A study on the MTI weather radar system for rejecting ground clutter.

Abstract

This paper describes a noncoherent MTI weather radar which makes it possible to reject ground clutter signals but to extract precipitation echo signals. Though the amplitudes of the ground clutter signal fluctuate somewhat due to radar system instabilities, antenna scanning and radar wave propagation effects, the frequency spectra are confined to a relatively narrow low region. Therefore, MTI signal processing is based on the fact that most of the energy of the clutter signal can be rejected by an MTI canceler which corresponds to a high pass filter. In this study, it was found that a no-feedback triple delay-line canceler is the most suitable for the standard weather radar of the Japan Meteorological Agency and that the attainable cancellation ratio is 35 dB, because the uniform noise cannot be rejected by the MTI canceler. Then the MTI residues which cannot be rejected by the canceler will be completely removed by introducing two additional techniques. The first one is an areal integration which makes it possible to decrease some of the MTI signal intensity levels below the threshold level, and if this is not enough, the second one will be used to reject compulsorily the residues of the clutter beyond the specified value of cancellation ratios. Meanwhile, since the frequency spectra of the precipitation echo signal are rather uniformly distributed in half of the pulse recurrence frequency which corresponds to the signal processing frequency band, part of the energy of the signal can be extracted through the canceler. Therefore, by considering the deviation from Rayleigh distribution of the signal in amplitude and the randomness of the amplitude fluctuation, it was found that the MTI signal processing can be used quantitatively for radar rainfall measurements referring to the normal signal processing in the noncoherent radar. This paper further discusses the rejection of sea clutter. It is found that the proposed MTI signal processing system will make it possible to rapidly obtain objective radar data, as compared with the manual data handling of ground clutter rejection which has been conventional so far. And it will contribute to the development of a system of short-period rainfall prediction based on the radar data transmitted from each radar site in the country to the large-sized computer in JMA.