Michael S. Grant

Demonstration of Bistatic Radar for Ocean Remote Sensing Using Communication Satellite Signals

Remote sensing of ocean roughness using reflected signals from digital communication satellites is demonstrated in an airborne experiment. Transmitted data are approximated as an infinitely long sequence of random bits, which is experimentally a hypothesis confirmed for the S-band XM radio signal. On July 2, 2010, a signal recorder was flown at an altitude of 3.17 km off the coast of Virginia, collecting ocean-reflected signals from both geostationary satellites identified as “Rhythm” and “Blues,” which were broadcasting the XM radio signal. Direct and reflected signals from the same channel were cross-correlated, producing a waveform that agreed well with a model generated at the 7.5-m/s wind speed reported from the Chesapeake Lighthouse. Adjusting this model to fit the experimental data produced an optimal estimate of 6 m/s. A Monte Carlo approach predicted errors of 0.5% from the simulated reflected XM radio signals and 2%-10% from simulated reflected Global Navigation Satellite System (GNSS-R) signals. This improvement was attributed to the higher ( ~ 30 dB) power in the XM radio signal. The availability of communication satellite transmissions, in all frequency bands used for remote sensing, opens the possibility of using signals of opportunity as low-cost alternatives to radiometry or scatterometry.

Estimation of Sea Surface Roughness Effects in Microwave Radiometric Measurements of Salinity Using Reflected Global Navigation Satellite System Signals

In February-March 2009, an airborne field campaign was conducted using the Passive Active L- and S-band (PALS) microwave sensor and the Ku-band Polarimetric Scatterometer to collect measurements of brightness temperature and near-surface wind speeds. Flights were conducted over a region of expected high-speed winds in the Atlantic Ocean, for the purposes of algorithm development for sea surface salinity (SSS) retrievals. Wind speeds encountered during the March 2, 2009, flight ranged from 5 to 25 m/s. The Global Positioning System (GPS) delay mapping receiver from the National Aeronautics and Space Administration (NASA) Langley Research Center was also flown to collect GPS signals reflected from the ocean surface and generate postcorrelation power-versus-delay measurements. These data were used to estimate ocean surface roughness. These estimates were found to be strongly correlated with PALS-measured brightness temperature. Initial results suggest that reflected GPS measurements made using small low-power instruments can be used to correct the roughness effects in radiometer brightness temperature measurements to retrieve accurate SSS.

Three-dimensional wind profiling of offshore wind energy areas with airborne Doppler lidar

Abstract. A technique has been developed for imaging the wind field over offshore areas being considered for wind farming. This is accomplished with an eye-safe 2-μm wavelength coherent Doppler lidar installed in an aircraft. By raster scanning the aircraft over the wind energy area (WEA), a three-dimensional map of the wind vector can be made. This technique was evaluated in 11 flights over the Virginia and Maryland offshore WEAs. Heights above the ocean surface planned for wind turbines are shown to be within the marine boundary layer, and the wind vector is seen to show variation across the geographical area of interest at turbine heights.

GPS Remote Sensing Measurements Using Aerosonde UAV

In February 2004, a NASA-Langley GPS Remote Sensor (GPSRS) unit was flown on an Aerosonde unmanned aerial vehicle (UAV) from the Wallops Flight Facility (WFF) in Virginia. Using direct and surface-reflected 1.575 GHz coarse acquisition (C/A) coded GPS signals, remote sensing measurements were obtained over land and portions of open water. The strength of the surface-reflected GPS signal is proportional to the amount of moisture in the surface, and is also influenced by surface roughness. Amplitude and other characteristics of the reflected signal allow an estimate of wind speed over open water. In this paper we provide a synopsis of the instrument accommodation requirements, installation procedures, and preliminary results from what is likely the first-ever flight of a GPS remote sensing instrument on a UAV. The correct operation of the GPSRS unit on this flight indicates that Aerosonde-like UAVs can serve as platforms for future GPS remote sensing science missions.

Combined GPS Reflected Signal and Visual Imagery for Unsupervised Clustering and Terrain Classification

Use of the Global Positioning System (GPS) L-band (1.57542 GHz) surface-reflected signal for remote sensing is a relatively recent development. In the current study, GPS satellite reflected-signal surface reflectivity, computed along the spectral point ground track across an agricultural area, is combined with 1m x 1m orthorectified, grayscale imagery to form a 2-dimensional (2D) space where unsupervised data clustering and classification is performed. Unlike intensity-only (1D) clustering and classification of the visual data, the moisture-sensitive GPS reflected signal allows water features to be readily differentiated from heavy vegetation and cultivated fields. To our knowledge, this study is the first use of the reflected GPS signal in combination with visual imagery for remotely sensed terrain data clustering and classification.

