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43
Mitigation of Clutter and Range Estimation Using
DSSS-Signalling
Nirmalendu Bikas Sinha1, Md. Zeeshan Hussain2 and Subhashis Bhunia3
Abstract— Inspired by recent success of Direct sequence spread-spectrum (DSSS) signal processing communications,
we introduce the DSSS based pulse radar concept for radar signal analysis and processing. In this paper the authors
deal with the range estimation of target and effects of clutter limits on detection range using DSSS technique under a
wide range of environmental conditions. This technology should bring the benefits of both: high sensitivity extended
effective range, robustness to ambient or jamming noise, strong resistance to counter measures and interception over
continuous wave techniques. Finally this paper addresses how the performance of radar improves by using ground
and air-borne DSSS-based signalling techniques for mitigation of clutter by detecting target and clutter radar crosssections (RCS).
Index Terms — DSSS, RCS, SCR, and SNR.
—————————— ——————————
1.
Radars
INTRODUCTION
are
very
complex
electronic
and
electromagnetic systems. Often they are complex
Township, Purba- Medinipur, 721171, W.B., India.
range gate from different pulses. Recorded
mechanical systems as well. Radar systems are
measurements from pulse-Doppler radars, therefore,
composed of many different subsystems, which
are normally in the form of range-pulse maps. Any
themselves
different
analysis of this form of data will need to encompass
components. There is a great diversity in the design of
the observed behaviour in the returns over time within
radar systems based on purpose, but the fundamental
individual range cells (temporal analysis), as well as
operation and main set of subsystems is the same. But
the behaviour from range cell to range cell (spatial
Pulse-Doppler radars typically process bursts of pulses
analysis). This approach will necessarily lead to
to provide range and Doppler information of the
models more realistic than those based on temporal or
environment.
spatial analyses of the data alone.
are
composed
of
many
Range information is provided by
range-gating of the pulse returns whilst Doppler
In case of DSSS based pulse radar signal occupies a
information is provided by coherent integration of
bandwidth in excess of the minimum necessary to
samples from the same
send the information; the band spread is accomplished
by means of a code that is independent of the data,
————————————————
• Prof. Nirmalendu Bikas Sinha, corresponding author is
with the Department of ECE and EIE , College of
Engineering & Management, Kolaghat, K.T.P.P
Township, Purba- Medinipur, 721171, W.B., India.
1
• 2Md. Zeeshan Hussain is with the Department of ECE,
College of Engineering & Management, Kolaghat, K.T.P.P
Township, Purba- Medinipur, 721171, W.B., India.
and a synchronized reception with the code at the
receiver is used for dispreading and subsequent data
recovery. Spread-spectrum techniques continue to use
the capabilities of communication and have been
applied to radar applications. In this paper we focus
on spread-spectrum signalling because of its many
advantages: selective addressing, multiple user access,
signal hiding, anti-jamming, interference rejection, and
high-resolution ranging . One main benefit of spread-
• 3Subhashis Bhunia is with the Department of ECE,
College of Engineering & Management, Kolaghat, K.T.P.P
spectrum signalling is more energy can be transmitted
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44
without sacrificing range resolution—increasing both
another,
range
predictable. Clutter echoes are random and have
and
precision
simultaneously.
With
this
while
volume
may
more
thermal
goal of this work is to bring the knowledge of how
individual
RCS is used to detect the reflected signal from the
random phases and amplitudes. In many cases, the
scatterer and how RCS pattern varies in case of clutter
clutter signal level is much higher than the receiver
and target with distance. Further the experimental
noise level. Thus, the radar’s ability to detect targets
results show that the detection performance can be
embedded in high clutter background depends on the
improved
for
Signal-to-Clutter Ratio (SCR) rather than the SNR[2].
Clutter is a term used to
The object of this paper is to predict the range
describe any object that may generate unwanted radar
detection capability and effects of SCR of a pulse
returns
radar
Doppler radar using DSSS technique where the
operations. Clutter can be classified into surface clutter
Doppler filter bandwidth is much wider than the
and volume clutter. Surface clutter includes trees,
bandwidth of each spectral line in the received signal.
spread-spectrum
mitigation of clutter.
