Lloyd A. Linstrom

Delay-Doppler analysis of bistatically reflected signals from the ocean surface: theory and application

We present a new stochastic theory for delay-Doppler mapping of the ocean surface for bistatic scattering. This stochastic theory should complement nicely the previous theories for the Global Positioning System (GPS) reflected signals from ocean surfaces, especially that of Zavorotny and Voronovich (2000). We quantify the Doppler spread of the reflected signal before interpreting the delay. Our theoretical results compare very well to Doppler spectra computed using data collected during an airborne campaign. The bandwidth of the spectra is linked to the geometry and to the ocean roughness. The bulk of the Doppler spread is caused by the rms slope and not by the surface orbital velocity. Our stochastic theory is generalized to include the delay mapping made possible by the existence of the pseudorandom noise code on the GPS L-band carrier. These results can be seen as a generalization of Woodwards theorem for FM signals to delay-Doppler analysis of more complicated signals. Our formulation is amenable to inversion for the determination of geophysical parameters such as surface wind vector and mean sea level. Another novelty in our approach is the inclusion of the sea state.

Special sensor ultraviolet spectrographic imager: an instrument description

We describe the Special Sensor Ultraviolet Spectrographic Imager (SSUSI). This instrument consists of a scanning imaging spectrograph (SIS) whose field-of-view is scanned from horizon to horizon and a nadir-looking photometer system (NPS). The SIS produces simultaneous multispectral images over the spectral range 1 150 to 1800A. The NPS consists of three photometers with filters designed to monitor the airglow at 4278A and 6300A and the terrestrial albedo near 6300A. SSUSI will fly on the DMSP Block 5D3 satellites S-16 thru S-19. The instruments will be calibrated at the Applied Physics Laboratorys Optical Calibration Facility.

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 Maxwells equations shows good agreement even for relatively rough surfaces.

Doppler analysis of GPS reflections from the ocean surface

The basic physics that governs the scattering of GPS signals from a wind-roughened sea surface (both delay and Doppler characteristics) is reviewed. We then describe the results of recent experimental campaigns in which reflected GPS signals have been recorded in the vicinity of coastal buoys or research vessels where independent in situ measurements were available. Comparisons between the measured and predicted Doppler spectra at both left- and right-hand circular polarization are presented and discussed. These polarization differences in the Doppler characteristics could lead to a more robust estimation of various geophysical parameters related to sea-surface roughness.