Dallas Masters

GPS signal scattering from land for moisture content determination

Stemming from research using GPS bistatically scattered signals to remotely sense ocean surface wind speeds, the authors provide proof-of-concept results for determining soil moisture from GPS reflected signatures. They present land-reflected power measurements from an aircraft-mounted, modified GPS receiver and recorded in different moisture environments. The estimated peak signal power is shown to correlate with land features. As a first attempt to correlate reflected power to soil moisture, the mean reflected power is compared to in situ soil moisture. Recommendations for further analysis to retrieve soil moisture and linking to a GIS database are proposed.

Seasonal polarimetric measurements of soil moisture using tower-based GPS bistatic radar

The results of GPS L-band (L1, /spl lambda/ = 19 cm) surface reflection measurements observed using multiple polarizations and receiving antenna gains are described. The measurements were performed using the 300 m tall ETL Boulder Atmospheric Observatory (BAO) tower during summer through fall of 2002. In this experiment, the first seasonal measurements of bare soil moisture from a stationary location using bistatic reflection of signal of opportunity were performed. Several receiving antennas offering various gain and polarization sensitivities were used. Theoretical modeling of bistatic surface scattering shows that the magnitude and width of the reflected waveform depend on the dielectric permittivity of the soil, vegetation cover, and soil roughness. By observing from a fixed tower over low grass, the roughness of the reflecting area remains constant, hence variations in the signal are uniquely related to changes in the dielectric permittivity, and therefore, to soil moisture. To investigate polarization sensitivity of the reflected signal to soil moisture, four endfire (/spl sim/ 12 dB) antennas with complete circular and orthogonal polarization sensitivities were used. The high-gain antennas increased the received dynamic range and reduced surface multipath radio wave interference. Seasonal retrievals of soil-moisture content from multi-polarization GPS reflection data is presented and compared with in-situ soil moisture measurements.

Airborne GPS bistatic radar soil moisture measurements during SMEX02

To further investigate the potential for remotely sensing soil moisture using the L-band GPS bistatic radar concept, a GPS bistatic radar participated for the first time in airborne measurements during the Soil Moisture Experiment 2002 (SMEX02) in Ames, Iowa. A 12 channel GPS navigation receiver was modified to perform bistatic radar measurements and mounted on the JPL PALS instrument. The reflected GPS signal-to-noise ratio measurements generated a ground track which was sensitive to the surface characteristics. Assuming surface roughness and vegetation cover to be constants over the duration of the study period, the temporal changes in the measured signals were suspected to be proportional to varying soil moisture content. The bistatic signal measurements were interpolated to a UTM grid to produce daily maps of relative change of surface soil moisture over the study region. The maps of the study region showed a transition from very dry surface soil moisture conditions to very wet conditions following precipitation events occurring in the middle of the study period. Additionally, the maps showed sensitivity to localized rainfall in areas without precipitation. The scattered signal measurements were also compared with in situ soil moisture measurements at 32 field sites and found to follow the general soil moisture trend as a function of time. These positive initial results from the first controlled experiment of GPS bistatic radar foe measuring soil moisture were encouraging. Additional analyses with the present data set and comparison with other remote sensing instruments (PALS) are planned as well as participation in future campaigns.

Measurement of Backscattered GPS Signals

This paper describes a technique utilizing GPS ground reflections (GPS bistatic radar) to detect objects with a significant radar cross section located on the surface of the earth. GPS bistatic radar has been shown to be effective as a radar altimeter and for characterization of the reflection surface but has thus far not been shown to be effective for object detection. The technique uses ground reflections with longer path delay than the shortest path specular reflection. Data was collected using a COTS software receiver and post-processed using an in-house tool. Regions with ground reflections were overlayed on aerial imagery to identify possible sources. I. INTRODUCTION The use of GPS signals as a passive radar system is becom- ing increasingly popular as an alternative to radar altimetry. Previously, GPS bistatic radar has been used for aircraft altimetry (1)-(3), remote sensing of ocean parameters (4), measurement of soil moisture content (3), and object detection (2). Thus far, the focus has been on using the shortest path specular reflection. This paper presents results from an airborne GPS bistatic radar experiment. The focus is on how this method can be used as a means of detecting objects on the ground. The technique described make use of further delayed reflections to detect objects. In this section a summary of the data collection campaign and an introduction to the GPS bistatic radar concept is given. A. Bistatic GNSS Radar Concept Unlike monostatic radar where a single system transmits and receives reflected radio frequency (RF) energy, bistatic radar uses geographically separated radar transmitters and receivers. In GPS bistatic radar the transmitter is the GPS satellite and the receiver is located on another satellite or airborne plat- form. This technique compares the direct and ground-reflected signals received from the GPS satellite for remote sensing purposes. This is shown in Figure 1 where a reflecting object is also present. The correlation delay between the two paths when combined with the GPS satellite elevation angle gives a measure of the aircrafts height above ground level (AGL). The strength and shape of the reflected signals correlation waveform provides insight into the ground surfaces reflection properties.

Comparison of sea surface wind speed estimates from reflected GPS signals with buoy measurements

Reflected signals from the Global Positioning System (GPS) have been collected from an aircraft at approximately 3.7 km altitude on 5 different days. Estimation of surface wind speed by matching the shape of the reflected signal correlation function against analytical models was demonstrated. Wind speed obtained from this method agreed with that recorded from buoys to with a bias of less than 0.1 m/s, and with a standard deviation of 1.3 meters per second.

GPS-based remote sensing of ocean-surface wind speed from space

Previous papers have demonstrated the ability to determine wind speed and direction from ocean-reflected GPS signals observed at aircraft altitudes to within 2 m/s and 20 degrees of in situ measurements. This paper presents a preliminary step towards extending GPS-based wind vector retrievals to spacecraft altitudes. The work takes the form of visibility, sensitivity, and noise analyses. The visibility analysis reveals the number of ocean-reflected GPS signals visible per day, a value that drives the uncertainty in wind vector estimates. The sensitivity analysis determines which delay/Doppler cells of a glistening zone visible from space are most sensitive to changes in wind speed and direction. Finally, the noise analysis uses results from the previous two analyses to determine the uncertainty in wind vector retrievals using the cells identified in the sensitivity analysis.

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.