give information about synthetic aperture radar system
The synthetic aperture radar system includes a multi-beam antenna having many reception beams in different direction from one another, the multi-beam antenna being adapted to receive radar echoes from objects. synthetic aperture radar system can detect a low-speed moving object without degradation of the cross-range resolution, enable the low-speed moving object to display without positional error, and does all these without reduction of the transmission antenna gain.
This describes a synthetic aperture radar system utilizing the Doppler effect caused by the movement of a radar platform to improve the cross-range resolution and also the capacity to ddetect a moving object. The width the reception beams is selected in such a way that the band width of a Doppler shift in the radar echo of a moving target is broader than that of the Doppler shift in the radar echo of a stationary target. This radar echo is pulse compressed to increase the range resolution. the radar echo is filtered to separate the ones of the moving and that of the stationary objects from each other. These radar echoes subjected to Fourier transform with respect to the distance between the moving platform and the objects.As a final step, the spectrum of the radar echo from the moving object is further shifted such that the center frequency of the Doppler shift due to the velocity of the object becomes zero. This algorithm of reception, pulse compression, frequency shift and Fourier transform are done for every reception beam.
SUMMARY of the instrument
It is a system mounted on a moving platform and includes:
1) a multi-beam antenna having reception beams in different directions.
2)cross-range resolution improving system containing moving object echo separating means, moving object spectrum transforming means, moving object spectrum synthesizing means , eference spectrum generating means, moving object side multiplying means , moving object spectrum inverse transform means.
the multi-beam antenna recieves the signals and a filtering enables the separation between the stationary and moving objects. The separation between the stationary and moving objects and the transform to the spectrum are carried out for each reception beam.Thus the apparent reception beam width is enlarged into the total width of the actual reception beams.Thus this apparatus can detect a low-speed moving object without degradation of the cross-range resolution.
The whole system working can be described as:
First of all, the frequency of the radar echo acquired by the multi-beam antenna is shifted, for reception beam, depending on the center frequency of the Doppler shift contained in the echo of the stationary object which his removes the Doppler shift due to the velocity of the moving platform from the radar echo. beam is high-pass filtered after the frequency shifting and echo of moving object is extracted from each reception beam. If low pass filtering is done, you get the echo of the stationary object. This is resampled to reduce the amount of data and then transformed into a spectrum.. Doppler shift due to the velocity of object is compensated. Then the echoes are multiplied by the respective reference spectrums. Finally, The result of inverse transform from the moving object spectrum inverse transform means is transformed as into information suitable for use in display.
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