7. To use the contrast filter, select Input Product Filter > Contrastor.
For target detection, the contrastor is necessary in order to pick-out an airborne
target from weak clear air or weather echoes.
The contrast filter provides a way to eliminate relatively weak echoes so that only the
embedded stronger echo cores remain.
It first computes a smoothed version of the DWELL (the background echo) and then
subtracts this from the original input data to get the final "contrasted" version of the
DWELL.
The DWELL algorithm runs twice: the first time to compute the background field and
the second time during which the background field is used to threshold the original
input data. For more information on the contrast algorithm see Configuring COMP
(page 132).
The parameters to configure the contrast filter are as follows:
• Thresh
Each input product data pixel must be greater than the corresponding pixel in the
average background field by this amount, else it is discarded.
• % Coverage
The background field is produced by a smoother that averages data pixels in a 10 km
by 10 km (6.2 mi by 6.2 mi) box that is passed over all the pixels in the first pass
DWELL product. If the percentage of valid (non-thresholded) pixels in this box does
not exceed the % Coverage limit, then a value of "thresholded" is assigned to this
box, that is, there are not enough points to compute the average.
8. If your input product consists of PPI products at dierent elevation angles, you can
select Sep PPI's.
For example, a PPI made with a * in the elevation angle filed has every angle in the
volume scan. In this case, the background field can be in one of the following ways:
• Default method
All of the angles are merged together to compute the background field.
• Sep PPI's enabled
The background field is computed separately for each PPI elevation angle and the
contrast thresholding is performed separately for each angle before merging the
results.
This provides better results when there are vertical gradients in velocity or
reflectivity. If the vertical gradients are weak then it does not really matter which
approach is used.
IRIS Product and Display User Guide M211319EN-H
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