Processing low orbiting beacon satellite data has been a challenge largely because of the limited vehicle space for a VHF, UHF, L-band antenna system. The effective antenna patterns severely limit range of uniform illumination range with a common phase center. Low SNR, particularly at acquisition severely, constrains phase-locked-loop (PLL) performance. However, with post-pass processing substantially more processing can be devoted to the critical frequency tracking operation. Several schemes have been used effectively to construct a narrow band filter centered on the Doppler frequency defined over a local time segment. An analysis of these procedures was presented in a paper REF.
In the paper it was hypothesized but not demonstrated that typical PLL performance was degraded for SNRs below 10 dB to the point that direct frequency estimation was a preferable alternative. The note SoftwarePLL compares frequency estimates derived from a conventional PLL and direct frequency estimation.