CHAPTER 40 Advanced Radar Techniques
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Even though antenna size and average power may
be limited, detection sensitivity can be enhanced
considerably, by refining conventional radar features.
Among key possibilities are employing frequency diversity,
minimizing transmitted noise, widening the dynamic
range of the receivers, minimizing quantization noise,
and reducing the receiver noise figure.
Employing Frequency Diversity. As it closes on a
target, a radar bearing aircraft may slip into one of the
deep notches in the target’s RCS pattern and remain
there for some time. As a result, the target may not be
detected by the radar until it has closed to a much
shorter range than would be expected for the target’s
average RCS.
However, the locations of the notches vary with
the radio frequency of the radar signal illuminating the
target. The single-look probability of detection, there-
fore, may be substantially increased by changing the
operating frequency at the end of each coherent
integration period.
For best results, the frequencies should be separ-
ated by the bandwidth of a pulse whose length corres-
ponds to the size of the target.
The length of a pulse, you’ll recall, is roughly 1000 feet
per microsecond of pulse width. For a 100-foot target,
for instance, the corresponding pulse width,
τ, would
be 0.1 µs. The bandwidth of a pulse being roughly 1/
τ,
the optimum separation of frequencies for this partic-
ular target would be 1 / 0.1 µs = 10 MHz.
Minimizing Transmitter Noise. Inadvertent noise
modulation of a radar’s transmitted signal may produce
ground clutter strong enough to limit the detection of
weak signals. This clutter not only reduces detection
sensitivity against tail aspect targets but, being inher-
ently broadband, spreads over into the clutter-free
spectral region in which nose-aspect targets are
detected in high PRF operation.
Transmitter noise may be minimized by providing
the following:
• An exciter that produces spectrally pure signals
• A ripple-free power supply
• A low-noise transmitter
Whatever noise is generated in the transmitter may
largely be eliminated by adding a noise reduction loop
around it.
This loop detects any phase or amplitude variations in
the transmitter output and adjusts the phase and
amplitude of the input so as to reduce the variations
toward zero.
Providing Wide Dynamic Range. Another common
inadvertent source of clutter is saturation of the radar
receiver or A/D converters by strong clutter, as a re-
sult of insufficient dynamic range. Saturation gener-
ates modulation products which—like transmitter
noise—spread into otherwise clutter-free spectral
regions.
Saturation of the receiver may be avoided by
distributing the gain throughout the receiver chain with
successive steps of automatic gain control.
Minimizing Quantization Noise. Noise due to
quantization of the received signals may be avoided
by:
• Employing highly linear A/D converters
• Quantizing with a significant number of bits
• Employing high sampling rates
• Summing samples
Minimizing Receiver Noise Figure. Receiver noise
may be minimized by employing very low noise
preamplifiers (LNAs), minimizing all losses ahead of
them, and placing the LNAs as close as possible to
the radiating elements—as is done in active ESAs.
With advanced solid state devices, remarkably low
noise figures may be achieved.
INCREASING DETECTION SENSITIVITY
Through Conventional Design Refinements
Transmitter
RF IF DigitalA/DVideo
AGC AGC AGC AGC AGC
Radar Pulse
100 ft