Robust Closed-loop Tracking of Airborne Low-Elevation GPS Radio-Occultation Signals

GNSS signals during radio occultation (RO) events have been used to derive properties of the atmosphere. Current GNSS RO observations are mainly relied on open-loop (OL) tracking of low-elevation satellite signals. The reason is that GNSS signals at low elevations are adversely affected by severe multipath effects due to propagation through lower troposphere structures and Earth surface scattering. The conventional phase-lock loop (PLL) and delay-lock loop (DLL) may lose lock of these signals with strong multipath interference signatures. While OL is known for its robustness, the wide bandwidth associated with OL introduces large errors in the satellite signal parameters estimates. In this paper, we will present a Kalman-filter based closed-loop (KFC) tracking method for airborne GPS RO signals that has comparable robustness with OL tracking and improved accuracy. The KFC tracking robustness, accuracy, and the corresponding retrieval of bending angles are evaluated using real signals recorded during two experimental flights. The results are compared with that of OL tracking methods.