CCAR / University of Colorado

1

Airborne GPS Bistatic Radar in CLPX

Dallas Masters

University of Colorado, Boulder

Valery Zavorotny

NOAA ETL

Stephen Katzberg

NASA LaRC

CCAR / University of Colorado

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

CCAR / University of Colorado

3

GPS Bistatic Radar Geometry

Rough surface

glistening zone

Range cells

GPS Transmitters24 sats

L1: 1.5, L2: 1.2 GHzPRN coding

Direct SignalRCP

Ref

lect

ed S

igna

lLC

P

GPS Receiver

Zenith & nadir antennas

dd

d

dt

tt

nd

dd BkT

G

R

GP

P

PSNR

1

44

2

2

rr

r

rs

a

st

tt

nr

rr BkT

G

R

S

R

GP

P

PSNR

1

444

2

20

2

24 dt

tt

R

GP

dd

d

BkT

G 1

4

2

24 st

tt

R

GP

20

4 rs

a

R

S

rr

r

BkT

G 1

4

2

Specular point

CCAR / University of Colorado

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

Delay (range)

Delay (Altimetry)

Reflected SignalDirect Signal

Bistatic cross section(Soil Moisture)

Cor

rela

tion

Pow

er

Increasing Roughness

Delay (range)

Delay (Altimetry)

Reflected SignalDirect Signal

Bistatic cross section(Water Content)

Cor

rela

tion

Pow

er

Increasing Roughness

Specular point

CCAR / University of Colorado

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

CCAR / University of Colorado

6

GPS Bistatic Radar Instrument

Rackmount PC-104 GPS receiver LCP patch antenna

CCAR / University of Colorado

7

GPS Bistatic Radar Flight

• Typical GPS reflected signal flight lines (20030325)

Lake calibration

Low altitude area

Lat

itude

Longitude

SNR (dB)

CCAR / University of Colorado

8

CLPX03 Reflections/NP MSA

Low altitude area

Typical GPS reflection 1 sec waveforms showing quasi-specular and rough surface scattering

SNR transect of NP MSA showing reflectivity variations

CCAR / University of Colorado

9

CLPX03 Reflections/Frasier MSA

Lake calibration

Low altitude area

• Reflected SNR correlated with surface elevations

CCAR / University of Colorado

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

CCAR / University of Colorado

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