John Hutchison & Ronnie Milione
The Philadelphia Experiment Recreated!
This is a method which combines the high energy of a long pulse width with the high resolution of a short pulse width. The pulse is frequency modulated, which provides a method to further resolve targets which may have overlapping returns. The pulse structure is shown in the figure 1.
Now the receiver is able to separate targets with overlapping of noise. The received echo is processed in the receiver by the compression filter. The compression filter readjusts the relative phases of the frequency components so that a narrow or compressed pulse is again produced. The radar therefore obtains a better maximum range than it is expected because of the conventional radar equation.
The ability of the receiver to improve the range resolution over that of the conventional system is called the pulse compression ratio (PCR). For example a pulse compression ratio of 50:1 means that the system range resolution is reduced by 1/50 of the conventional system.
Alternatively, the factor of improvement is given the symbol PCR, which can be used as a number in the range resolution formula, which now becomes:
The compression ratio is equal to the number of sub pulses in the waveform, i.e., the number of elements in the code. The range resolution is therefore proportional to the time duration of one element of the code. The maximum range is increased by the PCR.
The minimum range is not improved by the process. The full pulse width still applies to the transmission, which requires the duplexer to remained aligned to the transmitter throughout the pulse. Therefore Rmin is unaffected.
Pulse compression with linear FM waveform
At this pulse compression method the transmitting pulse has a linear FM waveform. This has the advantage that the wiring still can relatively be kept simple. However, the linear frequency modulation has the disadvantage that jamming signals can be produced relatively easily by so-called „Sweeper”.
The block diagram on the picture illustrates, in more detail, the principles of a pulse compression filter.
Figure 1: Block diagram
The compression filter are simply dispersive delay lines with a delay, which is a linear function of the frequency. The compression filter allows the end of the pulse to „catch up” to the beginning, and produces a narrower output pulse with a higher amplitude.
As an example of an application
of the pulse compression with linear FM waveform the
Filters for linear FM pulse compression radars are now based on two main types.
Pulse compression with non-linear FM waveform
The non-linear FM waveform has several distinct advantages. The non-linear FM waveform requires no amplitude weighting for time-sidelobe suppression since the FM modulation of the waveform is designed to provide the desired amplitude spectrum, i.e., low sidelobe levels of the compressed pulse can be achieved without using amplitude weighting.
Phase-Coded Pulse Compression
Phase-coded waveforms differ from FM waveforms in that the long pulse is sub-divided into a number of shorter sub pulses. Generally, each sub pulse corresponds with a range bin. The sub pulses are of equal time duration; each is transmitted with a particular phase. The phase of each sub-pulse is selected in accordance with a phase code. The most widely used type of phase coding is binary coding.
Origonal content taken from radartutorial.eu
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