Spaceborne Coherent GNSS Reflection Signal Processing over Complex Terrain

Abstract

This paper presents an onboard complex waveform or Delay Doppler map (DDM) processing approach for spaceborne coherent-reflection GNSS signal reflected off complex terrain. A digital elevation map (DEM) is needed for the processing of land-reflection signals. For onboard processing, the slope of the land surface is usually not considered or accurately modeled due to the computation limit, and this will cause mis-modeling of the SP location and errors in the code phase and Doppler frequency models for signal reflected off complex terrain, such as in the mountains. Discrepancies of up to 3 chips of GPS C/A code are shown in this paper for the code phase models from the processing results of CYGNSS raw data. The complex waveform/DDM processing presented in this paper can be configured for various ranges and spacings of the code phase and Doppler frequency shifts to allow various levels of model uncertainties. An example is presented for GPS signal reflected off the Himalaya region. The complex waveform processing result shows coherent reflection measurements that may not be captured by the normally applied open loop (OL) tracking approach, and the result also includes probable observations of glacier extent and surface variations.