Background Field
The contrast filter allows relatively strong echoes such as reflectivity cores, birds or aircraft
to be isolated from relatively weak background echo. For the target warning algorithm, the
contrast filter is essential to detecting airborne targets in weak clear air or weather echoes.
The background field is created by first running the DWELL algorithm without the contrast
step and then smoothing the data with a 2D filter. Note that the Max and Min filters are used
to filter the inputs to generate the background field.
If the input product is PPIs at dierent elevation angles, you can generate the background
field separately for each PPI elevation angle instead of merging the PPIs regardless of angle.
The merge algorithm for the creation of the background field is the same as for the main
DWELL algorithm (that is, maximum value), except that for velocity, the average value of the
signed velocity is used. The reason for using the Max for everything except velocity is that
weather echoes have a natural power fluctuation and these are better removed by the
contrast filter if the background field is based on maximum, that is, the background field has
larger values so that fewer weather speckles are passed. In the case of velocity the average
value is used to characterize the background field since it does not make sense to use the
maximum when every velocity is equally likely.
After the merge, a smoothing filter is applied. This is a 2D averaging filter set to 10 by 10 km
(6.2 by 6.2 mi) or 60 × 60 pixels (whichever is less), centered at the smoother output pixel.
A triangular weighting is used to assign more weight to the center pixels. The filter requires
a selectable percentage coverage to produce an average value at the center pixel, else a
value of "thresholded" is assigned to the center pixel. The threshold is typically set to 20 %
coverage. This has the eect that isolated point targets such as birds or aircraft are excluded
from the background field so that they are not filtered by the contrast filter.
An example of a smoothed contrast filed is shown below for the same case in the examples
in Composite Algorithm (page 130).
Chapter 5 – Configuring and Scheduling Optional IRIS Products
RESTRICTED 145
To Next Page
Loading...