Radartutorial .eu
Doppler-Filter
Figure 1: Block diagram of an MTI- Systems in a monopulse radar with a digital receiver
Practical moving target detectors on a fully-coherent radar equipped with monopulse processing involve a Doppler filter bank. The I and Q output from the phase-sensitive analogue/digital converter (ADC) are fed to a filter bank which can either be implemented in a Fast Fourier Transformation or in a bank of transversal filters. The output of the filters is followed by frequency-domain weighting to reduce filter sidelobe levels, and the magnitude of the output in each spectral band is computed. The whole MTI-wiring is carried out for tree identically channels (Σ, ΔAz and ΔEl)
The familiar pseudo-coherent triggering of the COHO is not necessarily here. The stage called „COHO” in a fully-coherent radar is now a phase constant frequency counter only.
All Doppler filters shown in the figure 1 are similiar. The filter works with help of the Puls-Pair-Method: a fixed target suppression happens by the phase comparison of the echoes received by at least two pulse periods. The high expenditure of Doppler Filters is caused to reduce the influence of the Blind speeds too.
Figure 2: Radar- Time- Line with tree
short- range- pulse periods for MTI- processing.
Division of the Doppler frequency domain into N separate bands offers a very flexible approach towards discriminating against fixed and moving clutter. If moving clutter (such as that from weather or birds) appears with a non-zero mean Doppler shift, the thresholds at the outputs of the various Doppler filters may be raised accordingly.
Figure 3: frequency response characteristics of Doppler Filters as Low Pass
Figure 3: frequency response characteristics of Doppler Filters as Low Pass
Doppler Filter as Low Pass
The basic principle incorporated in all MTD systems is that of a Doppler filter bank, which enables targets and clutter to be separated in the frequency domain. The output of these filters have separate thresholds which optimise detection for the relevant part of the frequency band concerned. It will also be appreciated that the Doppler filters are inherently performing the process of coherent integration.
Ground clutter is shown mainly in the Zero Doppler filter. The Zero Doppler Filter may be carry out as a digital low pass filter. It detects targets with a Doppler frequency corresponding with a radial speed of less than 20 knots. The next filter detects targets with a Doppler frequency corresponding with a radial speed of less than 40 knots, like rain clutter or chaff.
Figure 4: The frequency response characteristics of a typical Doppler filter bank for the case N = 8.
Figure 4: The frequency response characteristics of a typical Doppler filter bank for the case N = 8.
Doppler Filterbank
Division of the Doppler frequency domain into N separate bands offers a very flexible approach towards discriminating against fixed and moving clutter. If moving clutter (such as that from weather or birds) appears with a non-zero mean Doppler shift, the thresholds at the outputs of the various Doppler filters may be raised accordingly.
