Radargrammetric Application with Cosmo-SkyMed Imagery: Definition and Implementation of an Orientation Model for Spotlight Stereo Pairs

The availability of new high resolution radar spaceborne sensors offers new interesting potentialities for the acquisition of the data useful for the generation of Digital Surface Models (DSMs). Two different methods may be used to generate DSMs from Synthetic Aperture Radar (SAR) data: the interferometric and the radargrammetric approaches.

Actually, due to the low resolution amplitude supplied by the spaceborne radar sensors available until now (at the level of tens of meters), usually the first approach has been used, being aware that the radar interferometry may suffer for lack of coherence (for example in vegetated areas).

At present, the importance of the radargrammetric approach is rapidly growing due to the new high resolution imagery (up to 1 m GSD) which can be acquired by COSMO-SkyMed, TerraSAR-X and RADARSAT-2 in SpotLight mode. In this sense, it seems useful to underline that the two approaches should be considered complementary, in order to obtain the best (accurate and complete) product. It is well known that the two main steps for DSMs generation from SAR imagery according to the radargrammetric approach are the stereo pair orientation and the image matching.

In this paper they are investigated the topics related to image orientation of SAR stereo pairs in zero doppler geometry acquired by COSMO-SkyMed sensor in SpotLight mode.
As regards the general geometric setup, unlike the interferometric approach, the radargrammetric one is based on the signal amplitude, exploits the stereoscopy similarity to optical methods and the baseline between the orbital tracks from which stereo pairs is acquired have to be long. To obtain good stereo geometry, the optimum configuration for the radargrammetric application is when the target is observed in opposite-side view; however it causes large geometric and radiometric disparities hindering image matching. A good compromise is to use a same-side stereo pair in order to increase the efficiency in the correlation image process. This geometry acquisition may be easily satisfied thanks to the constellation of satellites (presently three of four are active) supplied by COSMO-SkyMed.

As regards the radargrammetric orientation model, it has to be underlined that, starting from the model proposed in the classical book of Leberl (Radargrammetric Image Processing, 1990), a refinement of the orbital model is needed to comply with and to exploit the potentialities of the novel high resolution (both in azimuth and in range). Then, the defined and implemented model performs a 3D orientation based on two range and two zero-Doppler equations, allowing for some calibration parameters least squares estimation, related to satellite position and velocity.

The new model has been tested over a stereo pair acquired by COSMO-Skymed in SpotLight mode on the well known Mausanne (South France) test site and the results are reported and discussed.