SAR Geocoding

Contact persons

Keywords

SAR, Geocoding, Geolocation, Map, Range, Doppler, Terrain, Ellipsoid

Quick Reference

Synthetic Aperture Radar (SAR) Images are normally provided in radar geometries called slant range or ground range. Combination of the images with data from other sources, however, often requires conversion to a conventional map geometry.

The slant or ground range geometry images can include severe height-induced geometric and radiometric distortions, particularly for scenes with significant topography. Digital Elevation Models (DEMs) can be used to remove terrain-induced geometric distortions in Synthetic Aperture Radar imagery. Techniques for such image rectification have been developed within our group. The operational ERS satellite terrain-geocoding software used to rectify SAR imagery from the ERS-1 and ERS-2 satellites at the D-PAF and UK-PAF was developed at the University of Zürich. Examples of ESA ERS Products are available.

Recent research has focussed on ScanSAR imaging modes, and the heteromorphic nature of the radar imaging process, particularly in steep mountainous areas, where layover occurs.

Some Results

Two SAR images are shown below. On the left is a detected SAR image in slant-range geometry. On the right is the same image after terrain correction. The methodology is described below.

ERS-1 slant range image
(courtesy of ESA-ESRIN)

ERS-1 Image,
geocoded to Swiss map coordinates using DHM25,
(courtesy of Swiss Federal Office of Topography)

Methods

Given spacecraft position vector S = {Sx, Sy, Sz} and velocity vector vS, as well as DEM position P = {Px, Py, Pz} and velocity vector vP, the following equations relate positions in the radar image to geodetic positions on the Earth's surface:

Range equation:

Doppler equation:

The symbol lambda stands for the radar wavelength, while Rs and fref represent the reference slant range and image Doppler respectively.

In terrain geocoding, one iterates through a digital elevation model, converting each DEM position (e.g. Northing, Easting, height) into the global Cartesian coordinates {Px, Py, Pz}.

Given that position P, one can iterate along a model of the spacecraft's orbit (in the more general case, to include aircraft, one would say the platform's position history) until the Doppler equation is satisfied. That provides the spacecraft position vector S, and one can then use the range equation to solve for the local slant range Rs. This algorithm is known as backward geocoding, as it proceeds backwards from map geometry back to radar geometry, although the goal (and end product) is a transformation of the radar image forward from radar to map geometry. In the case of SAR interferometry, forward geocoding techniques are used to transform the radar image directly from radar to map geometry.

Beyond the proper geolocation of each point in a SAR image, image understanding also requires an exact modelling of the imaging process. In hilly areas, more than one DEM pixel can converge on a single (range, Doppler) coordinate in radar geometry.

SAR image simulation is used to calculate the local illuminated area for each pixel in a SAR image. An example is shown below, alongside its ERS counterpart. The technique is generally appliciable to any SAR imagery acquired, and has been applied to ERS-1/2, JERS-1, ENVISAT ASAR, and ALOS PALSAR data.

Slant Range SAR Image Simulation,
calculated using DHM25,
(courtesy of Swiss Federal Office of Topography)

Slant Range ERS-1 Image
(courtesy of ESA-ESRIN)

Partnerships

Research has been conducted in cooperation with the following organizations:

