A search for better methods to reduce range and velocity ambiguity in the WSR-88D radar data has resulted in the discovery by the National Severe Storms Laboratory in Norman, OK of a set of “optimal” systematic phase code sequences known as SZ codes. These codes were originally designed by Chu nearly 30 years ago for communications applications. A testbed WSR-88D (KOUN) located in Norman Oklahoma and the NCAR S-Pol 10cm radar located in Boulder Colorado have been equipped by the National Center for Atmospheric Research in Boulder Colorado with the necessary hardware to transmit these SZ phase coded pulse sequences, and record the I and Q time series signal returns. A particularly useful sequence is the SZ (8/64) code, which minimizes the correlation of adjacent-trip return echoes, using a sequence that repeats every 32 hits. With suitable processing, a second or Nth-trip echo can be “whitened” (2nd trip is split into eight equally spaced spectra in the radar’s Doppler frequency Nyquist interval), while the first-trip echo remains “cohered” for normal Doppler processing. This paper will describe tests of echo separation, using I and Q time series data collected with the KOUN WSR-88D radar and the NCAR S-Pol radar. Data collected includes a squall line at long range in northwest Oklahoma, which would normally produce a second trip echo that would overwhelm weaker 1st trip clear-air returns. Using a separation technique that will be described in this paper, a good separation of 1st and 2nd-trip echoes is usually obtained. The measured separation routinely exceeds 30db, and often exceeds 40dB.
We will discuss performance of this technique as determined by looking at a variety of data which includes combinations of first and second-trip echoes. Some of the data contains target signatures that are quite different from the theoretical data used to predict the performance of the SZ code. We will discuss algorithm considerations that help make the method more robust to these situations.