and other issues is available in Shrader and Gregers-Hansen
(2008), Nathanson (1991), and Schleher (2010).
External to the radar, the chief factor limiting MTI improvement factor is

simply the width of the clutter spectrum itself. Wider spectra put more clutter
energy outside of the MTI filter null, so that less of the clutter energy is filtered
out. This effect is evident in Eq. (5.47) and was illustrated numerically in Table
5.1. The effective clutter spectrum width can be increased by radar system
instabilities or by measurement geometry and dynamics. For instance, a scanning
antenna adds some amplitude modulation due to antenna pattern weighting to the
clutter return. The power spectrum of the measured clutter is then the

convolution in the frequency domain of the actual clutter power spectrum and
the squared magnitude of the Fourier transform of the amplitude modulation
caused by the antenna scanning. This convolution increases the observed
spectral width somewhat. In some cases, the clutter power spectrum may not be
centered on zero Doppler shift. A good example is rain clutter. Moving weather
systems will have a nonzero average Doppler representing the rate at which the

rain cell is approaching or receding from the radar system. Unless this average