Measurements of polarimetric parameters at low signal-to-noise ratios

Simultaneous transmission/reception of horizontally and vertically polarized waves has been implemented on the research and development WSR-88D in Norman OK. This is the recommended polarimetric configuration for the WSR-88D network. On the network, reflectivity factor and Doppler velocity are estimated at SNRs above 2 and 3.5 dB respectively. It follows that the polarimetric parameters should be measured at similar SNRs. Low SNRs make differential reflectivity (ZDR) and the copolar correlation coefficient (ñhv) susceptible to noise effects. Novel methods to measure ZDR and ñhv at low SNR are discussed. The methods are based on measurements of the 1-lag correlation functions instead of the powers. At low SNR, the methods have no asymptotic biases and exhibit lower standard deviation of estimates. Radar images of ZDR and ñhv will be presented to demonstrate advantages of the new methods.

Another problem with simultaneous transmission is 3-dB loss in SNR because of power splitting into two channels. The power division reduces the signal-to noise ratio in each channel and leads to some loss of sensitivity in the channels. Schemes of signal recovery at low SNR will be discussed. In one, coherent summation of digital signals in the H- and V-channels is accomplished after removal of differential phase. In the other, autocorrelations of the horizontally and vertically polarized returns are combined. Relative merits of these will be demonstrated on radar images.