COMPLEX MONOPULSE PROCESSING, A TECHNIQUE FOR HYDROGRAPHIC MAPPING
Sudha S. Reese
Raytheon Company, Submarine Signal Division
Portsmouth, Rhode Island 02871
Abstract
In the amplitude-sensing monopulse system, two beams, A and
B, are formed using the entire array and electrically steering each
beam. The maximum response axis O R A ) of the beam is offset
+ 8, degrees from boresight, so that the MRA for the A beam is
6A = ios
where Os is called the ‘squint’ angle. The B beam is offset by - Os
degrees and its MRA is
OB = - 8,
The complex monopulsetechnique is a processing approach
which uses simultaneous in-phase and quadrature data from two
beams to obtain amplitude and angle information from the backscattered acoustic signal.This approach is advantageous for hydrographic applicationsbecause the algorithm’s simple implementation
can allow rapid processing of the data. Another feature of this
technique is that the imaginary part of the monopulse ratio provides a measure of the spatial coherence of the signal under consideration and, hence, can be used to edit the received signal in order
to minimize the effects of interference dueto multipath returns. In
this paper, different implementations of the monopulse processing
are examined and the resulting performances are presented.
Assuming a Gaussian-shapedbeampattern for simplicity, the
beampattern response of the beam can be written as follows:
-
P (e
where the normalization constant
v =
RA (8) = e
1. Introduction
In recent years the need to provide bathymetric data over large
areas of Ocean bottom have prompted the developmentof efficient
signal processing techniques for bottom mapping applications. S.
Morgera in Reference 1 has reviewed several of these techniques.
Typically, most systems use a multiple beam configuration to obtain a suficient number of depth measurements to infer the bottom
topography. This paperis concerned with the discussion of a signal
processing concept which processes the outputs of two beams to
measure the backscatteredintensity, angle of arrivaland slant range
of wavefronts from isolated small swaths at different ranges on the
sea bed. Also available simultaneouslyis information which can be
used to infer the spatial extent
or compactness of the acousticwavefront. This can be a valuable aid in dealing with the localization of
submerged wrecks or natural obstructions presenting a hazard to
navigation.
-
es)2
(1)
-1m
00
Bo is the half-power beamwidth.
When an acoustic signal Acos(wot) is incident on the array at an
angle OT the output voltages from the two channels are:
(3)
a(t) = RA (e) Acos(wot)
b(t) = RB (e) Acos(wot)
(4)
The outputs from channels A and B are combined to give the
sum and difference channel outputs, S(t) and D(t), respectively, as
follows:
S(t)
=
=
I
RA
RA
=
D
(e)
-
1
(5)
1
(6)
RB (0) Acos(wot)
S (e) Acos(wot)
2. Theory of Monopulse
Quotingfrom Introduction to Monopulse byD.R.
Rhodes
(Reference 2), “Monopulse is a concept of precision direction finding of a pulsed source of radiation. When an incoming wave from
an isolated pulsed return at a sufficiently great rangeis received on
two different beamssimultaneously, the relative amplitudes or relative phases of the received signal can be measured from all sources
within the beam at each instant to estimate the angle of arrival.
It is
this instantaneous formation of this ratio upon reception of each
pulse that is a distinguishing characteristic of the monopulse concept. The returns from differentranges are separated by range gating, after which the angular position within the beam can be determined with a high degree of accuracy.”
(e) +
RB (e) Acos(wot)
(e) Acos(wot)
where S(B), D(e) represent the responses of the sum and difference
channels as a function of angle.
Substituting equations (1) and (2)in the above expression, we
get:
21
01577385/84!ooOo-0021 $1.00
,31984 IEEE
Taking the ratio, we obtain:
= 2b2eSe for s m d e
When the signal is from a single target, the outputs of the Sum
and Difference channels are in phase and the ratio is real. In the
case of returns from multiple unresolved sources,however, D and S
may have any relative phaseand their ratio is therefore complexin
general. Consider an exampleoftwo
unresolved signalreturns
where Sum and Difference components are SI, D l , S2, D2, respectively.
(9)
Figure 1 illustrates the response of a monopulse processor. We
note the sumbeamis a beam withmaximum sensitivity at boresight.
The difference beam goes through a null at boresight and lends
itself to angle determination. The ratio @/S) is an odd function of
the angle and is independent of the signal power.
