Suitability of Correlation Arrays and Superresolution for Minehunting ...

DSTO-TN-0443
1. Introduction
This work was carried out as part of a project, called ‘Minefinder,’ within the Maritime
Operations Division, DSTO. The aim of this project, in the first instance, is to do the
preliminary design for a hull-mounted sonar to detect and classify sea mines. That
design is to be followed by experimentation to prove any new concepts and to separate
feasible from non-feasible designs. This could lead to a state-of-the-art sonar being
constructed by a commercial firm.
It was decided to look at some new options for the sonar that might improve
performance, including options that have generally been avoided in the minehunting
context. One possibility is to beamform in elevation as well as in azimuth. As well as
giving information on the elevation of targets, this vertical resolution might help avoid
problems with multipaths.
It is intended to devote effort also to the use of coded signals, especially multiple,
orthogonal (or almost orthogonal) pseudo-random sequences. By allowing many
signals to be in the water at once, coded signals could help to remove the limitation
placed on the rate of data acquisition by the long time taken for the signal to travel to
the furthest range and return. The coded beams could be used to produce several
narrow beams with different elevation angles. But they could also be used to send a
few signals to probe at short ranges at the same time as a differently coded signal
probes at longer ranges.
The present report is in large part a literature survey, but also contains some
analysis of the implications of the survey for the proposed sonar.
The first broad area covered by the report is inspired by work in radio astronomy.
First, in long-baseline interferometry, two widely separated arrays are used to obtain as
good a resolution as obtained from a single array joining the two. This concept might
be used to obtain high resolution by mounting two arrays out to the side in ‘free’
water, one on each side of the ship’s bow. Second, consider the cross-shaped array,
constructed from two subarrays in the form of two arms perpendicular to each other,
one subarray having M elements and the other having N . When this arrangement is
used as a correlation telescope, in some sense the M + N elements achieve as good a
result as the MN elements that a conventional array would use. Applied to sonar, this
concept might greatly reduce the number of elements needed.
The second broad area, superresolution, is inspired by a general problem posed by
bow-mounted sonars. From the viewpoint of sonar performance, it is desirable for the
array to be long in the athwartships direction. But a long array produces high drag,
which is quite undesirable. As a result such sonars are generally made less than 2 m
long. It is known that under certain conditions the resolution achievable with an array
can be improved beyond that given by the Rayleigh criterion; this phenomenon is
called superresolution (SR). This suggests the question: Can superresolution
techniques be used in our application to increase the azimuthal resolution for an array
of fixed length? The aim in this second broad area is to draw together information
from the literature with a view to answering this question. It should also be noted that,
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