Suitability of Correlation Arrays and Superresolution for Minehunting ...

DSTO-TN-0443
9.4 Other Estimators
Among other methods for estimating the discrete angular spectrum we mention
ESPRIT [Gething 1991, p. 208–210; Naidu 1996, pp. 313, 380]. The original ESPRIT, due
to Paulraj et al. [1986], is described by Gething. Gething makes the significant comment
that ‘ESPRIT, like MUSIC, encounters difficulties in resolving coherent rays’ and
mentions methods that have been used to overcome this problem.
Haykin [1995, pp. 172, 185] discusses some variants of the standard ESPRIT
method, mainly UCA-ESPRIT, where UCA stands for ‘uniform circular array.’ He says
that ‘UCA-ESPRIT is a novel algorithm that represents a significant advance in the area
of 2D angle estimation. It is a closed-form algorithm that provides automatically paired
azimuth and elevation estimates for each source’ (italics in original). Here the term
‘closed-form’ indicates that an iterative search procedure is not required; the algorithm
provides directly-calculable estimates of the source directions.
Other algorithms related to MUSIC are discussed in Gething (pp. 210–213) and in
Haykin (Chapters 5, 6).
10. Possible Drawbacks of Superresolution
This section discusses possible drawbacks of SR. These include both undesirable
consequences of using SR when the situation is unfavourable, and requirements that
must be fulfilled if SR is to work. Such drawbacks are as follows.
1. As discussed at the end of Section 6.1, a high signal-to-noise ratio is thought to be
necessary for the successful application of SR. Ideal conditions for a high SNR
often occur when there are just a few point sources (or targets), as in radio
direction finding. (This is because there is little interference due to the signals
from the other sources.) In the Minefinder sonar problem, some comments are
appropriate. First, in the situation of mine detection at the longest ranges, where
the signal is barely seen above the noise, the use of SR seems inadvisable. SR
should, however, be good for detection and classification at the closer ranges.
However even here, the question must be raised as to whether interference
from other point targets in the scene (on other parts of the minelike object, for
example, in the case of classification) produces sufficient ‘noise’ to spoil the
image.
2. Correlation (coherence) between the returns from two targets can perturb the
angular spectrum, as discussed in Sections 8.1 and 9.1. Artefacts are produced
unless some method of decorrelation is found. In the case of active sonar, in
which range as well as direction is estimated, it is believed that such effects
occur only due to pairs of targets that lie within the same range resolution cell
(but different directional cells). On the other hand in active sonar, correlation
occurs between virtually all pairs of same-range targets. Thus in the
Minefinder situation (prior to any decorrelation), SR faces a problem due to
correlated targets. (This problem would presumably go away if there were but
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