Surface roughness estimation from GPS sea reflections

The research reported in this paper was motivated by a Global Positioning System (GPS) data set collected by researchers at The Johns Hopkins University, Applied Physics Laboratory (JHU/APL) in May 1999. Raw GPS signal data were collected from an aircraft equipped with two downlooking antennas for receiving both left-hand circularly polarized (LHCP) and right-hand circularly-polarized (RHCP) GPS reflections from the ocean surface. Concurrent measurements of the local wind and wave conditions were collected from a nearby research vessel. The measured (LHCP) waveforms were similar to those already reported. The reflected RHCP waveforms were about 10 dB below those from the LHCP antenna. As far as we know, detection of the (depolarized) RHCP waveform has not been previously demonstrated. Models for GPS surface scattering based on geometrical optics or the Kirchhoff approximation predict a very weak depolarized return. We have recently developed a general bistatic scattering model that yields the proper cross section for both the specular and resonant (Bragg) scattering limits and also predicts depolarization for scattering out of the plane of incidence. Comparison of this scattering model with the cross section obtained by exact integration of Maxwell´s equations shows good agreement even for relatively rough surfaces.