Suitability of Correlation Arrays and Superresolution for Minehunting ...

DSTO-TN-0443
satisfied. Yet with special experimental arrangements, effective incoherence can be
produced, as discussed by Steinberg and Subbaram [1991, Sections 11-3, 11-4, Chapters
5, 6].
S&S (Sections 11-7, 11-3, 11-4) list four methods of achieving decorrelation as
follows. The first method is to sequentially use transmitters at different locations; this
is called transmitter-location diversity. The second is to physically move the receiving
array to different locations; again the array locations are used sequentially. The third
method is to arrange for the target to be moved. The fourth method requires only a
single pulse from a single transmitter. It relies on a method of signal processing that
requires the receiving array to be periodic. The situation is as if the receiving array had
moved. Before discussing methods typified by the four above (Sections 11.2, 11.3), we
consider a group of miscellaneous methods of decorrelation (Section 11.1).
11.1 Methods Based on Real-Time Averaging
We discuss here two pieces of work, first mentioned in Section 6.4. The first is that of
Shearman, Bickerstaff and Fotiades [1973] (SBF). SBF discuss over-the-horizon radar
using reflections from the ionosphere. They used a two-antenna receiving instrument,
with variable separation. Since also the two outputs were multiplied and the result
averaged over time, the device was in essence a correlation array (see Sections 2 to 4),
but with an extra twist (see next footnote).
Prior to the experiment, SBF thought that ‘the random variations in the height of the
ionosphere would decorrelate the returns sufficiently’ (italics added). This additional
method of decorrelation (besides the methods of varying range and varying target
strength, suggested in Section 4.3.1) is worthy of note. The discussion of SBF (p. 416)
makes clear that the averaging is to be done in real time. 14 This feature contrasts with
the synthetic averaging approach to be developed in Sections 11.2 and 11.3. In the SBF
experiment, it turned out that the returns were not decorrelated sufficiently.
The response of SBF was that ‘the radar was modified to make the transmitter and
receiver frequency-hop from pulse to pulse randomly ... This would serve to decorrelate
echoes from targets differing in oblique range by a few km’ (italics added).
(Presumably, ‘differing in oblique range’ refers to targets at the same range but at
different bearings.) The principle that makes the frequency-hop method work is not
clear from their description. However the method has similarities with the frequencydiversity
method described in Section 11.2. The frequency-hop method appears to be
based, at least in part, on synthetic decorrelation as opposed to real-time averaging. At
the time of writing by SBF, encouraging experimental results had been obtained.
Thus SBF have proposed two methods. The first (the use of the random height)
suggests that in sonar, one might rely simply on fluctuations in properties of the water
such as the temperature to produce decorrelation (see also the work of Clarke below).
The second method, frequency hopping, might be copied across directly.
14 To complicate matters, there is also a ‘synthetic’ component to the experiment. As the
experiment proceeds, of the only two elements, one element is gradually moved away from the
other. Thus a synthetic aperture is produced.
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