corner reflectors for the australian · pdf fileborne synthetic aperture radars medhavy...

CEOS SAR Calibration Validation Workshop 7- 9 November 2011 Fairbanks Alaska

CORNER REFLECTORS FOR THE AUSTRALIAN

GEOPHYSICAL OBSERVING SYSTEM AND SUPPORT FOR CALIBRATION OF SATELLITE-

BORNE SYNTHETIC APERTURE RADARS

Medhavy Thankappan1, Mark L. Williams2 and John Dawson1

1Geoscience Australia 2Horizon Geoscience Consulting


Outline

• Background / Objectives • Considerations for determining corner reflector

characteristics • Target brightness based on accuracy

requirements for displacement monitoring and radiometric calibration

• Corner reflector dimensions vis-à-vis shape • Field deployment / orientation strategy


Background

• Geoscience Australia - national agency for geoscience information; earth observation capability – two ground stations and data processing facility www.ga.gov.au

• Space Policy Unit coordinates space activities across government www.space.gov.au

• Australian Government investment of $23 million in a new Australian Geophysical Observing System (AGOS)

• Geospatial component of AGOS: GNSS instrumentation; ~ 150 corner reflectors, and a repository of InSAR data

http://www.ga.gov.au/�

http://www.space.gov.au/�


AuScope GNSS Network


Why Corner Reflectors?

• Support for monitoring crustal deformation • Reliable means to perform radiometric, geometric,

calibration of satellite-based SAR instruments • Advantages of low maintenance and low cost

compared to active devices such as transponders; transponders require power; in remote locations

• High radar-cross-sections (RCS) for small size, maintained over a wide range of incidence angles - the high-frequency solution for RCS is accurate


SAR Satellite Systems -1

• Only X & C band SAR systems were considered for this project (constraint: size of reflectors)

• Signal-to-Clutter Ratio (SCR) important consideration; design to be suitable for deformation & calibration studies

• Modes with fine resolution considered, spotlight modes not considered (due to beam steering requirement)


SAR Satellite Systems - 2 • Sensor Freq. Mode Az. Rg. Inc. • (GHz) (m) (m) (deg.) • ASAR 5.3 IMS 5 9.5 15 – 45 • ASAR 5.3 APS 8.4 4.8 15 – 45 • Sentinel-1 5.4 - 5 5.0 20 – 45 • RADARSAT-II 5.3 Fine 7.7 5.2 30 – 50 • RADARSAT-II 5.3 Standard 7.7 9.0 20 – 52 • RADARSAT-II 5.3 Mlook Fine 4.6 3.1 30 – 50 • RADARSAT-II 5.3 Ultra Fine 2.8 1.6 20 – 49 • RCM 5.3 High Resn 5 5 19 – 54 • RCM 5.3 V High Resn 3 3 18 – 54 • RISAT-1 5.35 FRS1 3 3 18 – 33 • RISAT-1 5.35 FRS2 12 12 18 – 33 • COSMO-SkyMed 9.6 Himage 3 3 20 – 60 • COSMO-SkyMed 9.6 WideRegion 16 7 20 – 60 • COSMO-SkyMed 9.6 PingPong 15 15 20 – 60 • TerraSAR-X 9.65 Strip single 3.3 1.2 20 – 45 • TerraSAR-X 9.65 Strip (dual) 6.6 1.2 20 – 45 • Kompsat-5 9.66 Strip 3 3 20 – 45 – 55 • AstroSAR-Lite 9.65 Strip 3 3 28 – 60


Clutter, Terrain, and Incidence Angle

• Sites may have high clutter, artificial surfaces, grassy or rocky

• Clutter level at X band in general 2-3 dB > C band for same terrain and incidence angle

σ = σ°(λ,Terrain) Pa Pr / Sinθ


Signal-to-Clutter Ratio: Calibration

• Typical SCR around 30 dB for radiometric calibration; could be as high as 40 dB (e.g. Freeman 2000*); should not saturate the received signal

• SCR of 35 dB corresponds to target brightness at X-band of 38.8 dBm2, 40 dB corresponds to brightness at C-band of 45 dBm2

• Reflectors with peak brightness of 43 dBm2 and 56 dBm2 used for TerraSAR-X; reflectors with brightness of around 40 dBm2 used for ASAR

*A Freeman, Radiometric Calibration of SAR Image Data, Proc. ISPRS Congress XIX, 2000

