CCAR University of Colorado Airborne GPS Bistatic Radar

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

CCAR / University of Colorado 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 2

GPS Bistatic Radar Geometry CCAR / University of Colorado GPS Transmitters 24 sats L 1: 1. 5, L 2: 1. 2 GHz PRN coding Direc t Sig RCP nal P LC Re fle c ted Si gn a l GPS Receiver Zenith & nadir antennas Range cells Rou glis gh su r ten ing face zon e 3 Specular point

Bistatic Radar Measurement CCAR / University of Colorado 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 Correlation Power Delay (Altimetry) Bistatic cross section (Soil Moisture) (Water Content) Increasing Roughness Direct Signal Reflected Signal Delay (range) 4

CCAR / University of Colorado CLPX 03 Configuration • Delay mapping receiver (DMR) developed by Katzberg & Garrison (NASA La. RC), based on GEC-Plessey GPSBuilder 2 • 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: 20 x 15 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 5

CCAR / University of Colorado GPS Bistatic Radar Instrument Rackmount PC-104 GPS receiver LCP patch antenna 6

CCAR / University of Colorado GPS Bistatic Radar Flight Latitude Low altitude area SNR (d. B) Lake calibration Longitude • Typical GPS reflected signal flight lines (20030325) 7

CCAR / University of Colorado CLPX 03 Reflections/NP MSA Typical GPS reflection 1 sec waveforms showing quasi-specular and rough Low altitude area surface scattering SNR transect of NP MSA showing reflectivity variations 8

CCAR / University of Colorado CLPX 03 Reflections/Frasier MSA Low altitude area Lake calibration • Reflected SNR correlated with surface elevations 9

CCAR / University of Colorado 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 10

CCAR / University of Colorado • • 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 EGM 96/GTOPO 30 (1 km) surface model • Parameters of interest: reflected SNR, direct SNR, waveforms satellite parameters, aircraft parameters • Data sets available by day in HDF format (~30 MB/day) • Data available directly from http: //ccar. colorado. edu/~dmr/data or through a link at NSIDC 11