Analysis of the correlation properties of digital satellite signals and their applicability in bistatic remote sensing

This paper presents a study of relevant correlation properties of signal transmitted from commercial communication satellites in order to evaluate their potential use as “signal of opportunity” for bistatic remote sensing. The ambiguity function for the XM radio satellites was computed analytically from published information on the modulation schemes and bandwidth, under the assumption that the data modulation is random. The model was then experimentally tested by recording the received signals from these satellites. Next, a cross-correlation waveform for digital signal reflected from random rough surface was simulated. Scattering model that were originally developed for Global Navigation Satellite System (GNSS-R) signals was applied to the modified simulator to incorporate the derived ambiguity function. The simulator was then used to generate synthetic waveform with a realistic signal to noise ratio (SNR). Retrieval algorithms for ocean surface roughness and reflectivity that were derived originally for GNSS-R, were applied to these simulated signals. Non-linear least square methods were applied to invert a scattering model and estimate the slope variances of the probability density function (PDF), which best fits the measurements of the reflected XM signal waveform. The SNR for the experimental data was found to be within 0.5dB of the theoretically calculated SNR.

Anisotropy in ocean scattering of bistatic radar using signals of opportunity

This paper present experimental results demonstrating the use of “signals of opportunity” from digital communications satellites (XM radio) as bistatic radar for ocean remote sensing. This builds upon the previous work which demonstrated that the shape of the cross-correlation “waveform” of reflected XM radio signals is sensitive to the roughness of the ocean surface. In these new results, we compare this sensitivity between the waveforms produced from the two XM radio satellites, viewed simultaneously at different azimuths, and show that a small discrepancy exists in the mean square slope (MSS) retrievals obtained from each of them. We then investigate the hypothesis that this discrepancy is the result of neglecting anisotropy in the model for the probability density function (PDF) of surface slopes and that this discrepancy might be useful for sensing the wind direction. In order to do so, a two-stage estimation process was applied to data collected on an airborne experiment that recorded the direct line of sight and reflected XM radio signals. In the first step, an isotropic normal distribution was assumed for the PDF and the mean square slope (MSS) was fit to the measured waveform data from each satellite independently. Since the two satellites are located at different azimuths, a difference between the two MSS estimates were observed. The second step involved using a bidirectional normal PDF with MSS constrained to that obtained from the first step, and a value was assumed for the ratio of upwind and crosswind slopes. The direction of the principal axes was varied to minimize the total residuals for both satellites. The results were compared with Chesapeake Lighthouse recordings of the local wind direction.

Application of reflected Global Navigation Satellite System (GNSS-R) signals in the estimation of sea roughness effects in microwave radiometry

In February-March 2009 NASA JPL conducted an airborne field campaign using the Passive Active L-band System (PALS) and the Ku-band Polarimetric Scatterometer (PolSCAT) collecting measurements of brightness temperature and near surface wind speeds. Flights were conducted over a region of expected high-speed winds in the Atlantic Ocean, for the purposes of algorithm development for salinity retrievals. Wind speeds encountered were in the range of 5 to 25 m/s during the two weeks deployment. The NASA-Langley GPS delay-mapping receiver (DMR) was also flown to collect GPS signals reflected from the ocean surface and generate post-correlation power vs. delay measurements. This data was used to estimate ocean surface roughness and a strong correlation with brightness temperature was found. Initial results suggest that reflected GPS signals, using small low-power instruments, will provide an additional source of data for correcting brightness temperature measurements for the purpose of sea surface salinity retrievals.

Selected CERES electronic component survivability under simulated overvoltage conditions

In August, 1998 a Clouds and the Earths Radiant Energy System (CERES) instrument telemetry housekeeping parameter generated a yellow warning message that indicated an on-board + 15V Data Acquisition Assembly (DAA) power converter deregulation anomaly. An exhaustive investigation was undertaken to understand this anomaly and the long-term consequences which have severely reduced CERES operations on the Tropical Rainfall Measuring Mission (TRMM) spacecraft. Among investigations performed were ground tests that approximated the on-board electronic circuitry using a small quantity of flight identical components exposed to maximum spacecraft bus over-voltage conditions. These components include monolithic integrated microcircuits that perform analog signal conditioning on instrument sensor signals and an analog- to-digital converter (ADC) for the entire DAA. All microcircuit packages have either a bipolar silicon design with internal current limiting protections or have a complementary metal oxide semiconductor (CMOS) design with bias protections. Ground tests that have been running for approximately 8 months have indicated that these components are capable of withstanding as much as twice their input supply voltage ratings without noticeable performance degradation. These data provide CERES operators with confidence of being able to continue science operations over the remaining life of the TRMM mission. This paper will discuss this anomaly and some possible causes, a simulator of affected electronics, test results, prognosis for future CERES operations, and conclusions.