Results from trials of a planar, incoherent, synthetic aperture sonar for underwater imaging
ABSTRACT Synthetic aperture sonar (SAS) techniques can yield high-resolution images with a small physical array. Their application in the underwater environment is usually confined to the deployment and synthesis of linear apertures. Incoherent SAS processing is a suboptimal approach compared with coherent SAS processing as the absence of phase information results in an inferior along-track resolution and a higher sidelobe level. The absence of phase information implies that incoherent processing is not constrained by phase-aliasing. Incoherent SAS can accommodate non-uniform inter-ping spacing and is tolerant to trajectory estimation errors of the order of the range-compressed pulse length. This presents new opportunities for a robust surveying system. Apertures of nonlinear shapes can be synthesised depending on the nature and requirement of the specific application and environment. Surveys can be conducted with minimal hardware deployment, such as by a diver. On the basis of this concept, tank trial results of a 3-dimensional incoherent SAS technique utilising the synthesis of 2D apertures are presented. Broadband pulses are employed to achieve optimal survey resolution. The feasibility of this technique is demonstrated for both monostatic and single-transmit, multiple-receive configurations. Practical results are shown for arbitrary-surface apertures sampled at non-uniformly separated positions.
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ABSTRACT: Over the past 20 years, sonar imaging technology particularly for the high-technology sector has been a focus of research, in which many developed countries, especially those with coast lines, have been competing with each other. It has seen a rapid development with increasing widespread applications that has played an important and irreplaceable role in underwater exploration with great prospects for social, economic, scientific, and military benefits. The fundamental techniques underlying sonar imaging, including multi-beamforming, synthetic-aperture and inverse synthetic-aperture sonar, acoustic lensing, and acoustical holography, are described in this paper. This is followed by a comprehensive and systematic review on the advantages and disadvantages of these imaging techniques, applicability conditions, development trends, new ideas, new methods, and improvements in old methods over recent years with an emphasis on the situation in China, along with a bold and constructive prediction to some development characteristics of sonar imaging technology in the near future in China. The perspectives presented in this paper are offered with the idea of providing some degree of guidance and promotion of research on sonar imaging technology.Chinese Journal of Oceanology and Limnology 06/2013; · 0.58 Impact Factor
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Published in IET Radar, Sonar and Navigation
Received on 16th March 2006
Revised on 8th August 2007
doi:10.1049/iet-rsn:20060036
ISSN 1751-8784
Results from trials of a planar, incoherent,
synthetic aperture sonar for underwater
imaging
K.Y. Foo P.R. Atkins T. Collins
Department of Electronic, Electrical and Computer Engineering, University of Birmingham, Edgbaston, BirminghamB15 2TT,
UK
E-mail: andrewfoo@gmail.com
Abstract: Synthetic aperture sonar (SAS) techniques can yield high-resolution images with a small physical array.
Their application in the underwater environment is usually confined to the deployment and synthesis of linear
apertures. Incoherent SAS processing is a suboptimal approach compared with coherent SAS processing as the
absence of phase information results in an inferior along-track resolution and a higher sidelobe level. The
absence of phase information implies that incoherent processing is not constrained by phase-aliasing.
Incoherent SAS can accommodate non-uniform inter-ping spacing and is tolerant to trajectory estimation
errors of the order of the range-compressed pulse length. This presents new opportunities for a robust
surveying system. Apertures of nonlinear shapes can be synthesised depending on the nature and
requirement of the specific application and environment. Surveys can be conducted with minimal hardware
deployment, such as by a diver. On the basis of this concept, tank trial results of a 3-dimensional incoherent
SAS technique utilising the synthesis of 2D apertures are presented. Broadband pulses are employed to
achieve optimal survey resolution. The feasibility of this technique is demonstrated for both monostatic and
single-transmit, multiple-receive configurations. Practical results are shown for arbitrary-surface apertures
sampled at non-uniformly separated positions.
1Introduction
The need to map extended sea bottom targets with a
relatively small physical array calls for the application
of synthetic aperture sonar (SAS) techniques. The
concept of SAS is based upon synthesising the aperture
of a large array by making multiple transmissions on a
pre-determined path. The received echoes are spatially
integrated to form a beam with resolution comparable
to that of a large physical transducer. Coherently
synthesising the aperture, by utilising phase information
in the process of integration in the along-track
direction, yields a feature resolution patch size that is
proportional to the dimension of the transmit
transducer and independent of frequency and cross-
track range [1, 2]. For a narrowband system, this is
achieved provided that the trajectory of the towed
platform is accurately estimated to the order of a
fraction of the wavelength and that the inter-ping
distance is not larger than half the physical length of
the towed array [3]. In order to meet these
conditions, the mapping rate would be limited, or a
longer array needs to be deployed. The system would
also require expensive inertial navigation systems to
track and compensate for platform instability.
