Signal processing.

The above measurements were obtained with a relatively simple PC-based digital signal processor described by Rison et al., 1993. The processor was originally developed for the CaPE program in 1991 and first used to obtain electrical alignment measurements of ice crystals (Krehbiel et al., 1996). Two Motorola 56001 digital signal processors averaged the signals from 32 transmitted pulses (16 ms at a 2 kHz pulse repetition frequency), at each of 250 one-microsecond range gates (37.5 km range). (The radar transmitted pulses at a 4 kHz rate, a holdover from its original operation as an alternating right- and left-hand circular polarization system.) One DSP processed the digitized outputs of matched logarithmic IF amplifiers in each receiver channel to obtain W_H and W_V. The other DSP correlated the outputs of coherent, constant-phase amplitude limiters in the two receiver channels to obtain the magnitude and phase of $\hat \rho_{HV}$. The raw averaged data values were read out by the host PC and stored on disk for the post-processing. At the same time the data were processed by the host PC to generate a real-time display. Although the data shown is from post-processing, essentially the same software used to generate the real-time display. The different variables and presentation formats are therefore available in real time to guide the measurements. The real-time/post-processing software additionally smoothed each variable using a running 3-gate (450 m) range average and a 3-ray running average. Since each ray consisted of the average of 32 transmitted pulses to begin with, a total of $9 \times 32 = 288$ samples were averaged. (An additional 3-gate running average was used to further smooth the range-differentiated $K_{\rm dp}$ values.)