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Transactions on Networks and Communications - Vol. 9, No. 4
Publication Date: August, 25, 2021
DOI: 10.14738/tnc.94.10512.
Neelakanta, P., & Groff, D. D. (2021). Assessment of RCS-specific SNR and Loglikelihood Function in Detecting Low-observable
Targets and Drones Illuminated by a Low Probability of Intercept Radar Operating in Littoral Regions. Transactions on Networks
and Communicaitons, 9(4). 1-22.
Services for Science and Education – United Kingdom
Assessment of RCS-specific SNR and Loglikelihood Function in
Detecting Low-observable Targets and Drones Illuminated by a
Low Probability of Intercept Radar Operating in Littoral Regions
Perambur Neelakanta
Department of Electrical Engineering
Florida Atlantic University, Boca Raton, Fl. 33431
Dolores De Groff
Department of Electrical Engineering
Florida Atlantic University, Boca Raton, Fl. 33431
ABSTRACT
The objective of this study is to deduce signal-to-noise ratio (SNR) based
loglikelihood function involved in detecting low-observable targets (LoTs)
including drones illuminated by a low probability of intercept (LPI) radar operating
in littoral regions. Detecting obscure targets and drones and tracking them in near- shore ambient require ascertaining signal-related track-scores determined as a
function of radar cross section (RCS) of the target. The stochastic aspects of the RCS
depend on non-kinetic features of radar echoes due to target-specific (geometry
and material) characteristics; as well as, the associated radar signals signify
randomly-implied, kinetic signatures inasmuch as, the spatial aspects of the targets
could fluctuate significantly as a result of random aspect-angle variations caused by
self-maneuvering and/or by remote manipulations (as in drones). Hence, the
resulting mean RCS value would decide the SNR and loglikelihood ratio (LR) of
radar signals gathered from the echoes and relevant track-scores decide the
performance capabilities of the radar. A specific study proposed here thereof refers
to developing computationally-tractable algorithm(s) towards detecting and
tracking by an LPI radar, the hostile LoTs and/or drones flying at low altitudes over
the sea (at a given range, R) in littoral regions. Estimation of relevant detection- theoretic parameters and deciding track-scores in terms of maximum likelihood
(ML) estimates are presented and discussed.
Key Words: Low-observable/Stealth Target, Radar Cross-section (RCS), Low Probability
of Interception (LPI) Radar, Ultrawideband (UWB) Radar, Loglikelihood Function,
Maximum Likelihood (ML) Estimation
INTRODUCTION
The study proposed here refers to modeling the performance details of radars capable of
detecting low-observable targets (LoTs) and drones. Concomitantly the test radar is presumed
to possess the adjunct efficacy of low-probability of interception (LPI) by hostile
reconnaissance systems. In all, relevant modeling being addressed refers to an LPI radar system
commissioned to detect effectively, the low-observable targets (LoTs) as well as, they are
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Transactions on Networks and Communications (TNC) Vol. 9, Issue 4, August-2021
Services for Science and Education – United Kingdom
designed to counter serious threats from hostile electronic attacks with the ability “to see and
not to be seen” [1].
In addition, also considered in the proposed modeling is that the operational ambient of test
LPI radars refers to near-shore regions posing significant sea-clutter background; and, the
associated target echoes thereof are presumably submerged in the "diffused" scattering of
radar-sourced electromagnetic (EM) waves from the sea/land surface posing a unique class of
clutter prevailing in near-shore/littoral regions.
Typically, the LPI radars being considered do not easily lend themselves to be detected by
hostile electronic reconnaissance systems [2]. Such “quiet radars” can, however be designed to
possess effective performance capabilities characteristics in tracking LoTs with a high
probability of detection (Pd) even under adverse clutter environment.