An alternative approach is to implement incoherent
SAS processing, a scheme that does not rely on phase
information in the along-track integration of echoes.
The absence of phase information produces images
that are inferior in along-track (or azimuth) resolution
and in image fidelity. However, this technique is not
restricted by the requirement to satisfy the classical
constraints designed to avoid phase-aliasing encountered
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in coherent SAS [4]. As a result, the inter-ping spacing
can be extended to achieve a higher mapping rate and
the transmissions need not be uniformly spaced [5].
For a narrowband system, the trajectory only needs to
be tracked to the accuracy of a fraction of the pulse-
compressed length, compared with a fraction of the
wavelengthwithcoherent
broadband system is employed, it is worth noting that
the requirements for the positional accuracy of both
coherent and incoherent approaches are similar.
However, it is likely that incoherent systems would be
deployed for bottom-mapping
alternative to multi-beam echo sounders. Thus, the
nominal operating frequency would be lower than that
of a conventional SAS imaging system, thus further
relaxing the positional tracking accuracy requirements.
The ability to process incoherently offers flexibility in
terms of the dimension of the array being synthesised.
Furthermore, an incoherent system is also capable of
operating in near-shore regions where there are rapid
changes of sound-velocity profile, forming non-
homogenous layers which introduce phase distortion.
Such phase distortion would degrade the performance
of a coherent SAS system when not adequately
compensated.
processing.Whena
operations asan
The desire to implement 3D beamforming or multi-
aspect imaging in the underwater environment [6]
necessitates the use of planar or arbitrary-surface
apertures, as opposed to simple linear apertures.
‘Linear’ here refers to the synthetic elements or spatial
samples lying on a line, as opposed to being distributed
on more than one line. The implementation of this
concept with phase-coherent systems is demonstrated
in the work on interferometric SAS [7] and multi-
aspect SAS [8]. With incoherent SAS processing, the
complexity of deployment is lower for synthesising
arbitrary-surface apertures. The synthesis of arbitrary-
surface apertures is attractive in situations where the
nature of deployment prohibits travel in a linear
manner and at constant velocity, such as surveys
conducted by a diver.
This work examines the alternative incoherent SAS
technique anddemonstrates
incoherent SAS in producing 2D synthetic apertures,
with the purpose of performing 3D surveys in an
underwater environment. Section 2 identifies the
fundamentals of incoherent SAS processing, introduces
the concept of a planar synthetic aperture and describes
the time-domain integration process required to
reconstruct an image representation. Details of a tank
experiment are reported in Section 3, where two
tungsten spheres are imaged with a 2D planar, mono-
static, synthetic aperture. Section 4 describes a second
experimentwherean arbitrary-surface
aperture is formed using a single-transmit, multiple-
receive configuration to produce the representation of a
theapplicationof
synthetic
metal rod in the water. A further trial was performed
and detailed in Section 5 to demonstrate the robustness
of an incoherent SAS processing strategy under a more
practical set of operating constraints. Section 6
concludes this paper.
2
2.1 Suboptimal approach
Incoherent SAS processing
In coherent synthetic aperture processing, phase
differences between consecutive echoes are exploited
to produce images with good along-track resolution.
The echoes are accurately time-aligned with respect to
a desired position within the image and then summed
coherently [2]. This process requires an accurate
knowledge of the position of the sonar platform as a
function of time. The coherent summation of echoes
with both magnitude and phase information leads to
an optimal processing strategy. However, in incoherent
processing, the phase information is removed before
the echoes are summed [4].
With coherent synthetic aperture processing, the
destructive interference actively reduces the along-
track sidelobes from the individual pings. This is not
the case for incoherent processing where phase
information is absent. Fig. 1 shows a comparison of
the integrated point-energy response for an adequately
sampledsyntheticaperture
without phase information under ideal conditions.
Both plots are normalised by the maximum value,
hence the pixels representing the point target have
values approaching 0 dB. The integrated sidelobe level
is ?9 dB greater for the incoherent processing strategy
when compared with the coherent processing strategy
for the illustrative example shown in Fig. 1. The peak
sidelobe level is 26 dB for the incoherent processing
case and 213 dB for the coherent processing case.
When surveying the seafloor, the average sidelobe
level affects the shadow contrast, whereas the peak
sidelobe level is related to the image contamination
that contributes toward false targets and degrades
image fidelity. It is therefore worth noting that with a
higher sidelobe level, incoherent synthetic aperture
processing is a suboptimal approach, especially for
data sets that can be successfully processed with
phase-coherent methods.
processedwith and
Incoherent synthetic aperture processing can also be
applied in tandem with coherent processing. An
example of such an application is where a towed array
is deployed. For each ping, the returns from the
elements of the array are coherently summed using
phased-array techniques. The output from the phased-
array can then be summed incoherently across multiple
pings, as demonstrated in [9].
