This data set contains measurements of Global Positioning System (GPS) signals reflected from the Earths surface and collected on an airborne platform. The measurements were taken over the Soil Moisture Experiments 2002 (SMEX02) Walnut Creek Watershed area in Iowa, USA. The reflections collected were from flat agricultural land with mostly corn and soybean crops in mid-development. Data were collected on 19, 25, 27 June 2002 and 1, 2, 5-8 July 2002. 

The data are categorized by campaigns and dates, and are processed into levels. Level 0 files are text files directly from the receiver. Levels above 0 are output in Hierarchical Data Format (HDF) format, ordered in UTC time and gzipped. The HDF files contain metadata in the form of attributes that describe the data seta and variables. The variables also contain attributes to self-describe the individual data pieces.

A modified GPS Delay Mapping Receiver (DMR) tracks and measures the direct, line-of-sight, right hand circularly polarized (RHCP) signal of a GPS satellite. It also simultaneously measures the delayed, Earth- reflected, near-specular, left hand circularly polarized (LHCP) GPS signal. The DMR was mounted on the NCAR C-130 along with the Jet Propulsion Laboratory (JPL) Passive and Active L- and S-band (PALS) microwave instrument. GPS signal reflections were collected using the standard hemispherical RHCP antenna mounted on the roof of the aircraft, and a reversed polarization LHCP hemispherical antenna mounted on the belly. The receiver position is computed from either an external navigation source or the receiver's own GPS navigation solution, whichever is more accurate. The GPS surface reflections are georeferenced to a composite Earth consisting of the WGS-84 ellipsoid, the EGM96 geoid, and the GTOPO30 elevation model.

The primary objective was to collect land reflections for sensitivity of the reflected GPS power to temporal and spatial soil moisture and foliage attenuation. The measurements are a form of bistatic radar and can be used to estimate the surface scattering coefficient  (roughness and/or dielectric properties) and path delays between the direct and reflected GPS signals. Scattering coefficients can be used to estimate changes in surface soil moisture. 

These data were collected as part of a validation study for the Advanced Microwave Scanning Radiometer - Earth Observing System (AMSR-E). AMSR-E is a mission instrument launched aboard NASA's Aqua Satellite on 04 May 2002. AMSR-E validation studies linked to SMEX are designed to evaluate the accuracy of AMSR-E soil moisture data. Specific validation objectives include assessing and refining soil moisture algorithm performance; verifying soil moisture estimation accuracy; investigating the effects of vegetation, surface temperature, topography, and soil texture on soil moisture accuracy; and determining the regions that are useful for AMSR-E soil moisture measurements.