Figure 2 describes the scenario in which the Sum and Difference
vectors are the vector sums of the individual target components.
Forming the monopulse ratio(D/S) and taking the imaginary part,
we can show
S
where a is the ratio of the amplitudes of two targets
and 6 is the relative phase between the target signals.
b) DandSbemnr
a) AandBbeamr
Figure 2. Phasors of Two Unresolved Tmgefs
From Figure 3 we note that, whenthere are two unresolved targets of equal amplitudes and widely separated angles, the quadrature component can be significant.Hence, by observing the Q com-
ponent, one can detect multiple return signals as in the case of
multipath interference objects with vertical extent. A significant
imaginary component of the monopulse ratio in general suggests
the presence of interfering returns and the possibility of a large
angular uncertainty. A constraint based on thrzsholding the imaginary monopulse ratio can aid in reducing angle estimation errors.
ANGLE
00
c) D md S sensitivi~patterns for an array with squint angle
equal to half the 3 dB beamwidth
DIS
I
z
0
I
00
I
RELATIVE PHASES OF TARGETS = 90'
ANGULAR SEPARATION OF TARGETS = 10'
RELATIVE LEVELS OF THE TARGETS IN DB
ANGLE
d) Corresponding D/S curve
Figure 3. Vmiation of Quahatwe Components of Monopuke
Ratio with ReMive Levels of the Unresolved Twgefs
Figure I . Responses of Amplitude Monopulse Processor
22
Figure 5 shows an alternative configuration where the receiving
array is split in two halves and each beam is formed using the subarray channels. This is called the “phase sensing” monopulse approach. The beam patterns of the A and B beams will differ very
little and only the phases which vary as a function of the angle of
arrival will be different. The Sum and Difference channels are
formed similarly but, we note, the difference channel output is rotated in phase by 90” before the subsequent operations. It is shown
in Reference 3, when the phasecenter separation is equal to half the
array length, the normalized angular variance is
3. Implementation
A functional block diagram of the system using the amplitude
sensing monopulse concept is shown in Figure 4. Real samples are
taken of the two squinted beam outputs and processed through
Hilbert transform filters to obtain the imaginary components. Sum
and Difference channels are formed and from these SS* and DS*
values are generated, followed by pulse length integration.
QUADRATURE
ZSS’
ANGLE
REAL
Figure 4. Amplitude-Sensing Monopulse Processor
OUAORATURE
The various output statistics are estimated as follows:
2 ss*
Amplitude,
Figure 5. Phase-Sensing Monopuke Processor
4. Conclusion
A method has been introduced which provides rapid measurements of backscattered intensity and angle of arrival of acoustic
signal returns throughout the side-scanned area. An important feature of this technique is the quadrature component of the monopulse ratio which provides spatial information about the signal.
The technique is easy to implement and exhibits predictable behavior. The concept has been used successfully in radar signal processing to combat various environmental conditions including multipath, glint, and target multiplicity.
Q component,
where y is slope of the monopulse curve in volts per radian.
The root mean square error in the estimate of angle of arrival
using the monopulse will be many times less than thebeamwidth of
the sum channel. For largesignal-to-noise ratios, an estimate of
that error is given by (Reference 3)
Acknowledgment
The author wishes to thank Dr. S. G. Chamberlain for helpful
discussions on the subject.
References
I . S. Morgera, “Digital Signal Processing for Precision Wide
Swath Bathymetry,” IEEE Journal of Ocean Engineering. VOE
- 9 No. 2, April, 84;pages 73-84.
where Bo is the half-power beamwidth
S/N is the signal-to-noise ratio in the sum channel
C is theslope of themonopulsecurveinvoltsper
beamwidth.
2. D. R. Rhodes, Introduction to Monopuke, McGraw-Hill Book
Company, Inc.
For an amplitude monopule system with squint angle equal to
the 3 dB beamwidth, the slope is approximately 1.5 and the nor-
3. L. K. Amdt, “Responses of Arrays of Isotropic Elements in
Detection and Tracking,” Research Report, U.S. Navy Electronics Laboratory, San Diego, California, 21 April 1967.
malized rms angular variance is:
(+)( i )
2
-..222
4. D. K. Barton, Radars, VoL I-Monopuke
House, Inc., 1974.
O
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Radars, Artech