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2.2 Along-track resolution of incoherent
SAS
The along-track resolution of an incoherent processing
scheme can be derived from the geometry as shown in
Fig. 2. Ra and Rb are two receiver positions with
23 dB half-beam angle ub. The dotted lines are iso-
range ambiguity loci with respect to the receiver
positions, and they correspond to the matched-filtered
responses from a point target marked ‘x’. The point
target matched-filtered responses range from 2Dr to
Dr, as a result of the limited system bandwidth
available. The azimuth resolution of the system is
defined in terms of the correlation of the overlapping
responses. This resolution is taken to be twice the
azimuth increment that causes the correlation to drop
to one-half of its peak value, considering only a slice
along the azimuth dimension of the azimuth-range
ambiguity function. Thus, the azimuth resolution, Px,
can be expressed as
Px¼ xb? xa
(1)
By trigonometric relationships and assuming ro? Dr,
one can derive that, at the 23 dB level [4]
Px¼4:5 Dr L
l
(2)
where L is the physical dimension of the receiver
transducer, l the wavelength of the signal and 2 Dr
the range resolution. Unlike the expression for the
azimuth resolution of a classical coherent SAS, the
incoherent azimuth resolution is also proportional to
the range resolution of the signal. The use of
broadband (i.e. linear frequency modulated or linear
period modulated) signals and pulse compression
therefore improves both the range and azimuth
resolution in incoherent processing.
2.3 Data collection
The main difference between incoherent and coherent
SAS signal processing functions is the removal of phase
information before SAS integration is performed on
the received ping returns. With an incoherent
strategy, the spacing between ping instances need not
be uniform and the tracking accuracies may be
reduced. Data collected with a phase coherent system
can be processed incoherently, but data collected with
an incoherent strategy will probably yield poor results
when processed coherently.
The trials presented in Sections 3 and 4 were
conducted with the aid of an X–Y positioning table
and therefore provided an idealised navigational setup.
This enabled the evaluation of the performance of an
incoherentSASimaging
conditions to verify that the performance matched the
theoretical predictions. Section 5 details a trial in
which a fully incoherent strategy was employed. The
robustnessof incoherent
demonstrated for the situation where the ping spacing
is non-uniform and wider than required in a coherent
system. No mechanical positioning system was used
and the localisation of the pings was based entirely
upon the time-of-flight ranging of pings sent out by
the transmit transducer.
systemunder optimal
SASprocessing is
Figure 2 Geometry of matched-filtered responses from
target ‘x’ resolved by two receiver positions, Raand Rb
Figure 1 Comparison of the integrated point-energy
response for an adequately sampled synthetic aperture
processed with and without phase information under
ideal conditions
a Point spread function obtained using phase-coherent
integration
b Using incoherent integration
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2.4 Image reconstruction
To perform image reconstruction, two separate
matrices, Idataand Iimage, are introduced to represent
thecollecteddataand
integration on the collected data, respectively. The
positionofeach of
corresponding to Idata is assumed to be known by
independent techniques.
the result ofazimuth
thetransmissionpings
Consider a broadband continuous-time signal, s(t),
being transmitted. Assuming a noise-free propagation
model with no Doppler scaling and no phase errors,
the received signal from a point target, r(t), will
be a delayed and attenuated version of the transmitted
pulse
r(r) ¼ Ams(t ? ta) (3)
where Amis the attenuation factor (which also accounts
for target reflectivity) and tathe two-way propagation
time delay of
s(t).
representations of the transmitted and received signals
are definedas
S( f )
The matched filter is performed by multiplication of
the received signal spectrum by the conjugate of the
replica signal spectrum. This is then converted into
the time domain (via the inverse Fourier transform)
to give
ð
The complex time-domain samples from the output of
the matched filter are then converted to a modulus-
squared form. This removes the phase information and
leaves an energy representation:
Thefrequency-domain
and
R( f ),respectively.
G(t) ¼R( f)S ? ( f)ej2pftdf
(4)
G2(t) ¼ G(t) G?(t)(5)
The discrete-time samples are then appended to the 2D
data matrix Idata, where the rows correspond to the ping
number and the columns correspond to the sample
number (range cells).
