Capella payload demonstrates InSAR capability

SAR Interferometry is a powerful remote sensing technique that allows users to measure geophysical parameters, such as surface topography, ground changes and terrain deformation [https://elib.dlr.de/82313/1/SAR-Tutorial-March-2013.pdf]. This capability enables the collection of critical insights for a variety of application fields such as defense & intelligence, civil engineering, mining, oil and gas, disaster management and climate science. InSAR techniques use the phase information from repeated SAR collects to measure changes in distance between collects with the accuracy of a fraction of the radar wavelength (~3 cm for Capella X-Band radar [https://www.capellaspace.com/data/sar-imagery-products/] ). 

Depending on the collection geometry and the processing technique applied, InSAR can be used for different purposes. Some of the most common applications are measurements of changes with Coherent Change Detection (CCD), derivation of Digital Elevation Models (DEMs), and estimation of terrain deformation or subsidence. This information cannot be derived by very-high resolution SAR images alone when only the amplitude of the pixels in the image is used in the analysis. InSAR instead fully exploits the complex signal that coherent radar inherently measures. 

Figure 1: The phase coherence, obtained by combining Single Look Complex image pairs, is high (close to 1) for hard targets such as building and the airport runway that are not changing in time. As expected, a lake can be recognized in the scene with the low coherence, close to 0.  Coherence is a measure of the similarity of the signals in an interferometric pair

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Figure 2: Fringes

Figure 3: DEM with a measured vertical accuracy better then 1 meter.

Capella teams recently utilized operational radars to demonstrate that the Capella radar payload is capable of InSAR measurements. Sarmap SA processed collects from our data archive to generate a high-resolution DEM. For this proof of concept, the normal baseline between the collects was 617.4 m, which is less than 10% of the estimated critical baseline, and the incidence angle difference was 0.023 degrees. The Doppler Centroid difference was 44 Hz and the PRF was 6238 Hz. The height ambiguity for this geometry is 10.057 m. The spatial resolution of the generated DEM is better than 2.5 m. The height precision of the DEM is estimated from interferometric coherence, the baseline, and height of ambiguity, and for most of the scene was found to be equal or better than 1m.

The results demonstrated that InSAR techniques benefit from very high-resolution SAR imagery from Capella and precise attitude information provided in the associated metadata. 

In addition to this, the DEM generated is a unique example of InSAR applied to non-polar orbit SAR collects, and such orbital configuration could lead to interesting measurements of the North-South component of terrain displacement when Differential InSAR (DInSAR) techniques are applied. 

At Capella, we aim extract the deep information content that make the radar signal unique among other remote sensing techniques. We are actively developing a number of  InSAR compatible data products and InSAR based applications. Our InSAR products will gradually become operational in the months to come as we deploy more and more satellites.