This paper is organized as follows: Described in the following section is a typical operational
ambient of LPI radars in near-shore regions. In the subsequent section, the LPI radar related
signal characteristics and associated radar cross section (RCS) features of fluctuating LoTs
being tracked are described. Relevant statistical aspects of underlying detection-theoretics are
identified and described subsequently along with pertinent analytical details on signal-to-noise
ratio (SNR) and loglikelihood function involved; and, related analytical considerations on
maximum likelihood (ML) estimations are furnished along with computational assessment of
track-score details.
NEAR-SHORE DEPLOYMENT OF LPI RADAR
Illustrated in Figure 1 is an LPI radar that views a low-altitude flying LoT (or stealth target) at
low-grazing angles in a littoral, near sea-shore region. The LoT being tracked poses fluctuating
RCS due to its kinematic states of varying aspect angles. Further, its non-kinematic details of
RCS include stealth attributes due to geometry, as well as, low observance deliberations
rendered via EM energy absorption by target-surface and/or by active (deceptive) on-board
jamming. In modern contexts, drones (unmanned aerial vehicles or UAVs) and high-altitude,
long-endurance (HALE) stealth drone are also deployed in warfronts causing perceivable
threats [3] [4]. Such drones denote targets of small size and low RCS. Further, in the design of
the passive or active systems dedicated for drone detection, the RCS value of such obscure
potential targets is not known a priori. [5]-[7]
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Neelakanta, P., & Groff, D. D. (2021). Assessment of RCS-specific SNR and Loglikelihood Function in Detecting Low-observable Targets and Drones
Illuminated by a Low Probability of Intercept Radar Operating in Littoral Regions. Transactions on Networks and Communicaitons, 9(4). 1-22.
URL: http://dx.doi.org/10.14738/tnc.94.10512
Figure 1. Conceived operational ambient of an LPI radar viewing a low-altitude flying LoT (or
stealth target) at low-grazing angles in the vicinity of littoral, near sea-shore regions. The LoT
being tracked poses fluctuating RCS due to the kinetics of its varying aspect angles; further, the
shape/geometry of the hostile target under surveillance could possess low observance features
improvised via radar absorbing materials (RAMs) on the target-surface and/or by deploying
active (deceptive) on-board jamming.
Illumination of the LoT being tracked results in echo signatures at the LPI radar (mono- or bi- static version) constituted by specularly-reflected fluctuating echoes as well as, signals due to
diffused scattering of EM waves from statistically-rough sea surface. In summary, the echo
signature being received and processed at the LPI radar system depicts a random signal
constituted by: (i) fluctuating target scattering details plus an interference pattern due to
multipath echo-reflections modulated by stochastic attributes of the sea-scatter (from the
clutter-cells).
Further, the system considerations of LPI radar deployed towards tracking LoTs can be briefly
stated as follows: The LPI radars are intended to survive countermeasures envisaged by the
hostile systems to detect the presence of radars (especially in battlefield ambient) via electronic
reconnaissance techniques designed to intercept EM/RF emissions from the radars. That is, the
LPI radars are enabled to mask their presence so as not to be seen but, at the same time
possessing high performance capabilities “to see” the target by detecting and ranging it even
under low observable target obscurity improvised.
In general, LPI radars are rendered to escape detection by hostile interception by using
antennas that have a transmit radiation pattern with sidelobes of ultra-low levels of − 45 dB
[8]; and, the radiated energy of the LPI radar is spread over a wide spectrum of frequencies so
that the intercepting hostile receiver is compelled to search a large bandwidth to find/locate
the LPI radar. Further, the LPI radar buries itself in the environmental noise by exploiting time- bandwidth product to reduce its peak transmitted power. As such, with a mismatch in
waveforms for which the intercepting hostile system is tuned, the LPI radar effectively masks
itself and becomes invisible [9] [10].
RF Interception
of LPI Radar by
Hostile
LoT
LPI Radar (Mono- or Bi-static)
Platform Operating in a Littoral
Region
LoT being
Tracked
Radar Echo Signature
of Fluctuating RCS of
LoT being Tracked