Low Elevation Measurements of GPS Ocean Reflections

High altitude GPS measurements made at the Haleakala Summit on Maui, Hawaii, have revealed the spectral characteristics of the direct satellite signal and the ocean reflected signal. The altitude of the observation site (above 3000 meters) gave a long line-of-sight view over the ocean to the horizon with multi-paths primarily due to the ocean surface roughness. The characteristics of the reflected signal depend on the scattering properties of the sea surface and the footprint of the reflection zone. The footprint size and shape in turn depends on the signal incidence angle and the relative velocities of transmitter and receiver to the reflection point. The scattering properties of the sea surface are related to the roughness, which depends on sea wave characteristics. We present the spectral properties of the signals as received by a high precision GPS instrument, simultaneously in both phase-locked mode and open-loop raw mode in separate receiver channels. The instrument setup consists of separate L1 and L2 antennas both oriented with the main gain lobe toward the horizon. The use of directive antennas pointed towards the horizon enables signal recordings down to the lowest layers of the atmosphere. The measurements of the low elevation grazing GPS signals reveal the incoherent scattered process in the reflection zone. Thus a radio occultation retrieval technique for the phase differences between the direct and reflected signal has been applied combined with a statistical retrieval method. The results are derived through a sequential Bayesian estimation method, where the retrieval algorithms are based on a particle filtering technique. The horizontal size of the probability density function, which uniquely describes the ocean reflection zone using the recursive particle filter method, totals from 200 to 500 meters for all data sets.