Bistatic system and baseline calibration in TanDEM-X to ensure the global digital elevation model quality Jaime Hueso González ⇑ , John Mohan Walter Antony, Markus Bachmann, Gerhard Krieger, Manfred Zink, Dirk Schrank, Marco Schwerdt German Aerospace Center (DLR), Microwaves and Radar Institute (HR), 82234 Wessling, Germany article info Article history: Available online 18 June 2012 Keywords: Accuracy Calibration Digital Interferometer SAR Satellite Space abstract TanDEM-X is an operational satellite mission with the goal of generating a high quality global digital ele- vation model (DEM) based on synthetic aperture radar (SAR) interferometry in X-band. In order to ensure the quality of the DEM, the differential range measurements and knowledge of the interferometric base- line have to be extremely accurate. In this paper, the bistatic system calibration strategy implemented in TanDEM-X to achieve the desired DEM quality will be described, focusing on the baseline calibration pro- cedure. The results of the tests, which were performed in parallel to the operational DEM acquisition, ver- ify the suitability of this approach. Ó 2012 International Society for Photogrammetry and Remote Sensing, Inc. (ISPRS) Published by Elsevier B.V. All rights reserved. 1. Introduction The currently operational TanDEM-X mission (Krieger et al., 2007) consists of two twin satellites provided with X-band SAR instruments, TerraSAR-X (TSX) and TanDEM-X (TDX), flying in a controlled helix formation, with typical baseline lengths of 250– 1000 m. This configuration enables the systematic acquisition of single-pass cross-track interferograms of the Earth, in the pursuit of generating a high quality global DEM. This mission is imple- mented in the framework of a public–private partnership between the German Aerospace Center (DLR) and Astrium GmbH. TDX was launched into orbit in June 2010, joining TSX, already operational since 2007. During the summer of 2010, the first part of the com- missioning phase took place, in which both satellites were ar- ranged in a safe 20-km distance pursuit monostatic formation, in order to calibrate TDX for the monostatic operation. In October 2010, the mean along-track distance was eliminated and both sat- ellites started to orbit in a close helix formation. This was also the beginning of the so-called bistatic commissioning phase, where different bistatic issues were tested (Hueso González et al., 2010c). 2 months later, in mid December, the nominal DEM data acquisition started in order to fulfill the mission schedule. How- ever, some calibration activities continued in parallel to monitor this complex system, and to store vital information for the health of the mission, like the aspects related to DEM calibration. In order to ensure the quality of the DEM, the differential range measure- ments have to be precise enough to avoid missing the right inter- ferometric ambiguity interval. The accuracy in the knowledge of the baseline between the two satellites is critical, as well, since it has a direct impact on the final DEM quality. This paper is structured as follows. After this introduction, Sec- tion 2 describes the driving requirements related to the DEM accu- racy, and the flow of the DEM calibration processing chain. These requirements motivate the development of the calibration proce- dures also mentioned here. Section 3 focuses on the baseline prob- lematic and describes the impact of this potential error on the resulting DEM. In Section 4, the algorithm to compensate the base- line errors is described in detail. The measurements of the opera- tional mission will be shown in Section 5, which verify the suitability of this approach. Finally, Section 6 summarizes all the corrections applied to the system and evaluates the fulfillment of the requirements of this highly complex radar imaging mission, prior to the final DEM calibration and mosaicking process (Hueso González et al., 2010a). 2. TanDEM-X driving requirements 2.1. DEM calibration flow In early mission design stages, it was assumed that the knowl- edge of the baseline vector between TSX and TDX, as provided by differential GPS measurements, was accurate enough to result in moderate systematic height errors that could be cancelled by 0924-2716/$ - see front matter Ó 2012 International Society for Photogrammetry and Remote Sensing, Inc. (ISPRS) Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.isprsjprs.2012.05.008 ⇑ Corresponding author. E-mail address: desperta@yahoo.com (J. Hueso González). ISPRS Journal of Photogrammetry and Remote Sensing 73 (2012) 3–11 Contents lists available at SciVerse ScienceDirect ISPRS Journal of Photogrammetry and Remote Sensing journal homepage: www.elsevier.com/locate/isprsjprs