An adaptive inter-frequency aiding carrier tracking algorithm for the Mountain-top GPS radio occultation signal

Structures in atmospheric refractivity in the moist lower troposphere can cause scintillation of GPS radio occultation (RO) signals. The RO signal scintillation is characterized by rapid fluctuations in signal carrier phase and deep amplitude fading, which prevent RO receivers from maintaining lock of the signals at low altitudes. This paper presents an initial implementation of an adaptive inter-frequency aiding carrier tracking (I-FACT) algorithm to enable improved tracking performances of the scintillating RO signals. To evaluate the performance of the algorithm, real intermediate frequency (IF) data collected during a mountain-based radio occultation experiment is processed using the I-FACT algorithm as well as a conventional phase lock loop (PLL), an open loop (OL), and a single-frequency adaptive carrier tracking (S-FACT) algorithm. The improved performances of the I-FACT over the other algorithms as well as its limitations are presented in this paper.