Page 412 - Fundamentals of Radar Signal Processing

centered at its unambiguous range of 17.3 km, but the velocity has aliased from

its actual value of 93.4 m/s to an ambiguous velocity of 3.4 m/s (93.4 – 3 × 30).
The SLC is heavily aliased in velocity but fades with range so that most targets
do not compete with it. If MTI filtering is applied to suppress the MLC there
will be blind speeds every 30 m/s in the affected range bins.
Part c of the figure is the medium PRF case. The MLC is ambiguous in both
range and Doppler, having aliased to a range of 2.3 km (17.3 – 15) and a

velocity of –56.6 m/s (93.4 – 150). Notice that the MLC and its sidelobes now
wrap around in the range dimension and that the SLC wraps around in Doppler.
SLC is now present at essentially all ranges and Dopplers, though in varying
amounts and patterns in different range cells. The SLC also wraps in range but
this is less evident.
Part d of the figure is the high PRF case. The MLC again wraps to the
ambiguous range of 2.3 km (17.3 – 3 × 5) but is spread fairly uniformly

throughout the short 5 km unambiguous range. It is located at its unambiguous
velocity of 93.4 m/s. The narrow spread of the MLC in velocity allows it to be
filtered out with a relatively narrow MTI or other notch filter with little risk of
filtering out moving targets. The clutter out to 75 km has folded over 15 times to
“fit” into the 5 km unambiguous range at this PRF. There is now significant and
relatively constant SLC at all ranges, though the AL and other near-in clutter is

still visible beginning at just over 2 km. On the other hand, the radar is now
unambiguous in Doppler and the full SLC spread in velocity of ±134.1 m/s can
be seen. In addition, there is now a clear region in the Doppler spectrum for
velocities having a magnitude between 134.1 and 225 m/s that was not present
in the other figures, enabling noise-limited detection of targets at these high
relative velocities.
Table 5.4 summarizes the major strengths and weaknesses of low, medium,

and high PRF operation, especially from the viewpoint of an airborne radar.
Broadly speaking, low PRF modes are very effective for ranging, mapping, and
imaging modes, but poor at detection of moving targets due to the lack of a
sizable clear region. High PRF operation is complementary to low PRF
operation in both its strengths and weaknesses. High PRF modes are good for

detection of high-Doppler shift targets (e.g., rapidly closing aircraft or missiles)
in high clutter due to the large clear region, but poor at detection of low-
Doppler targets (slow-moving closing targets or opening targets) due to high
sidelobe clutter and little or no range gating capability. Medium PRF operation
is a compromise that retains most of the strengths of each without the inherited
weaknesses becoming too severe. In-depth discussion of the properties and
processing for all three regimes is given in Alabaster (2012), Morris and
Harkness (1996), and Stimson (1998).