Filling the Nadir Gap of a Synthetic Aperture Sonar
Dr Marc Pinto
Chief Technology Officer
Kraken Sonar Systems Inc.
www.krakensonar.com
Summary: Image resolutions of 5cm x 5cm are required for effective shadow classification and ATR
performance in MCM operations. All of today’s high resolution side-looking sonars, including Synthetic
Aperture Sonars (SAS), leave a gap near nadir. Traditional methods of filling the gap include either an
additional gap-filler sensor or a change in the survey pattern to overlap successive tracks. Overlapped
tracks are best suited to SAS, because it allows full area coverage with the constant high resolution of the
SAS. As the area coverage of a SAS is proportional to its physical array length, the area coverage can be
maintained by increasing this length by approximately 30%. On the other hand the design of a gap-filler
sonar which matches the SAS resolution is very difficult, if at all possible. Fortunately for applications
requiring high resolution, such as MCM, the overlapped tracks method is very effective, especially for AUVs
which have a small turn radius.
Today’s high resolution side-looking sonars and related automatic target recognition (ATR)
algorithms rely mainly on acoustic shadows for target classification. They are designed to
operate over the range of grazing angles over which shadow classification is effective, typically
a maximum grazing angle close to 45° - 50° and a minimum grazing angle close to 6° - 8°.
At grazing angles higher than 45°, the reduction in shadow length and the degradation of range
resolution, dramatically reduce classification performance. At grazing angles lower than 6° - 8°,
the shadow length becomes excessive and increased natural shadowing appears over rough
terrain, leading to target masking. Therefore all side-looking sonars leave a nadir gap at high
grazing angles.
For a sonar altitude of H, a maximum grazing angle of 45° and a minimum grazing angle of 5.7°,
which correspond to the design parameters of the AquaPix InSAS systems of Kraken Sonar
Systems Inc., the minimum plan range is Xmin=H and the maximum plan range is Xmax=10H.
The nadir gap is therefore equal to G=2H in plan range and the single sided swath is D=XmaxXmin=9H. Exact nadir gap sizes depend on environmental conditions and system configuration
as well as sonar and ATR design, but gap sizes from 1.5H to 3H in plan range are typical for all
side-looking sonars.
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A low cost option to cover the gap is the so-called overlapped tracks method (see Fig. 1) which
allows the coverage of the whole survey area at the price of additional survey legs and
therefore reduced area coverage. As seen from Fig. 1, the effective two-sided swath is now
given by S= (3D+G)/2. For the above values for the AquaPix SAS, for which D=9H and G=2H, one
has S=29H/2=14.5H. The effective swath S and therefore the area coverage rate is reduced by a
factor 14.5/20, or 27.5%, compared to the maximum value of 2Xmax=20H, which ignores the
nadir gap. This reduction does not depend very much on the size of the gap as for G=3H (resp.
G=1.5H) one gets S=14.25H or a 29% loss (resp. S=14.6 or a 27% loss). The reduction in area
coverage is due mainly to the existence of a gap and not so much to its size.
Dedicated sonars, known as gap-filler sonars can also be used to ensonify the nadir region and
allow to reach the maximum swath of 2Xmax=20H. These are typically Forward-Looking Sonars
(FLS) operated at a maximum grazing angle of 45° to allow for shadow classification. The main
issue with such FLS is that they cannot achieve the same high resolution as side-looking SAS,
which is on the order of 5cm x 5cm as required for effective shadow classification and ATR
performance.
A popular FLS used for gap-filling1 has angular resolutions of 0.5° - 1°, which are acceptable only
when operating at very low altitudes above the seafloor. For instance at H=2.65m the
resolution in the nadir gap is of the order of 3.7cm for a 0.5° FLS and 7.4cm for a 1° FLS.
