Abstract
A linear characteristic signal, which is converted from the output amplitude of a logarithmic receiver, can be used for MTI weather radar's signal processing.
This paper describes the input-to-output characteristics (normal and MTI signals) of precipitation echoes due to the technique investigated by simulation. There are two errors affecting the system's performance, that is, the limitation of the receiver dynamic range and the quantization effect of A-D conversion.
The amplitude ratio between the adjacent levels which gives the A-D conversion's quantization resolution is uniform for the whole input range of the logarithmic receiver. Therefore, the reproducibility in the small amplitude region is essentially superior to that of the linear receiver technique. However, since the following log-lin conversion makes the sensitivity decrease in the small amplitude region, it is necessary to increase the bit number of the conversion by 4 bits, by which the accuracy can reach the limit decided by the A-D conversion's resolution of the log receiver itself.
Concerning the linearity of the characteristics of the system, in the small signal region, the normal signal causes positive errors and the MTI signal negative errors. In the saturation region, on the contrary, the MTI mean amplitude decreases for the input signal amplitude beyond some specified value, because the MTI signal is processed from the amplitude difference between the adjacent pulses. Therefore, the overall characteristics of this technique are determined by the MTI signal processing. In cases where the bit numbers of the system dynamic range and the A-D conversion are both set in 8 bits, the minimum measurable rainfall intensity is 0.46 mm/hr and the unambiguous rainfall intensity reaches 138 mm/hr.
It is stated that the MTI signal amplitude can be calibrated with the measured normal signal amplitude even though it is also accompanied with the system's quantization error.
