Clutter- Doppler Filter (2)
Another method for clutter suppression is called “Moving Window” method too.¹) In this method, two complete pulse periods (PRP) are stored unweighted. In the third period they are compared with each other.
Figure 1: Functional block diagram of a Doppler-Filter using “Moving Window” techniques
Figure 1: Functional block diagram of a Doppler-Filter using “Moving Window” techniques
Figure 1: Functional block diagram of a Doppler-Filter using “Moving Window” techniques, (interactive picture)
In the third PRP, all three data streams are weighted, added and rescaled. The weighting happens in the same manner to as in the algorithm “Integrate & Dump”. E.g. Data from the first and the third PRP will be multiply with the factor 0.5; the second PRP with the factor -1. Echo signals having in these three period the same phase and amplitude are summarized to zero. If the echo signal fluctuates (by origin of a moving target) then these three periods are summarized to a nonzero value.
Here again may arising from the carry additional bits. These must be converted back to its original format by a rescaler. Often, just the bits with the smallest value will be truncated. This has the disadvantage that very weak echo signals are even smaller. Sometimes are used more complex rescaling algorithms, therefore.
The usable difference compared with the Integrate & Dump- procedure is that there regardless of the number of pulse periods the MTI-wiring produces only one output signal. Here the number of pulse periods used (output signal) is limited by the circuitry. This results in N-2 output signals by N MTI processing periods. This procedure is primarily used when the radar requires a higher number of hits per scan for pulse integration.
In-phase & Quadrature Data
The input signals are digital data with real part (in phase) and imaginary (quadrature) of the echo signal.
Both signals are processed here still separated.
Weighting Multiplier
The single echo pulses in subsequent pulse periods are multiplied by a given factor, that the intended goal,
constant pulses of three pulse repetition periods become zero, can be achieved mathematically.
These factors are called weight.
Weights
In this example given the weights are defined:
in the first pulse period to as +0,5
in the second pulse period to as - 1
in the third pulse period to as +0,5
Memory
In this memory, the full pulse period is stored in their entire bit width.
Often these are organized as a serial memory (FIFO - First In, First Out),
having a memory cell for each range cell.
Adder
The adder summarizes all three values in the third PRP.
Fixed targets will substantially cancel each other.
Moving targets will have an output signal.
Rescaler
The output signal is growth in the bit width due to the many processes of arithmetic (Carry bit!).
Usually it is rescaled by insignificant bits are omitted and only the original bit width is forwarded.
There are also more complex rescaling algorithms possible.
¹) Please, don't confuse it with the Sliding Window in the plot extractor!