Attila Komjathy

Wind speed measurement using forward scattered GPS signals

Instrumentation and retrieval algorithms are described which use the forward scattered range-coded signals from the global positioning system (GPS) radio navigation system for the measurement of sea surface roughness. This roughness has long been known to be dependent upon the surface wind speed. Experiments were conducted from aircraft along the TOPEX ground track and over experimental surface truth buoys. These flights used a receiver capable of recording the cross-correlation power in the reflected signal. The shape of this power distribution was then compared against analytical models, which employ a geometric optics approach. Two techniques for matching these functions were studied. The first recognized the most significant information content in the reflected signal is contained in the trailing edge slope of the waveform. The second attempted to match the complete shape of the waveform by approximating it as a series expansion and obtaining the nonlinear least squares estimate. Discussion is also presented on anomalies in the receiver operation and their identification and correction.

GPS Signal Scattering from Sea Surface: Wind Speed Retrieval Using Experimental Data and Theoretical Model

Abstract Global Positioning System (GPS) signals reflected from the ocean surface have potential use for various remote sensing purposes. Some possibilities are measurements of surface roughness characteristics from which wave height, wind speed, and direction could be determined. For this paper, GPS-reflected signal measurements collected at aircraft altitudes of 2 km to 5 km with a delay-Doppler mapping GPS receiver are used to explore the possibility of determining wind speed. To interpret the GPS data, a theoretical model has been developed that describes the power of the reflected GPS signals for different time delays and Doppler frequencies as a function of geometrical and environmental parameters. The results indicate a good agreement between the measured and the modeled normalized signal power waveforms during changing surface wind conditions. The estimated wind speed using surface-reflected GPS data, obtained by comparing actual and modeled waveforms, shows good agreement (within 2 m/s) with data obtained from a nearby buoy and independent wind speed measurements derived from the TOPEX/Poseidon altimetric satellite.

Mediterranean Balloon Experiment: ocean wind speed sensing from the stratosphere, using GPS reflections

Abstract The MEditerranean Balloon EXperiment (MEBEX), conducted in August 99 from the middle–up stratosphere, was designed to assess the wind retrieval sensitivity of Global Navigation Satellite Systems Reflections (GNSSR) technology from high altitudes. Global Positioning System reflected signals (GPSR) collected at altitudes around 37 km with a dedicated receiver have been inverted to mean square slopes (MSS) of the sea surface and wind speeds. The theoretical tool to interpret the geophysical parameters was a bistatic model, which also depends on geometrical parameters. The results have been analyzed in terms of internal consistency, repeatability and geometry-dependent performance. In addition, wind velocities have been compared to independent measurements by QuikSCAT, TOPEX, ERS/RA and a Radio Sonde, with an agreement better than 2 m/s. A Numerical Weather Prediction Model (NWPM, the MM5 mesoscale forecast model) has also been used for comparison with varying results during the experiment. The conclusion of this study confirms the capability of high altitude GPSR/Delay-map receivers with low gain antennas to infer surface winds.

Retrieval of Ocean Surface Wind Speed and Wind Direction Using Reflected GPS Signals

Global positioning system (GPS) signals reflected from the ocean surface can be used for various remote sensing purposes. Some possibilities include measurements of surface roughness characteristics from which the rms of wave slopes, wind speed, and direction could be determined. In this paper, reflected GPS measurements that were collected using aircraft with a delay mapping GPS receiver are used to explore the possibility of determining ocean surface wind speed and direction during flights to Hurricanes Michael and Keith in October 2000. To interpret the GPS data, a theoretical model is used to describe the correlation power of the reflected GPS signals for different time delays as a function of geometrical and sea-roughness parameters. The model employs a simple relationship between surface-slope statistics and both a wind vector and wave age or fetch. Therefore, for situations when this relationship holds there is a possibility of indirectly measuring the wind speed and the wind direction. Wind direction estimates are based on a multiple-satellite nonlinear least squares solution. The estimated wind speed using surface-reflected GPS data collected at wind speeds between 5 and 10 ms 21 shows an overall agreement of better than 2 m s 21 with data obtained from nearby buoy data and independent wind speed measurements derived from TOPEX/Poseidon, European Remote Sensing (ERS), and QuikSCAT observations. GPS wind retrievals for strong winds in the close vicinity to and inside the hurricane are significantly less accurate. Wind direction agreement with QuikSCAT measurements appears to be at the 308 level when the airplane has both a stable flight level and a stable flight direction. Discrepancies between GPS retrieved wind speeds/directions and those obtained by other means are discussed and possible explanations are proposed.

Sea ice remote sensing using surface reflected GPS signals

This paper describes a new research effort to extend the application of Global Positioning System (GPS) signal reflections, received by airborne instruments, to cryospheric remote sensing. Their experimental results indicate that reflected GPS signals have potential to provide information on the presence and condition of sea and fresh-water ice as well as the freeze/thaw state of frozen ground. They show results from aircraft experiments over the ice pack near Barrow, Alaska indicating correlation between forward-scattered GPS returns and RADARSAT backscattered measurements.

Extraction of sea state and wind speed from reflected GPS signals: modeling and aircraft measurements

Currently, the wind retrieval from GPS ocean reflections uses the geometric optics model. This model could be insufficient in situations when quasi-specular reflections are affected or dominated by Bragg scattering. A small-slope approximation of the lowest order that accounts for both mechanisms is used for modeling of bistatic cross-section of a rough surface. The paper reports comparison between modeled and measured waveforms of the reflected GPS signal, and the impact of this factor on wind retrieval is discussed.

An improved high precision ionospheric total electron content modeling using GPS

Space-based radio navigation systems such as the Global Positioning System (GPS) can provide us with a unique opportunity to study the effect of the ionosphere as the signals propagate from the satellites to the GPS receivers. Based on a modified version of the University of New Brunswicks (UNB) DIPOP software package, the authors enhanced the algorithm to model ionospheric total electron content (TEC) using dual frequency GPS observations from stations of the International GPS Service (IGS). The algorithm uses a spatial linear approximation of the vertical TEC above each IGS station using stochastic parameters in a Kalman filter estimation to describe the local time and geomagnetic latitude dependence of the TEC. The authors demonstrate that the enhanced UNB algorithm in conjunction with an IRI-95 update procedure is capable of modeling the diurnal variation of TEC even during a solar storm period in the low latitude region using a higher order surface estimation technique for each GPS station processed.

Student Reflected GPS Experiment (SuRGE)

SuRGE is a low-Earth orbit satellite proposed for an 8-month mission to establish the possibility of spaceborne bistatic GPS remote sensing of ocean surface wind speed/direction, sea surface elevation and soil moisture. Equipped with two GPS reflection instruments and three downward-looking antennas, SuRGE is optimized to validate this measurement technology. Through a nadir facing high-gain (/spl sim/ 20 db) antenna, a delay and Doppler mapping receiver will continuously record and process GPS reflections. Using this same antenna an analog translator will capture GPS reflections, downconvert the signals and rebroadcast them to properly equipped ground stations where the data can be post-processed to also retrieve the ocean and land surface properties.