Publications

SMALL D., ROSICH B., SCHUBERT A., MEIER E. [2006]:
ENVISAT ASAR Geometric Validation,
Proc. of CEOS SAR Cal/Val Workshop, Edinburgh, Scotland, Oct. 3-6, 2006 (in press).
SMALL D., ROSICH B., SCHUBERT A., MEIER E., NÜESCH D. [2005]:
Geometric Validation of Low and High-Resolution ASAR Imagery (PDF, 1.6 Mb),
Proc. of the 2004 ENVISAT & ERS Symposium, Salzburg, Austria, Sept. 6-10, 2004 (ESA SP-572, April 2005). 9p.
SMALL D., HOLZNER J., RAGGAM H., KOSMANN D., SCHUBERT A. [2003]:
Geometric Performance of ENVISAT ASAR Products,
Proc. of IGARSS'03, Toulouse, France, July 21-25, 2003, pp. 1121-1123.
SMALL D., SCHUBERT A., KRÜTTLI U., MEIER E., NÜESCH D. [2003]:
Preliminary Validation of ASAR Geometric Accuracy,
Proc. of ENVISAT Validation Workshop, Frascati, Italy, Dec. 9-13, 2002 (ESA SP-531, August 2003). PDF (PDF, 1.6 Mb)
SMALL D., KOSMANN D., HOLZNER J., RAGGAM H., PIRRI M., SCHUBERT A., KRÜTTLI U., HUMMELBRUNNER W., FRANKE M. [2002]:
ASAR Level 1 Geolocation,
Proc. of ENVISAT Calibration Review, Noordwijk, The Netherlands, Sept. 9-13, 2002. PDF (PDF, 1.0 Mb)
SMALL D., MEIER E., NÜESCH D. [2001]:
Efficient Geolocation and Image Simulation for Extended SAR Strip Maps,
Proc. of IGARSS'2001, Sydney, Australia, July 9-13, 2001, pp. 3152-3154.
SMALL D., BIEGGER S., NÜESCH D. [2000]:
Automated Tiepoint Retrieval through heteromorphic Image Simulation for Spaceborne SAR Sensors,
Proc. of ERS-ENVISAT Symposium 2000, ESA Publication SP-461, Gothenburg, Sweden, Oct. 16-20, 2000.
SMALL D., BIEGGER S., NÜESCH D. [2000]:
The Topology of SAR Imagery in Rough Topography,
Proc. of EUSAR'2000, Munich, Germany, May 23-25, 2000, pp. 501-504.
SMALL D., HOLECZ F., MEIER E., NÜESCH D. [1998]:
Absolute Radiometric Correction in Rugged Terrain: A Plea for Integrated Radar Brightness,
Proc. of IEEE-IGARSS'98, Seattle, USA, July 6-10, 1998, pp. 330-332.
SMALL D., HOLECZ F., MEIER E., NÜESCH D. [1998]:
Radiometric Normalization for Multimode Image Comparison,
Proc. of EUSAR'98, Friedrichshafen, Germany, May 25-27, 1998, pp. 191-194.
SMALL D., HOLECZ F., MEIER E., NÜESCH D., BARMETTLER A. [1997]:
Geometric and Radiometric Calibration of RADARSAT Images,
CDROM Proc. of Geomatics in the Era of Radarsat (GER'97), Ottawa, Canada, May 24-30, 1997. CSA -- Local PDF (PDF, 787 Kb)
SMALL D., PASQUALI P., FÜGLISTALER S. [1996]:
A Comparison of Phase to Height Conversion Methods for SAR Interferometry,
Proc. of IEEE-IGARSS'96, Lincoln, Nebraska, USA, pp. 342-344.
SMALL D., WERNER C., NÜESCH D. [1995]:
Geocoding and Validation of ERS-1 InSAR-derived Digital Elevation Models,
EARSeL Advances in Remote Sensing, Volume 4, Number 2, Oct. 1995, pp. 26-39, pp. I-II.
SMALL D., WEGMÜLLER U., MEIER E., NÜESCH D. [1994]:
Geocoded ERS-1 InSAR-derived Digital Terrain Information,
Proc. of CEOS SAR Calibration Workshop, Ann Arbor, Michigan, USA, pp. 184-190.
MEIER E., FREI U., NÜESCH D. [1993]:
Precise Terrain Corrected Geocoded Images.
Chapter in "SAR Geocoding: Data and Systems", ed. G. Schreier, 1993, Wichmann, pp. 173-186.
MEIER E. [1989]:
Geometrische Korrektur von Bildern orbitgestützter SAR-Systeme.
Remote Sensing Series, Vol. 15, University of Zürich, 1989, 137 pp.

Department of Geography

Research Projects