that
may
interfere
signalling
with
normal
vegetation, ground terrain, man-made structures, and
sea surface (sea clutter). Volume clutter normally has a
clutter
characteristics
be
improvement comes a richer signal design space. The
by
noise-like
clutter
components
because
(scatterers)
the
have
2. DSSS RADAR SYSTEM MODEL
large extent (size) and includes chaff, rain, birds, and
insects. Surface clutter changes from one area to
SCATTERRING
MEDIUM
Rx
Tx
Spreading
Signal
Fig.1 DSSS radar system model
The
radar
system
is
Descrambling
Scrambling
composed
Despreading
Received
signal
processed through the signal processor displays
of
many
data for the operator, through the Human
subsystems which include Radio Frequency
Subsystem,
Subsystem
Transmitter
and
Subsystem,
Signal
Receiver
Processing/Data
Processing/Control Subsystems. In a pulsed radar
Machine Interface. The antenna is generally
repositioned after a certain number of pulse
system, the transmission time is called the pulse
transmissions. In case of DSSS radar the signal is
width. A pulse is transmitted at regular intervals,
transmitted through spread spectrum technique.
the repetition interval being termed as the pulse
A schematic of the DSSS radar system is shown in
repetition interval (PRI). The waveform generated
Fig.1.
at the transmitter is passed to the RF system,
3. PERFORMANCE ANALYSIS
through which the waveform is transmitted into
the medium of propagation. A waveform on
reaching the target reflects back towards the
radar, the reflected echo being intercepted by the
RF system. This received signal after being
In this section, we investigate the best achievable
performance of DSSS Radar system parameters in
presence of clutter for surface and volume and
determine the range estimation of target.
3.1 Radar equation for area clutter-ground based radar
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45
The received power from clutter
10
4 / is the clutter
RCS , x is the sum of RCS of side lobe and main
Clutter RCS in dBsm
Where, conventional radar
DSSS based radar
5
0
-5
-10
-15
lobe and other symbols have their usual
-20
0
significance.
20
40
60
Slant Range in Km
80
100
120
Fig.4Clutter RCS vs. slant range for area clutter
Fig.2:Geometry for ground based radar clutter
In order to calculate the total clutter RCS, we compute
clutter areas for both the main beam and the side-lobes
whose
Target RCS in dBsm
5
x 10
13
4
3
DSSS based radar
2
1
0
0
geometry is shown in Fig.3. The angles
represent the antenna 3-dB azimuth and elevation
conventional radar
20
40
60
Slant Range in km
80
100
120
Fig.5Target RCS vs. slant range
beam-widths, θA and θE respectively. The radar height
(from the ground to the phase center of the antenna) is
In Fig.4and Fig.5 we have successfully implemented
denoted by hr, while the target height is denoted by ht.
the behavior of the clutter RCS and Target RCS vs.
The radar slant range is R , and its ground projection
slant range for conventional radar and DSSS based
isRg . The range resolution is ∆R and its ground
radar signalling. It is observed that the clutter RCS
projection is ∆Rg. The main beam clutter area is
decreases significantly in case of spread spectrum
denoted by AMBc and the sidelobe clutter area is
technique. So DSSS radar is most useful for mitigation
denoted by ASLc.
of clutter.
3.2 Radar equation for volume clutter
In case of volume clutter, we can use the Rayleigh
approximation of a perfect sphere to estimate the rain
droplet’s RCS(Fig.6). Because in this region the
Fig.3: Clutter geometry for ground based radar
Finally, in order to account for the variation of the
clutter RCS versus range, calculate the total clutter
RCS as a function of range.
object size is on the order of a wavelength
and the RCS is transitioning from being
dependent upon both object size and
wavelength to being dependent mainly on
object size.
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46
1.4
10
x 10
-17
Volume SCR in dBsm
1.2
1
2
σ/( π r )
Rayleigh
region
Resonance or Mie region
Optical
region
0.1
1
conventional radar
0.8
DSSS based radar
0.6
0.4
0.2
0
0
.0.01
2
4
6
Slant Range in km
8
10
12
4
x 10
Fig.8: Variation of Signal to clutter ratio with range
0.001
0.1
1
10
20
2π r/λ
In Fig. 7and Fig.8 indicated that the performance of
DSSS radar improves the quality of range resolution of
Fig.6 Normalized RCS of a sphere vs.
Normalized Size
targets by minimizing the SCR.
As indicated in this figure, the RCS can often
appear to be larger than dictated by object
size. Typical objects that could be in the
resonance region would be birds, bullets,
artillery shells, some missiles and raindrops.
The clutter RCS of rain drop(considering
volume clutter) is 59.5
#
!" .