IA / σt ΓA = K [1 + (σclutter / σt ΓA)] IA - Image intensity; σt - theoretical peak RCS; ΓA - Impulse response; σclutter - observed mean pixel intensity near target; K - calibration constant


Line-of-sight (LOS) DInSAR height errors as a function of signal-to-clutter ratio (SCR) estimated using the standard deviation of the single-look phase distribution

SCR (dB) X-band C-band 30 0.24 0.43 35 0.14 0.26 40 0.08 0.15 45 0.05 0.09 50 0.03 0.05

Estimated LOS height errors (in mm) for different bands as a function of signal to clutter ratio

Signal-to-Clutter Ratio: Deformation Studies

The distribution of interferometric phase in the presence of noise for an SCR of 3.7 dB and look numbers 1,4,9 and 16


Incidence Angle, SCR and Reflector Brightness

Estimates of reflector brightness for anticipated SCR and incidence angles at C and X band


SCR thresholds and Reflector RCS

• A minimum SCR of 38 dB at X-band and 44 dB at C-band to ensure an accuracy of ~0.1mm for LOS deformation; for radiometric assessment an SCR of 30 - 40 dB at both bands required

• For DInSAR at X-band, RCS in the range of 38 to 46 dBm2 (~ targets used by DLR)

• At C-band RCS should be between 47 dBm2 and 54 dBm2

• Single reflector design for two frequency bands (X&C) implies compromised performance


Reflector Design: Shape

• Shape and implication on RCS; traditional canonical radar reflectors considered; small size practical, large permits greater margin of error; well-defined phase centre

• Trihedral reflectors compact & yield high RCS for a given size; triangular trihedrals: high structural strength; square trihedrals: brightest for a given dimension; circular: between the above two Target Peak RCS

Flat Plate Diplane Trihedral


Scattering Pattern & Aspect Angle

Scattering pattern for a triangular trihedral

Scattering pattern for a square trihedral

Gridded trihedral for polarimetric SAR: traditional corner reflector with one surface replaced by a wave guide, alters the reflection coefficient for one of the polarisations to yield a cross-polar return

Gridded Square Trihedral reflector dimensions for Polarimetric SAR

Courtesy: Tom Ainsworth, NRL


Reflector Dimensions and Tolerances

Trihedral reflector dimensions for X band as a function of shape and peak RCS

Tolerances* Inter-plate Orthogonality ~ 0.2 deg Plate Curvature ~0.75 mm Plate Surface Irregularities ~0.5 mm *For X-band triangular trihedrals based on work done at DLR

Trihedral reflector dimensions for C band as a function of shape and peak RCS


Saturation

• Advice on maximum brightness of corner reflectors sought from ESA, DLR, CSA, ASF

• Envisat: transponders with RCS of 62 dBm2 used; TerraSAR-X: triangular trihedrals with RCS of 55 dBm2 used; Radarsat 2 and RCM: target brightness of 60 dBm2


Reflector Orientation

• Knowledge of sensor orbits, imaging characteristics, and target locations


Orbits

• ENVISAT, RADARSAT-2, TerraSAR-X inclinations 98.49o, 98.58o, and 97.45o, differing by only 1.13o

• ERS-2 <-> ENVISAT, RADARSAT-1 <-> RADASAT-2

• Lower orbit for TerraSAR-X - 514 km • Sentinel-1, different orbits from ERS-2 and

ENVISAT • Test CR prototypes for final deployment


Orientation Calculations • TLE + SGP4 propagator model • Typically right-looking; Asc – West; Desc - East • Seek “broadside” solution within swath on

correct side of sensor

Distance between reflector and the satellite as a function of azimuth angle showing the broadside solution is at 105.3 degrees for Alice Springs


Summary • SCR requirements identified using system

properties of SAR sensors & expected clutter at reflector sites

• Reflectors deployed for AGOS should have RCS in the range of 38 dBm2 to 46 dBm2 at X-band & between 47dBm2 and 54 dBm2 at C-band

• Trihedral corner reflectors recommended, single size could provide a solution for both X and C band reflectors, with compromise in performance

• Dimensions of such a reflector would be 2.2 m for a triangular, 1.6 m for a circular & 1.3 m for a square trihedral

• Target orientation calculations being tested • Deployment strategy under development;

prototypes to be tested in field


Thank you

[email protected]

corner reflectors for the australian · pdf fileborne synthetic aperture radars medhavy...

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