User manual

Code: ENIGMA III User Manual
Edition: 1 Revision: 4
Date: 19/12/2008
Page: 21/97
2.4 Doppler clutter filters
Doppler clutter filtering is based on the valid assumption that stationary ground targets appear as strong
narrow peaks around zero in the spectrum (sampled over the PRT).
The doppler clutter filter (both time- and frequency- domain processing) attenuate the radar signal around
the zero frequencies. Time domain processing applies a configurable highpass filter to the radar data. This
attenuates all DC components in the signal, regardless if they are due to clutter or weather. The frequency
domain filter work on the above mentioned assumption and are able to separate weather and clutter
targets.
2.4.1 Time domain filtering
The pulse-pair processing performs as a first stage filtering of the input data. It has an implementation of
the digital infinite impulse response (IIR) filters. There are seven 40 dB and seven 50 dB 4-order
Chebyshev highpass filters. The coefficients can be chosen with the parameter 'ClutterFilter'.
0 and 8 – no filtering;
from 1 to 7 – one of 40 dB filter;
from 9 to 15 – one of 50 dB filter.
Additional parameter are ZeroFilter and FilterDelay. When using PRF staggering it is recommended to
use the ZeroFilter and FilterDelay parameter as consecutive rays are sampled with a different sampling
frequency (PRF). The recursive filters must be reset to avoid artifacts.
GAMIC Proprietary
Illustration 13: Typical spectrum showing weather and clutter targets