1. Airborne GPS Bistatic Radar in CLPX Dallas Masters University of Colorado, Boulder Valery Zavorotny NOAA ETL Stephen Katzberg NASA LaRC

2. Review of GPS Bistatic Radar • CLPX 02 & 03 was first piggyback test of GPS bistatic radar over snow and mountainous terrain • Uses simple, modified GPS receiver to measure signals scattered from the land surface • Receives GPS L-band signal @ 1.5 GHz • Bistatic radar measures forward scattered power rather than back scattered power; functions as a scatterometer • Antennas: • Zenith RCP hemi patch for direct signal tracking, navigation • Nadir LCP hemi patch (wide field) for reflected signal measurement • Footprint is range-limited by GPS pseudo-random code, but land surface may look “specular” for smooth to moderate roughness

3. GPS Receiver Zenith & nadir antennas GPS Bistatic Radar Geometry GPS Transmitters 24 sats L1: 1.5, L2: 1.2 GHz PRN coding Direct Signal RCP Reflected Signal LCP Range cells Specular point Rough surface glistening zone

4. Bistatic Radar Measurement • GPS bistatic radar measurements: • Delay of reflected signal  receiver height above surface • Magnitude of reflected power reflectivity water content • Distribution of reflected power  surface roughness  Specular point Delay (Altimetry) Delay (Altimetry) Bistatic cross Bistatic cross section section (Soil Moisture) (Water Content) Correlation Power Correlation Power Increasing Roughness Increasing Roughness Reflected Signal Reflected Signal Direct Signal Direct Signal Delay (range) Delay (range)

5. CLPX03 Configuration • Delay mapping receiver (DMR) developed by Katzberg & Garrison (NASA LaRC), based on GEC-Plessey GPSBuilder2 • 5 channels operate in a nominal zenith tracking mode • 7 channels operate open loop, measuring the scattered power at specified chip offsets with respect to the direct signal • Operates autonomously w/ PC-104 • Size: 20x15x15 cm chassis • Flew on NASA P-3 • Collected measurements: 02/21,23,24; 03/25,30,31 • Aircraft height at ~5000 m AGL • Auto selection of highest elevation sat (nearest nadir incidence) • Incidence angles between 0-35 deg • Footprint size varies: Fresnel zone ~ 80 m to 3 km depending on specularity of reflection and receiver height

6. GPS Bistatic Radar Instrument Rackmount PC-104 GPS receiver LCP patch antenna

7. GPS Bistatic Radar Flight Low altitude area SNR (dB) Latitude Lake calibration Longitude • Typical GPS reflected signal flight lines (20030325)

8. CLPX03 Reflections/NP MSA Typical GPS reflection 1 sec waveforms showing quasi-specular and rough surface scattering Low altitude area SNR transect of NP MSA showing reflectivity variations

9. CLPX03 Reflections/Frasier MSA Low altitude area Lake calibration • Reflected SNR correlated with surface elevations

10. Working with GPS Bistatic Radar • GPS measurements should be considered EXPERIMENTAL • Calibration issues: • GPS receiver is uncalibrated in absolute sense • Assume noise is constant and estimate SNR • Assumptions for first order analysis: • Surface roughness, incidence angle, receiver height constant • Estimate reflected SNR • Maps of SNR tracks sensitive to surface Fresnel reflectivity and roughness • Need to compare with other data sets, imagery

11. CLPX GPS Summary • Collected data sets in 02 and 03 campaigns • Reflected signals were quasi-specular • First-order reflectivity maps show spatial variations of reflectivity • CLPX data sets: • Ground tracks georeferenced to EGM96/GTOPO30 (1km) surface model • Parameters of interest: reflected SNR, direct SNR, waveforms • satellite parameters, aircraft parameters • Data sets available by day in HDF format (~30MB/day) • Data available directly from http://ccar.colorado.edu/~dmr/data or through a link at NSIDC