To reconstruct the SAS image, for each point on
Iimage, a sample will be chosen from each cross-track
slice of Idatabased on the back-projected ranges, and
then summed in the along-track direction, such that
Iimage[na, nr] ¼
X
N
n¼1
Idata[n, nd[n, na, nr]]w[n, na, nr]
(6)
where [na, nr] is the coordinate of the point of interest in
the along-track and cross-track direction, respectively
on Iimage, N the total number of pings in the
synthesised aperture, nd the sample delay that
determines which sample on the nth cross-track slice
is selected for summation and w[n] the weighting
matched to the angular response of the element with
respect to the point of interest.
Extending this to an arbitrary-surface synthetic
aperture, the coordinates in a 3D image environment
are represented by the x–y–z axes instead of the
along-track and cross-track axes. The z-axis represents
the depth from the surface, whereas the x- and y-axes
are perpendicular to the z-axis. Equation (6) now
becomes
Iimage[nx,ny,nz]¼
X
N
n¼1
Idata[n,nd[n,nx,ny,nz]]w[n,nx,ny,nz]
(7)
where [nx, ny, nz] is the current point in the 3D image
volume on which focusing is performed and nd
the sample delay from the current point of interest
to the corresponding arbitrary-surface synthetic array.
The sample delay ndis
nd[n,nx,ny,nz]¼2fsRn[nx,ny,nz]
c
(8)
where the range function Rnis a 3D vector dot product
between the location of the point of interest [nx, ny, nz]
and the location of the point P[a, b, x] on the arbitrary-
surface synthetic aperture, fsthe sampling frequency and
c the speed of sound. The arbitrary-surface synthetic
aperture may use a different coordinate system to that
of the image coordinate system.
In incoherent processing, obtaining a spatially
balanced energy distribution around a point target
improves the image representation and reduces the
occurrence of false targets. The energy distribution
can be observed from a range–loci plot (Fig. 3) and
is affected by the position of elements (linear
aperture) and the symmetry of the array (arbitrary-
surface aperture). This is taken into account by
the weighting function, w, which determines the level
of contribution from each element with respect to
a point of interest. Extending this to an arbitrary-
surface synthetic aperture, the weighting function is
expressed as
n
w[n,nx,ny,nz]¼wTsT[n],nx,ny,nz
o
wRsR[n],nx,ny,nz
no
(9)
wherewTandwRaretheweightingfunctionscorresponding
to the transmitting and receiving locations, respectively,
in order to take account of spatially separated
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transmit–receive configurations sT[n] and sR[n] are the
position vectors of the transmitting and receiving
elements, respectively, at ping n.
In the tank trials, a weighting function was used to
minimise the contribution from synthetic elements
that corresponds to angles close to, or exceeding, the
23 dB beamwidth with respect to a point of interest
while maximising the main-lobe contribution. This
effectively matched the spatial integration process
to the response of the transducer. In the second
trial with a spatially separated transmit–receive
configuration, a bi-lateral weighting was applied to
both the projector and the receiving elements. By
using only the effective, matched-weighted, synthetic
elements with respect to a point of interest, the point-
target energy response of the synthetic aperture is
improved. This is as a result of minimising the energy
spilling over from adjacent targets, which would
otherwise contribute to the noise floor of the current
point of interest.
3
aperture tank trial
Amonostaticconfigurationreferstotheco-locationofboth
thetransmittingandreceivingelements,wherethedistance
between them is either negligible or appropriately
compensated. Each transmitting synthesised element
location corresponds to a similar synthesised receiving
element location on the synthetic aperture. The trial was
conducted in a water tank with dimensions of width
1.5 m, length 1.5 m and depth 1.5 m. The bottom and
side walls of the tank were lined with acoustic absorbers.
A tungsten carbide sphere, 20 mm in diameter, was hung
by a thin string 0.2 m below the water surface, as
illustrated in Fig. 4. Two transducers with a resonant
frequency of 500 kHz were used, operating in transmit
and receive modes, respectively. The transducers were
guided by an X–Y positioning table.
Monostatic planar synthetic
A total of 75 measurements were taken in each
direction of the x–y plane, thus forming a 75 ? 75
element planar synthetic aperture with a uniform
separation between pings of 5 mm. The transducers
were maintained stationary during the transmit–
receive period. The transmitted signal was a linear
frequency modulated pulse with a bandwidth of
200 kHz, centred on 500 kHz and weighted by a Von
Hann window function. The pulse duration was 0.1 s.
The received signals were matched-filtered and
appended onto a data matrix Idata. Along-track
integration was then performed to obtain the total
energy return for each point of interest in order to
construct an image representation, Iimage, as defined
by (7). All elements were used in the integration to
Figure 4 Illustration of the water tank trial with a spherical
target suspended in water and the synthetic aperture
elements facing the bottom of the tank
Figure 3 Energy distribution
a Uniform range–loci spacing
b Point-energy response for non-uniform spacing
c Point-energy response for uniform range–loci spacing
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