Altimetry with reflected GPS signals: results from a lakeside experiment

In the fall of 1999 an experiment was performed at Crater Lake, Oregon, to demonstrate the feasibility of surface altimetry with GPS. A GPS antenna was directed at the lake its axis pointing slightly downward-from a rocky precipice. This arrangement allowed collection of both the direct GPS signal as well as the signal reflected off the lake surface. The relative delay and carrier phase rates between direct and reflected signals are used to infer the height of the lake surface. The site was chosen for its elevation, resulting in clear separation of direct from reflected signal waveforms much of the time. The paper discusses the experimental setup, the data processing steps and the findings of the investigation to determine feasibility and accuracy of this new type of altimetric measurement. Thermal-noise error contributions of 1 cm in 40 seconds can be inferred based on the analysis of carrier phase signals. Delay measurements using the Coarse Acquisition (CA) signals give 1-cm thermal-noise error in about 13 hours. These measurements uncover the systematic instrumentation, processing, and modeling errors germane to future airborne and spaceborne measurements over the ocean.