National Oceanic and Atmospheric Administration Environmental Technology Laboratory

Bistatic GPS Signal Scattering from an Ocean Surface: Theoretical Modeling and Wind Speed Retrieval from Aircraft Measurements

V. U. Zavorotny

Cooperative Institute for Research in Environmental Sciences,
University of Colorado/NOAA, Environmental Technology Laboratory
325 Broadway, Boulder, CO 80303-3337
email: Valery.Zavorotny@noaa.gov
Phone:303-497-6616; Fax: 303-497-6181

This work was supported by NASA Headquarters under the Physical Oceanography Program RTOP 622-47-55

Workshop on Meteorological and Oceanographic Applications of GNSS Surface Reflections: From Modeling to User Requirements July 6th 1999, De Bilt, The Netherlands

Acknowledgments
The presentation is based on results obtained by:

• P. Axelrad
• G. Born
• J. Garrison
• S. Katzberg
• A. Komjathy
• A. Voronovich
• V. Zavorotny

Outline of talk

• Introduction
• GPS as a bistatic radar scatterometer
• Modeling of GPS bistatic surface scattering
• Aircraft experiments with the delay-mapping GPS receiver
• Wind-speed retrieval using experimental data and theoretical model
• Future directions: delay-Doppler mapping, satellite-borne instruments, other geophysical parameters
• Conclusions

Introduction

• GPS gave birth to many new applications
• The bistatic geometry, circular polarization and special temporal structure of GPS signals provide unique opportunities for remote sensing
• Currently the feasibility of an airborne GPS delay- mapping scatterometer is shown
• Ocean-surface state and wind speed retrieval has been demonstrated using this scatterometer
• A satellite GPS scatterometer is under the consideration
• A concept of the high-resolution, fully-polarized, delay-Doppler-mapping, bistatic GPS radar is proposed
• Other applications: wave height, wind vector, wetland, salinity, ice state determination

Cox&Munk photographs of the sun's glitter (to give an idea how surface reflections form a glistening zone)

Geometry of multistatic GPS radar system

By employing code-range measurements, a receiver can distinguish particular annulus zones on the ocean surface illuminated by GPS signals.

Modeling of GPS bistatic surface scattering

In the GPS receiver the signal at time t₀ is cross-correlated with a replica of PRN code a taken at a different time t₀ -

(1)

The average power density as a function of a time delay :

(2)

Eq.(2) is a bistatic-radar equation with the function D^{2 2} |S|²as a resulting radar footprint. An annulus zone appears due to the code-correlation process:

(3)

Function |S|² defines a Doppler zone. The scattering cross-section ₀ in the geometrical optics leads to quasi-specular scattering:

(4)

W is the PDF of surface slopes. The width of ₀ in terms of defines a glistening zone
For isotropic Gaussian slopes:

(5)

Model for ocean surface statistics

• Statistics of the slopes is assumed to be Gaussian, with anisotropic surface slopes having wind-dependent variances and correlations.
• These parameters can be derived from a solely wavevector spectrum of full surface elevations by integrating it over wave numbers smaller than a dividing parameter of the two-scale model

• Two spectral models for sea roughness were used in the case of well-developed seas: (a) J. R. Apel, JGR, 99, 16,269, (1994); (b)T. Elfouhaily, et al. JGR, 102, 15,781, (1997).

Waveforms for various receiver altitudes and wind speeds
at =45° (a) h = 1 km; (b) h = 10 km; (c) h = 300 km.

Click for full-size image.

Waveforms with and without taking into account Doppler spreading for h = 5 km, =45°, v_rec=0.17km/s.

Click for full-size image.

Waveforms obtained at different integration times T_i
(h=225 km, =45°, v_rec=7.9km, U=8m/s, _c=0.1 µs)

Click for full-size image.

Aircraft experiments with the delay-mapping GPS receiver

• For the receiver, a commercially available GPS-development kit from GEC Plessey (now MITEL) including a 2021 correlator chip fed by two 2010 RF front ends were used
• Two low-gain L-band antennas were used: a zenith-oriented RHCP antenna, and a nadir-oriented LHCP antenna
• Aircraft remote sensing experiments started since 1997
• November 1997: NASA Stennis Space Center (NASA Super King Air B-200 aircraft; Gulf of Mexico)
• May 1998: Wallops Island (C-130 aircraft; Gulf Stream )
• August 1998: Wallops Island (Virginia Space Grant Consortium Balloon Experiment)

LHCP antenna on bottom of fuselage of Langley B-200 aircraft.

Delay-mapping receiver hardware in the Langley B-200 aircraft.

The delay-mapping GPS receiver

The shape of the waveform versus code delay is a function of ocean surface roughness and hence wind speed.

The experimental waveforms for PRN21 agrees well with the theoretical model for wind speeds 8-10 m/s.

Scheme illustrating the formation of footprints.

Sensitivity to the wind direction: Function versus Doppler offset for six wind directions
(H=5km, U=8m/s, teta=90°, Ti = 10ms)

Click for full-size image.

Fully-polarized delay-Doppler mapping using GPS signals

• The current delay-mapping technique has several limitations: the footprint is an elliptical zone, a spatial resolution is low, a relatively low SNR, only a left-hand circularly polarized antenna for surface scattered signal
• Adding a second down-looking right-hand circularly polarized antenna makes the system fully-polarized
• Larger coherent integration times and scanning of Doppler compensation frequency make the system similar to the synthetic aperture radar
• The intersection of hyperbolic equi-Doppler zones and elliptical equi- range zones form a small footprint on the mapped surface
• A higher spatial resolution can be achieved using the P-code sequence of the GPS signal, or using a modified signal processing

Conclusions

• It has been demonstrated in aircraft experiments that GPS scattered signals reflected from the ocean surface can be used as a remote-sensing tool, particularly for wind-speed retrieval
• Theoretically modeled GPS surface scattering agrees well with experimental data
• Potentials of this technique can be enhanced by employing fully- polarized delay-Doppler mapping receiver
• Surface-reflected GPS signals can be used in various other remote-sensing applications: wind vector, wave height, salinity, ice studies, delineation of wetlands
• Satellite-borne instruments will give a unique opportunity to use GPS as a global scale remote-sensing tool to infer various Earth surface parameters

NOAA | ETL | Radar | Bistatic GPS Signal Scattering from an Ocean Surface: Theoretical Modeling and Wind Speed Retrieval from Aircraft Measurements: http://www.etl.noaa.gov/~vzavoronty