Suitability of Correlation Arrays and Superresolution for Minehunting ...

DSTO-TN-0443
The second problem is that the correlation array, at least in the form initially
envisaged, requires a comfortably long time over which to average the signal products.
An initial argument finds this requirement to be incompatible with the sonar’s
possessing the combination of good range resolution and low range sidelobes.
However the argument assumes that phase shifts in real time are being utilised to
remove correlations. If, on the other hand, a synthetic decorrelation method, such as
the FBLP method, exists that can be applied to correlation arrays, it seems likely that
this second problem goes away, as discussed in Section 11.4.
If these two major problems are satisfactorily dealt with, for satisfactory operation
of the correlation array one must still check on two points. First, one must ensure that
the required high signal-to-noise ratio is achieved. Second, in the case where discrete
targets are modelled, one must ensure that the number of targets assumed is not too
high, otherwise spurious targets are produced.
The second major area discussed has been superresolution. Several techniques for
achieving SR have been briefly described, and references have been given to other
techniques. The techniques briefly described include Capon’s method, an
autoregressive moving average (ARMA) model and a specialisation of the latter, the
forward-backward linear prediction (FBLP) model.
An important pair of questions is: How good is each SR technique, and which of
the several SR techniques is best for the Minefinder sonar? Unfortunately, while each
method has had successes, the literature essentially fails to give general conclusions as
to the resolution achieved by each method and the conditions under which it may be
applied. Munier and Lacoume (see Section 12) make an attempt to answer such
questions. Regarding the second question above (i.e. conditions), the first step towards
answering it has been taken by Clarke and Mather [1985] (see Section 12). In particular,
they say that the ‘best’ method varies from problem to problem, depending on the
collateral or a priori knowledge. Each alternative algorithm applies a different
constraint, model or assumption.
Undesirable features of SR—that is, conditions that must be imposed and
unfortunate outcomes that can be produced—have been discussed in Section 10. Thus
a high signal-to-noise ratio seems to be essential and some means must generally be
found to decorrelate the returns from each pair of targets. An overzealous attempt to
increase resolution often leads to distortion of the image in the ways listed in Section
10. The fixed-array-weights methods of Section 6 have further problems.
In both the area of correlation arrays and that of SR, an important requirement for
the Minefinder application is the need to decorrelate the targets. Decorrelation
methods, applicable in the case of SR but possibly also adaptable to correlation arrays,
are discussed in Section 11. The theory of the FBLP method of decorrelating, discussed
by S&S, looks very encouraging; on the other hand, the experimental test results
presented so far are considerably less encouraging.
Finally we turn to overall prospects for the use of these two broad techniques in the
Minefinder sonar context. The prospects for correlation arrays and those for
superresolution are so similar that we discuss them together. First, a technique for the
decorrelation of targets must be made to work in the sonar context. Most likely, one
would adapt a method already developed for SR (perhaps for the radio context), such
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