Phase Codes for Mitigating Ambiguities in Range and Velocity
Abstract
We review cubic phase codes for mitigating ambiguities in range and velocity before introducing two specific codes. The two have periodicities of 5 and 7 samples for both the transmitted and the modulation code sequences. The short periods are suitable for generating codes of arbitrary length starting with about 15. We abbreviate the two codes with L5 and L7 and describe generation of the codes starting with kernels (i.e., minimum length sequences that repeat to generate the codes of desired lengths). The L5 modulation code produces 5 spectral replicas of the coded signal and the L7 produces 7. We apply the L7 code to a sinusoid and reveal spectra of the modulated signals from several ambiguous range intervals. Through simulation, we show application to weatherlike signals and construct examples whereby two weather signals and ground clutter are overlaid. Using theory, we define the operating region of the codes in the signal parameter space. The region covers a wide range of overlaid returned powers and spectrum widths; it is obtained from simulations involving the L codes and the SZ(8/64) code. The technique is effective in distinguishing the returns from two trip regions separated by no more than L − 2 ambiguous range intervals and reconstructing the corresponding spectral moments. The L5 and L7 codes protect from trip returns up to the fifth and seventh, making them suitable for short-wavelength (3 and 5 cm) radars as their PRTs must be relatively short to accommodate the expected spread of velocities in storms.
SIGNIFICANCE STATEMENT
We propose improved phase codes to mitigate the ambiguities in range for pulsed Doppler weather radars. The currently implemented code SZ(8/64) on the WSR-88D resolves signals from up to 4 range ambiguous intervals 4ra (typically 460 km). The proposed codes extend this to 5ra or 7ra, suitable for scans at elevation angles ≤ 0°. More significant improvements are for radars with shorter wavelengths, like the Terminal Doppler Weather Radar because their ra values are much shorter than on the WSR-88D. The new codes enable equal optimum retrieval (in errors and region of recovery) of Doppler velocities from each ra; the SZ(8/64) yields optimum performance for the first, second, and fourth ra intervals. The next step is their demonstration/validation on weather radars.
© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).
