Adopting Equivalent Radar Cross Section (ERCS) as the Radiometric Measurement Quantity for SAR Images
This thread is hosted by the SAR Subgroup of the CEOS Working Group on Calibration and Validation.
Problems with the Current Definition
Since the conception of the synthetic aperture radar (SAR) principle, radar cross section (RCS) has always been considered as the radiometric measurement quantity for SAR images (i.e. the quantity which allows to distinguish different gray scale values in a typical SAR image). The definition has certainly enjoyed great success in the last decades. With the advent of modern, high-resolution and high-accuracy SAR systems, several fundamental problems became apparent though:
- RCS is defined as a ratio of powers (scattered vs. incident power), signal phase has no influence. The pixel intensity in a SAR image, on the other hand, depends on magnitude and phase. The consequence is that two different SAR point targets with identical RCSs may appear differently bright in a SAR image (due to the phase response) despite all calibration efforts. Stated differently: A SAR instrument simply does not measure RCS.
In general, the RCS of point and distributed targets is angular and frequency dependent. For most older and some current systems, the relevant angular and spectral ranges are small and the backscatter can be approximated to be constant within the relevant spectral and angular range (think: metallic sphere). However, this approximation is flawed for modern high-resolution and high-accuracy SAR systems, where the angular and spectral dependence must be considered.
An important special case are calibration point targets like trihedral corner reflectors and transponders, which decisively do not look like ideal spheres (with a flat angular and spectral response). To achieve comparable radiometric measurement results across SAR missions and calibration point targets, the angular and spectral dependence must be considered.
The purpose of this Loomio thread is to discuss and prepare a CEOS SAR recommendation for a better definition of the radiometric measurement quantity, which eliminates the problems listed above.
For radiometric SAR measurements, the current definition Radar cross section (RCS) shall be substituted with the definition of equivalent radar cross section (ERCS).
The proposed ERCS definition is:
> The equivalent radar cross section (ERCS) shall be equal to the radar cross section of a perfectly conducting sphere which would result in an equivalent pixel intensity if the sphere were to replace the measured target.
- Sound definition, which is applicable for low-resolution and high-resolution SAR systems alike
- Adoption of concepts which proved successful in other fields of metrology:
- Black body ↔ metal sphere
- Standardized photometric scales in stellar photometry ↔ Standardized SAR passbands
- Traceable radiometric calibration becomes possible, therefore also:
- Applications in legal metrology (important for commercial customers)
- Mis-calibrations between systems can be avoided (consider Sentinel-1A/B, RADARSAT-2, RCM-1/2/3)
- Research results become reproducible across SAR missions
- Use the new ERCS definition in requirement specifications for upcoming SAR missions (so that the definition can be used in measurement uncertainty analyses).
- Calibration targets must be calibrated to a known ERCS (before: RCS). This means that the angular and frequency dependent RCS of a calibration point target (typically a transponder or a trihedral corner reflector) must be measured, and its ERCS must be computed (e.g. with a point-target SAR simulator, see Resources below).
- In the future: Discuss adoption of standardized passbands (i.e., standardized apodization functions).
The discussion and the result of the voting on this Loomio thread is indicative only. The Loomio platform is intended to facilitate opinion building outside and in preparation of the yearly CEOS SAR Workshops. Official recommendations are only ever passed at the summary session of a CEOS SAR Workshop.
- CEOS draft recommendation on ERCS (2015).
- CEOS presentations on ERCS: 2015, 2014, 2013.
- Björn J. Döring and Marco Schwerdt: The Radiometric Measurement Quantity for SAR Images. IEEE Transactions on Geoscience and Remote Sensing, vol. 51, no. 12, Dec. 2013. Links: 1, 2.
- Björn J. Döring, Marco Schwerdt: The SAR Passband Problem: Analytical Model and Possible Practical Solutions. IEEE Transactions on Geoscience and Remote Sensing (2015). Links: 1, 2.
- Björn J. Döring, Philipp Looser, Matthias Jirousek, and Marco Schwerdt: Reference Target Correction Based on Point Target SAR Simulation. IEEE Transactions on Geoscience and Remote Sensing, vol. 50, no. 3, 2012. Links: 1, 2.
Björn Döring (DLR) started a proposal Wed 17 Aug 2016
Adopt ERCS Closed Thu 8 Sep 2016
For radiometric SAR measurements, the current definition radar cross section (RCS) shall be substituted with the definition of equivalent radar cross section (ERCS).
The proposed ERCS definition is:
The equivalent radar cross section (ERCS) shall be equal to the radar cross section of a perfectly conducting sphere which would result in an equivalent pixel intensity if the sphere were to replace the measured target.
|Agree - 2|
|Abstain - 2|
|Disagree - 2|
|Block - 2|
Fri 19 Aug 2016
I have concerns about the applicability of a metal sphere as a theoretical reference. Doesn't work for small spheres. Just stick to current reference of isotropic scatterer (sigma 4 PI/lambda squared) and add phase equal to 0 deg.
Fri 19 Aug 2016
I have concerns about the applicability of a metal sphere as a theoretical reference. Doesn't work for small spheres. Just stick to current reference of isotropic scatterer (sigma = lambda squared/4PI) and add phase equal to 0 deg.