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83 
Generally, assessment procedures were developed for targets 
of opportunity, and test sites were not necessarily required. 
In the case of point target related measurements, a point 
target detector was set up to determine features that were point- 
like. The procedure was not automatic, a local search being made 
in the neighbourhood of a target determined initially by visual 
inspection. The detector searches for a high peak-to-background 
ratio over a local window, which can be adjusted to cope with 
different image parameters, when a point-like feature passes the 
detector test, then the target response is interpolated between 
image points, and measurements of response width, usually between 
points 3 dB down on the peak, are made in two orthogonal 
directions. This procedure is now improved to include the full 
2-dimensional interpolation around the target. This will often 
allow sidelobe structure to be discerned close to the main 
response, further confirmation of the presence of a point 
target. 
For measurements of radiometric resolution, a detector is 
used to test statistically that an area selected has the 
characteristics of a uniform distributed target. The detector 
calculates the rank correlation coefficients between rows and 
columns of a rectangular area of the distributed target. When 
the target area is oversampled in the image, nearest neighbour 
correlations are omitted. The probability distribution of the 
rank correlation coefficients can be shown to be approximately 
Gaussian for the number of rows and columns greater than, say, 
10, for a distributed target having no discernible structure or 
correlation. Hence, the test can be used to select the 
distributed target areas from the chosen areas. Clearly, the 
form of the test is not unique and alternatives are available. 
The distributed target intensities are then used to 
calculate the quantity: 
y = 10 10810 {y+ a) 
H 
where yu and c are the mean and standard deviation of the observed 
distribution. y is an estimate of the radiometric resolution, 
expressed in dBs, of the system. 
The SAR-580 data set has been used for further measurements 
within this facility [2]. With this data, the interest has not 
been in comparing processing, but in a comparative evaluation of 
the image quality from the three radars used on the Convair-580. 
d 
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