Abstract
Radiometric calibration adjusts the measured pixel intensity to a physical property, the radar cross section (RCS). After calibration, this relationship is defined over the entire backscatter range: from low image power (near noise) up to high reflections (below saturation). Based on a proper radiometric calibration, the measured radar backscatter for the Sentinel-1 synthetic aperture radar (SAR) satellite constellation is validated over a wide backscatter range using different target types. Therefore, the RCS derived from point targets and radar brightness from distributed targets are compared between Sentinel-1A (S-1A) and Sentinel-1B (S-1B) acquisitions over the same observation area for regions where a stable target backscatter is expected for a certain period of time. Low differences (in the order of 0.3 dB) are found between S-1A and S-1B for high and medium backscatter derived from point targets or rainforest regions, but higher differences for low backscattering regions like ice areas and lakes. For comparing radar brightness containing low backscatter targets, an accurate derived noise level has to be taken into account. In addition to the measured lower noise equivalent beta zero (NEBZ) level, higher transmit power was detected for S-1B compared to S-1A. The higher antenna gain of S-1B leads finally to a higher sensitivity for low backscattering areas of S-1B compared to S-1A and explains the found differences.
Highlights
One of the main reasons for building a space-borne radar constellation instead of operating a single instrument only is the higher revisit rate for acquisitions over the area of interest
The different target types focus on different backscatter regimes covering point targets with high signal-to-noise ratio (SNR) and distributed targets such as the Amazon rainforest with medium SNR, down to backscatter near noise found in ice areas and calm waters
The differences between the observed radar cross section (RCS) derived from synthetic aperture radar (SAR) images and the theoretical expected RCS of the DLR reference targets are depicted in Figure 1 for S-1A and S-1B as a function of time
Summary
One of the main reasons for building a space-borne radar constellation instead of operating a single instrument only is the higher revisit rate for acquisitions over the area of interest. The Radarsat constellation mission (3 satellites) was lunched recently in June 2019 [6]. Within virtual constellations [7] instruments from different platforms are combined; in particular, multi-temporal products can be derived by merging multiple SAR sources or even combining these with optical instruments [8]. SAR constellation missions using small satellites could reduce the mission costs dramatically. Such missions are planned for future developments [9] – related concepts are under investigation or within the development phase [10]
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