IEEE Aerospace and Electronic Systems Magazine - October 2020 - 24

Principles and Applications of Random FM Radar Waveform Design
experimentally illustrate the efficacy and tradeoffs involved with applying random FM to a stationary MTI mode. Similar behavior is expected
for airborne/space-based MTI, although the coupling between slow-time (Doppler) and fast-time
(range) discussed in the previous section may
lead to further interactions since a moving platform likewise induces coupling between Doppler
and the spatial domain (hence the use of STAP).
For SAR, where both waveform BT and the
number of pulses in the CPI are generally much
higher than for MTI, even greater freedom may
be possible because sensitivity-limiting RSM
Figure 5.
may instead become a benefit. Specifically, wellAutocorrelation response of individual PRO-FM and complementary (Compdesigned waveforms can achieve a peak sidelobe
FM) waveforms and combinations of subsets of 5 [7].
level on the order of À20 log10 (BT) dB [15] and
slow-time processing of M random waveforms
elicits an additional 10 log10 (M) dB in sidelobe
Fundamentally, the root cause of RSM is that we are
reduction because the sidelobes add incoherently [8].
attempting to perform standard radar receive processWhen these values are large, high dynamic range (if sideing, which involves range-domain pulse compression
lobe limited) can be achieved without the need for wellfollowed by processing in slow-time across the pulses in
known tapering approaches that impose degradation in
a CPI (or sweeps/segments if FMCW). Such an
both resolution and SNR. A preliminary simulation examapproach makes sense when the same waveform is
ple of SAR using FM noise waveforms was presented in
repeated but is actually not consistent with the emission
[7], with experimental effects still to be explored. Given
structure when employing nonrepeating waveforms. In
the high transmit power afforded by the FM structure and
the nonrepeating context, it would be more appropriate
the separability enabled by individual uniqueness, waveto perform joint fast-time/slow-time receive processing.
forms of this type could also prove useful for differentiatDifferent approaches to this manner of combined procing different range ambiguities that otherwise become
essing were considered in [20]-[22], with a subsesuperimposed on receive, particularly for space-based
quently reduced complexity method presented in [23]
applications.
that addresses the attendant high computational cost
Finally, polarimetric scattering is useful for many
that accompanies any form of joint domain processing
remote sensing applications of SAR and could likewise
(e.g., like space-time adaptive processing (STAP) for
aid in target discrimination for MTI. Like standard noise
clutter cancellation from a moving platform).
radar, the separability of random FM waveforms permits
Finally, at higher transmit powers, for which random
simultaneous dual-polarized operation (i.e., transmit on
FM waveforms are well suited, one can also expect to
both orthogonally polarized channels concurrently),
realize extended (if not completely eliminated) range
thereby avoiding the timeline tradeoff required to alternate
ambiguity to the degree that the waveforms are suffipolarizations. In [24], a simultaneous dual-polarized MTI
ciently separable on receive. Of course, this prospective
mode was experimentally demonstrated that provides
benefit also brings with it a more complicated arrangement
coherent versions of the HH, HV, VH, and VV channels,
for multiple time-around clutter, which may necessitate
which may be beneficial for subsequent polarimetric procmore sophisticated joint-domain methods for cancellation
essing. Of course, the fundamental limitation in this consuch as those noted above.
text is the achievable isolation (or purity) between the
dual-polarized channels due to their close proximity
(roughly À10 to À30 dB is typical, although further
APPLICATIONS OF RANDOM FM WAVEFORMS
enhancement is possible).

MTI AND SAR
Some of the applications of FM noise waveforms are the
same as those that have been examined for standard noise
radar, such as MTI and SAR. However, the FM property
permits operation at greater ranges through the use of efficient power amplifiers (class C or higher) operated in saturation. Figures 2-4 (see [5], [7], [9]-[11], [24])
24

COGNITIVE RADAR SPECTRUM SHARING
Waveform design to facilitate transmit spectral notching,
an ongoing topic of research for some time, has become
increasingly important within the context of cognitive
sensing as one of the means to address the demands for

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OCTOBER 2020
IEEE Aerospace and Electronic Systems Magazine - October 2020

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