Detlev Kosmann

Ensuring globally the TanDEM-X height accuracy: Analysis of the reference data sets ICESat, SRTM and KGPS-tracks

The TanDEM-X mission will derive a global digital elevation model (DEM) with satellite SAR interferometry. Height references play an important role to ensure the required height accuracy of 10m absolute and 2m relative for 90% of the data. In this paper the main height reference data sets ICESat (for DEM calibration), SRTM (for phase unwrapping) and kinematic GPS-Tracks (KGPS — for DEM verification) are analyzed regarding to their accuracy. For the ICESat data a reliable quality measure is developed. For SRTM an improved version adjusted to reliable ICESat data is presented and a concept for collecting and evaluating decimeter-precise kinematic GPS tracks is proposed.

Production chain towards first calibrated and mosaicked TanDEM-X DEMs

The main product of the TanDEM-X mission is an interferometric DEM product that is finally calibrated due to residual systematic offsets and tilts and where different, generally two coverages, are mosaicked. Above this, a water mask is provided to support later editing of rough water areas. In this presentation the commissioning phase work to set the DEM production chain into operation is described and the first commissioning phase products are shown.

Value added SAR products for ENVISAT

At the end of 1999 the European Space Agency (ESA) will launch the environmental satellite ENVISAT. The payload consists of different elements, e.g. ozone sensor, altimeter, spectrometer and an Advanced Synthetic Aperture Radar (ASAR). This ASAR is a further development of the well proved SAR system from ERS-1 and ERS-2. The standard processing of the huge amount of data will done in different European Processing and Archiving Centers (PAC). One of these PAC is set up at the German Remote Sensing Data Center (DFD) of the DLR at Oberpfaffenhofen. Additional to the standard products provided by ESA processors, national value added products will be available from the D-PAC. The DFD is setting up a system to support the user community with precise geocoded products. The experience of the geocoding system for ERS, GEOS, will be integrated into the new geocoding system for ENVISAT (EGEO). EGEO will be capable to geocode data from the different modes, like ScanSAR, alternating polarization and different resolution. Various types of geocoded products are under design: enhanced ellipsoid geocoded, terrain geocoded and mosaics. The geocoding system will get support from other DFD facilities. Very important is a topographic data base. This new database will support EGEO with Digital Elevation Models (DEM) from all over the world generated from InSAR, Ground Control Points (GCP) and other auxiliary information. A new processing environment is planned to adapt the system to state of the art hardware, operating system and software. The implementation will use high speed Unix workstations.

Geometric performance of ENVISAT ASAR products

We describe validation measurements of the geometric accuracy of ASAR images, measured redundantly via independent methods. Our tests include image (IM), alternating polarization (AP), and wide swath (WS) mode acquisitions over a variety of test sites. ASARs slant range products (IMS/APS) require a slightly different validation methodology than ground range precision (IMP, APP) and medium resolution products (IMM, APM, WSM). A third approach is required for ellipsoid-geocoded products (IMG, APG). The most highly accurate validation is possible with single look complex (SLC) data (IMS and APS products), as all other product types lose resolution during multilooking. For a library of ground control points (GCPs) including map features such as bridges or road intersections, as well as (where available) transponders and corner reflectors, we use surveyed or map-measured position information (together with the delay value in the case of transponders) to solve the zero-Doppler iteration and predict the position of the GCP as an azimuth and slant range coordinate in the radar image. In the case of ground range products (e.g. IMP, APP, IMM, APM, WSM) the predicted slant range value is additionally transformed by a slant to ground range transformation tro determine the predicted image coordinate. The GCP feature is then either measured by inspection of a detected image, or localized automatically within the neighborhood of the prediction. GCPs are measured within the radar geometry image products, derivative geocoded products, and topographic maps, providing their measured map, radar geometry, and nominally geocoded GTC locations. Radar image locations are compared to map reference values and statistics of differences are tabulated. We compare the accuracies of the estimates achievable using transponders and map GCPs. Based on the suite of products (and accompanying orbit information) available to us, we establish a methodology for estimating a preliminary sampling window start time bias. The multiple validation and estimation techniques used ensure robust determination of ASAR geolocation accuracy.