Generalized SZ phase codes to mitigate range and velocity ambiguities

It is well known that for Doppler radars transmitting uniformly spaced pulses there is a coupling between the maximum unambiguous range and velocity. That is, one can only be increased at the expense of a proportional decrease of the other. Because this fundamental limitation hinders observation of severe weather phenomena, the Radar Operations Center of the US National Weather Service has undertaken the implementation of evolutionary signal processing techniques to mitigate the effects of velocity and range ambiguities on the NEXRAD network. The first technique that was targeted for operational implementation is referred to as Sachidananda-Zrnić (SZ)-2 and has been in use since 2007.

The SZ-2 algorithm is based on systematic phase coding of the transmitted pulses with the SZ(8/64) code. Although the SZ(8/64) phase code results in a very effective recovery of weak overlaid signals, it leads to optimum performance only if the overlaid signal trip numbers differ by one. However, in the current operational implementation of the SZ-2 algorithm, overlaid signals can exhibit trip differences of up to three.

This paper introduces a family of systematic phase codes of the form SZ(n/64). A closer look into the performance of these generalized codes reveals a number of omissions in the early research work. Further, no single code is optimum for all overlay cases, and, surprisingly, the best overall phase code in the SZ family is not the SZ(8/64). Simulations and real weather data are used to illustrate the performance of these codes.