IEEE Aerospace and Electronic Systems Magazine - September 2020 - 9

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product of the bandwidth B and the integration time T of
the correlator is the time-bandwidth product BT, which is
a particularly significant parameter for noise radars. The
benefit of using modulated signals is not only the decoupling of the pulse duration from the range resolution, but
also the fact that the peak power of the radar pulse can be
reduced by an amount equal to BT without compromising
the range resolution. This lowered transmit power in
many cases allows for the use of solid-state transmitters.
With simple noise-like signals, the ratio between the peak
of the compressed signal and the mean level of its range
sidelobes is also equal to BT. This ratio, called the peak
side lobe ratio (PSLR), is particularly important for continuous wave noise radars, where the sidelobes of large
returns extend over the whole range swathe.
NRT emissions are ideally homogeneously spread in
frequency and infinitely lasting over time. This ideal continuous white noise is only of theoretical interest as in
practice both B and T are limited. The main limitations
on the available bandwidth are set by frequency regulations. For example, X-band marine radars can only operate
in the band from 9300 to 9500 MHz and S-band air traffic
control radars have to stay within 2700-2900 MHz.
When the regulations are less restrictive, other practical
considerations, such as hardware limitations also need to
be taken into account when deciding how much bandwidth
to use.
There are three principal limitations on the integration time. The first source of limitation is the radar
scene, which changes over time, for example, a
target's position may change due to its motion. The
second limitation is the rate at which the radar must
SEPTEMBER 2020

deliver information to the operator. If the radar is scanning, the integration time is also limited by the time
for which it can dwell in a particular direction. The
third limitation is that echoes from moving targets
have typical coherence times of the order of 100 ms,
while the need to search over a wide angular sector
may also limit the antenna dwell time to less than
100 ms. With a typical bandwidth of 50 MHz as considered here, and an exemplary integration time of 20
ms, the processing gain is of the order of one million.
In consideration of the foregoing, band-limited finiteduration noise is a very useful radar waveform. It can be
treated as a nonrepetitive signal code known solely to the
radar, but unpredictable to any third party.
In this article, we present an overview of technical
concepts for NRT and demonstrate the recent developments. We begin with introducing the basics for NRT and
clear advantages of NRT over deterministic radars in the
"Introduction." In the section "NRT Waveform Design
Evolution," the characteristics of noise waveforms have
been discussed including a discussion on tailored noise
waveforms. Many of the key factors in verifying the performance of noise radars that make NRT a challenge have
been covered at an appropriate level in the section "Noise
Radar Characterization." This is followed by several
descriptions of NRT programs and historical NRT developments within certain technical organizations around the
world in "NRT Implementations Used in NATO Research
Group Trials." Finally, in "Conclusions and Outlook," we
provide our insight on NRT applications and trends, along
with a brief outlook for the future development path of
NRT.

IEEE A&E SYSTEMS MAGAZINE

IEEE Aerospace and Electronic Systems Magazine - September 2020

Table of Contents for the Digital Edition of IEEE Aerospace and Electronic Systems Magazine - September 2020