Synthetic Aperture Radar (SAR) - Radiometric Calibration

Contact Persons

David Small

Erich Meier

Keywords

synthetic aperture radar, SAR, calibration, validation, corner reflector, transponder, backscatter, sigma nought, beta nought, gamma, noise equivalent

Quick Reference

Radars measure the ratio between the power of the pulse transmitted and that of the echo received. This ratio is called the backscatter. Calibration of the backscatter values is necessary to enable intercomparison of radar images acquired with different sensors, or even of the same sensor if acquired in different modes or processed with different processors.

Methods

The base of the radiometric calibration is the radar equation. The formulation of the received power for distributed scatters Pd for a scattering area A can be written as:

In the spaceborne case, given highly accurate orbit state vectors (available for ENVISAT ASAR and ALOS PALSAR), due to the stability of the satellite platform, ideal models of Doppler variations with time correspond well to the actual antenna positions.

In the airborne case, the SAR data must also be focussed considering an ideal flight path. Each pixel is terrain geocoded by means of the usual range-Doppler equation referring to the ideal position. However, in radiometric terms, the pixel refers to the actual antenna position, i.e. to the location where the antenna effectively transmitted and received. The antenna elevation & azimuth angles, as well as the local incidence angles in the range and azimuth directions must be determined taking into account the actual antenna position, its pointing direction and the position of the scatterer on the ground. Antenna attitude (i.e. roll, pitch and yaw) variations are taken into account. An overview of SAR data calibration is provided in Figure 1.

Figure 1: SAR data calibration overview

Results

Figure 2 shows geometrically terrain corrected as well as radiometrically terrain corrected SAR data with and without terrain normalisations to radiometry. The technique is generally applicable to all SAR imagery, and has been applied to the following SAR systems: ERS-1/2, JERS-1, Radarsat-1, SIR-C, AeS-1, ENVISAT ASAR, and ALOS PALSAR.

 

 

Figure 2: Geocoded uncalibrated (above) and radiometrically terrain corrected / calibrated (below) ASAR Wide Swath (WS) SAR images of Switzerland -- ASAR data ©ESA

Partnerships

Research has been conducted in cooperation with the following organizations:

• European Space Agency
• German Aerospace Centre (DLR)
• Canadian Space Agency
• AeroSensing, now Intermap

Publications

• SMALL D. [2011]:
  Flattening Gamma: Radiometric Terrain Correction for SAR Imagery,
  IEEE Trans. on Geoscience and Remote Sensing, 2011, Vol. 49(8), pp. 3081-3093.
• SMALL D., MIRANDA N., ZUBERBÜHLER L., SCHUBERT A., MEIER E. [2010]:
  Terrain-corrected Gamma: Improved Thematic Land-cover Retrieval for SAR with Robust Radiometric Terrain Correction,
  Proc. of ESA living planet symposium, Bergen, Norway, June 28 - July 2, 2010, 8p.
• SMALL D., MIRANDA N., MEIER E. [2009]:
  A Revised Radiometric Normalisation Standard for SAR,
  Proc. of IGARSS 2009, Cape Town, South Africa, July 12-17, 2009, pp. 566-569.
• SMALL D., SCHUBERT A., ROSICH B., MEIER E. [2007]:
  Geometric and Radiometric Correction of ESA SAR Products (PDF, 3.5 Mb),
  Proc. of the Envisat Symposium 2007, Montreux, Switzerland, Apr. 23-27, 2007 (ESA SP-636, July 2007). 6p.
• 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., MEIER E., NÜESCH D. [2004]:
  Robust Radiometric Terrain Correction for SAR Image Comparisons (PDF, 3.0 Mb),
  Proc. of IGARSS 2004, Anchorage, Alaska, USA, Sept. 20-24, 2004, pp. 1730-1733.
• SMALL D., JEHLE M., MEIER E., NÜESCH D. [2004]:
  Radiometric Terrain Correction Incorporating Local Antenna Gain (PDF, 833 Kb),
  Proc. of EUSAR 2004, Ulm, Germany, May 25-27, 2004, pp. 929-932.
• 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).
• 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.
• HOLECZ F., PASQUALI P., MOREIRA J., and NÜESCH, D. [1998]:
  Rigorous Radiometric Calibration of Airborne AeS-1 InSAR Data, Proc. of IGARSS'98, Seattle, USA, July 6-10, 1998.
• SMALL D., HOLECZ F., MEIER E., and NÜESCH, D. [1998]:
  Absolute Radiometric Correction in Rugged Terrain: A Plea for Integrated Radar Brightness, Proc. of IGARSS'98, Seattle, USA, pp. 330-332, July 6-10, 1998.
• SMALL D., HOLECZ F., MEIER E., and 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., and 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 Abstract & PDF --  Local PDF (PDF, 787 Kb)
• HOLECZ F., MICHEL T., and KELLER, M. [1996]:
  Calibration and Classification of SIR-C SAR Data in Moderate Slope Areas, Proc. of IGARSS'96, Lincoln, USA, 1996