CONCLUSION
We investigated and compared the detection and
range estimation performance of DSSS based pulse
radar over conventional radar. We demonstrated that
the DSSS radar detection performance outperforms the
other types of pulse radar because of its selective
addressing, anti-jamming, interference rejection, and
Where, ri = radius of rain drop.
To minimize the SCR (signal to clutter ratio)
of a volume clutter for effective detection of
target RCS, we formulate the following
equation (1) using DSSS technique instead of
conventional based pulse radar and analyzed
the subsequent results ( Fig. 7 and Fig.8 ) .
8
)
$%&' … … 1
*+, high-resolution ranging. So superiority of DSSS radar
Where, * */ *0
Research M (PIER M), Vol. 1, pp. 185-195, 2008.
detection has been established for range estimation
and scintillation of RCS fluctuation in case of surface
and volume clutter.
REFERENCES
[1] N.B.sinha, D.Kandar and R.bera, “Measurement of
target
parameters
using
the
DSSS
Radar”,
International Journal Progress in Electromagnetics
[2] R.Bera, D. Kandar, N.B. Sinha, S. Dhar, M. Mitra,
10
x 10
19
“Design, Modeling and Implementation of Wireless
Embedded System for Intelligent Transport System
Volume RCS in dBsm
conventional radar
8
Application”, published in
DSSS based radar
6
IEEE 3rd International
Conference on Industrial and Information Systems, IIT
4
Kharagpur, INDIA, (8-10 Dec. 2008).
2
[3]. Nirmalendu Bikas Sinha, M. Sonal, R.Bera and
0
0
M.Mitra,
2
4
6
Slant Range in km
8
10
12
4
x 10
of
Digital
Radar
Technology for Imaging and Remote Sensing in
Intelligent
Fig.7:Clutter RCS vs.slant range
“Implementation
Transport
System”,
published
in
International Journal Progress in Electromagnetic
Research C (PIER C), Vol.11, and PP. 213-228, Dec.
2009.
JOURNAL OF TELECOMMUNICATIONS, VOLUME 6, ISSUE 1, DECEMBER 2010
47
[4]. Nirmalendu Bikas Sinha, Manish Sonal, R.Bera and
international journals including IEEE.
M.Mitra “Modelling and Implementation
of ITS Channel estimation and Imaging
Radar : An Impact on Remote Sensing
and
ITS
System”,
published
in
International Journal of Research and
Reviews in Applied Sciences (IJRRAS), Vol. 1, No.2,
pp. 118-126, November 2009.
[5] Urazghildiiev et al, “Vehicle Classification Based on
the Radar Measurement of Height Profiles”, IEEE
transactions on intelligent transportation systems, vol.
8, no. 2, june 2007,pp 245-253.
[6] Tsang et al, “Advance Path Measurement for
Automotive Radar Applications”, IEEE transactions on
intelligent transportation systems, vol. 7, no. 3,
september 2006, pp 273-281.
Prof. Nirmalendu Bikas Sinha received the B.Sc
Md. Zeeshan Hussain is
(Honours in Physics), B. Tech, M. Tech degrees in
Department
of
Electronics
Radio-Physics
Engineering
at
College
and
Electronics
from
Calcutta
pursuing B.Tech in the
&
of
Communication
Engineering
and
2001,
Management, Kolaghat, under WBUT in 2011, West
respectively. He is currently working towards the Ph.D
Bengal, India. His areas of interest are in Microwave
degree
/Millimeter wave based Broadband Wireless Mobile
University,
in
Calcutta,India,in1996,1999
Electronics
and
and
Telecommunication
Engineering at BESU. Since 2003, he has been
associated with the College of Engineering and
Management, Kolaghat. W.B, India where he is
currently an Asst.Professor is with the department of
Electronics
&
Electronics
&
current research
Communication
Instrumentation
Engineering
Engineering.
&
His
Interests are in the area of signal
processing for high-speed digital communications,
signal
detection,
MIMO,
multiuser
communications,Microwave /Millimeter wave based
Broadband
Wireless
Mobile
Communication
,semiconductor Devices, Remote Sensing, Digital
Radar, RCS Imaging, and Wireless 4G communication.
He has published more than 50 papers in different
international Conference, proceedings and journals.He
is presently the editor and reviewer in different
Communication and digital electronics.
Subhashis Bhunia is
pursuing B.Tech in the
Department
of
Electronics
Engineering
at
College
of
&
Communication
Engineering
and
Management, Kolaghat, under WBUT in 2011, West
Bengal, India. His areas of interest are in Microwave
/Millimeter wave based Broadband Wireless Mobile
Communication and digital electronics.