An Adaptive Carrier Tracking Algorithm for Low Altitude Mountain-based GPS Radio Occultation Measurement

Abstract

Sharp gradients 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 phase and deep amplitude fading, which prevent RO receivers from retrieving meaningful data at low altitudes. To facilitate the development of advanced RO receiver technologies, mountain-based RO (MRO) experiments were conducted and raw IF samples of RO signals were collected. In this paper, an adaptive carrier-tracking (ACT) algorithm is applied to the MRO IF data. The adaptive carrier-tracking loop performance is compared with an open loop (OL) tracking and conventional closed phase lock loop (PLL). The results demonstrate that the ACT can maintain lock of RO signals and generate meaning Doppler frequency estimations at lower altitudes compared to the other two methods. The algorithm and comparative processing results of the MRO data are presented in the paper.