However this low altitude limits the range of the side-looking sonar to 10H, or 26.5m, so that
the overall area coverage of the system is 0.28 km2/hr which is very low compared to InSAS
systems even when those use the overlapped track method (see Table 1). Multibeam echo
sounders have also been proposed2 but their operation at high grazing angles precludes the use
of shadow classification and therefore the most current ATR algorithms.
It is seen therefore, that for a SAS, there are additional benefits of the overlapped track method
and additional drawbacks of a dedicated gap-filler sonar. As the resolution of a SAS is very high,
and does not degrade with range, the overlapped track method allows full coverage of the area
with the same high image resolution and therefore the same excellent detection and
classification performance, which motivates the use of SAS in the first place.
1
http://www.blueview.com/media/Atlas-Hydroid-Gapfill-12_01_Final_(3).pdf
Mine detection using swath bathymetry sonars: Tools and Techniques, LtCdr M. Brissette, Mosaic Hydrographic
services (see www.mosaichydro.com).
2
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In addition, the area coverage rate of a SAS is known to be directly proportional to the length of
the physical array so that the drop in area coverage by a factor 14.5/20 due to overlapping can
be compensated for by increasing the array length by the inverse factor 20/14.5 (see Table 1).
The increase in cost and power consumption due to the increase in array length is generally a
small fraction of that of the corresponding FLS due to the much lower channel sampling density
of a narrow field of view side-looking system compared to than a wide field of view forwardlooking system.
While the array length is ultimately limited by the maximum payload capacity of the platform,
the maximum area coverage offered by a SAS is a step change over real aperture systems, even
when these real aperture systems use gap-filler sonars (2.1 km2/hr vs. 0.28 km2/hr).
It is very challenging, and often impossible, to design a gap-filler sonar which offers the same
resolution as a Synthetic Aperture Sonar. A typical high resolution SAS delivers along/across
track resolution of 5cm x 5cm - or better - up to a range of 250m, obtained by operating at an
altitude of H=25m.
As an example, the AquaPix InSAS2 system delivers 3cm x 3cm image resolution up to a range
of 265m (swath width of 530m), when operated at 3 knots. Achieving less than 5cm x 5cm
resolution at 45° grazing angle with an FLS operated at H=26.5m is impossible to achieve with
practical FLS sizes suitable for small diameter AUVs.
When the SAS is operated at H=26.5m, the gap-filling FLS lead to very coarse resolution, of the
order of 37cm to 74cm in the nadir gap of the SAS, which is now more 10 - 20 times worse than
the resolution of the SAS!
This order of magnitude degradation leads to a significant degradation in operational
performance for MCM and negates the benefits of operating a high resolution side looking
sonar in the first place. One must expect a much higher false alarm density in areas surveyed
only by the gap-filler so that it is very possible, even likely, that any gain in survey time due to
the use of gap-filler sonar is lost, and possibly leads to more lost time from prosecuting false
positives following the initial survey.
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Conclusion
A gap-filler that could match a Synthetic Aperture Sonar in resolution remains a desirable
feature operationally. However it is not easy to see how this can be achieved at a reasonable
cost. Fortunately for applications requiring very high resolution, such as MCM, the overlapped
tracks method is very effective, especially for AUVs that have a small turn radius.
G
D
Figure 1. Overlapped track method of gap-filling. Adjacent tracks are paired so that gaps can be covered. This paired pattern
is then repeated to cover the whole area.
SSS
InSAS1
InSAS2
H(m)
2.65
13
26.5
Xmin(m)
2.65
13
26.5
Xmax(m)
26.5
132.5
265
v(m/s)
1.5
1.5
1.5
ACR (km2/hr)
0.28
1.43
2.1
Scenario
With gap-filler sonar
Overlapped tracks
Table 1. Area coverage rates obtained using the AquaPix InSAS1 with a dedicated gap-filler sonar and the AquaPix InSAS2
with the overlapped track method. The ACR of InSAS2 is seen to be higher than that of InSAS1+gap-filler sonar while offering
much higher operational performance since InSAS2 avoids any drop in image quality in the gap.
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