Astrid Gruber

The TanDEM-X DEM Mosaicking: Fusion of Multiple Acquisitions Using InSAR Quality Parameters

Since 2010, TanDEM-X and its twin satellite TerraSAR-X fly in a close orbit formation and form a single-pass synthetic aperture radar (SAR) interferometer. The formation was established to acquire a global high-precision digital elevation model (DEM) using SAR interferometry (InSAR). In order to achieve the required height accuracy of the TanDEM-X DEM, at least two global coverages have to be acquired. However, in difficult and mountainous terrain, up to five coverages are present. Here, acquisitions from ascending and descending orbits are needed to fill gaps and to overcome geometric limitations. Therefore, a strategy to properly combine the available height estimates is mandatory. The objective of this paper is the presentation of the operational TanDEM-X DEM mosaicking approach. In general, multiple InSAR DEM heights are combined by means of a weighted average with the height error as weight. Apart from this widely used mosaicking approach, one big challenge remains with the handling of larger height discrepancies between the input data, which are mainly caused by phase unwrapping errors, but also by temporal changes between acquisitions. In the case of inconsistencies, the TanDEM-X mosaicking approach performs a grouping into height levels. A priority concept is set up to evaluate the different groups of heights considering the number of DEMs and several InSAR quality parameters: the height error, the phase unwrapping method, and the height of ambiguity. This allows the identification of the most reliable height level for mosaicking. This fusion concept is verified on different test areas affected by phase unwrapping errors in flat and mountainous terrain as well as by height discrepancies in forests. The results show that the quality of the final TanDEM-X DEM mosaic benefits a lot from this mosaicking approach.

Validation of tie-point concepts by the DEM adjustment approach of TanDEM-X

The aimed accuracies for the final TanDEM-X DEM of 10m absolute and 2m relative height error will be ensured by calibration data. One crucial data set for the relative accuracy is tie-points that connect adjacent DEM acquisitions in the approximately 4km-overlap-area with each other. In this paper an improved concept for tie-point candidates is presented that is based on averaging a larger region instead of comparing single points. This concept should be more robust against noise. It is validated by applying the DEM calibration on a simulated test area, as real TanDEM-X data was not yet available. Also, the DEM calibration will be validated for the first time on a larger “real” test site by applying the TanDEM-X processing scenario.

Validation of the absolute height accuracy of TanDEM-X DEM for moderate terrain

In September 2013 the production of TanDEM-X digital elevation model (DEM) started. As the data acquisition for difficult terrain lasted until April 2014, final DEM production started for flat to moderate terrain regions where two final coverages surfice. This paper focuses on a first validation of moderate terrain to prove the absolute height accuracy. In a detailed comparison three DEM tiles from different continents are chosen to validate the TanDEM-X DEM by computing differences to GPS tracks, ICESat validation points, and SRTM. On a global scale all TanDEM-X DEMs produced so far are compared with ICESat and GPS tracks. Both validations presented here for the first time indicate that the absolute height error for moderate terrain for TanDEM-X is below 2m and therefore much better than the specified 10m/LE90.

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.

TanDEM-X DEM calibration: Correction of systematic DEM errors by block adjustment

This paper gives an overview of the DEM adjustment within the TanDEM-X mission. The DEM adjustment estimates residual, systematic height offsets and deformations of each single interferometric DEM acquisition. The challenge of calibrating the TanDEM-X DEMs lies in the magnitude of the systematic errors: these errors are in the same order like the random error of about 2 m. For the estimation of the corrections a least-squares adjustment of adjacent, overlapping interferometric DEMs over a certain earth region is described in this paper. Adjustment results on simulated DEM data are shown to validate the approach. The tests are carried out for different dense ground control point configurations. Further the improvements by a combined adjustment of the two coverages are demonstrated.

Automatic Extraction of Water Bodies from TerraSAR-X Data

Medium resolution SAR satellite data have been widely used for water and flood mapping in recent years. Since the beginning of 2008 high resolution radar data with up to one meter pixel spacing of the TerraSAR-X satellite are operationally available. The improved ground resolution of the system offers a high potential for water detection. However, image analysis gets more challenging due to the large amount of image objects that are visible in the data. Water body detection methods are reviewed with regard to their applicability for TerraSAR-X data. Flood detection approaches for rapid disaster mapping are presented in this paper.

TanDEM-X: DEM Calibration Concept

The TanDEM-X mission will derive a global digital elevation model (DEM) with satellite SAR interferometry. The aimed accuracies are an absolute height error of 10 m and a relative height error of 2 m for 90% of the data. This requires a correction of the elevation heights after interferometric processing by residual systematic DEM errors. The estimation and correction of these errors is called DEM calibration. This paper gives an overview of the DEM calibration strategy within the TanDEM-X mission. First, the error sources and their influence on the DEM are determined by a functional description. Then, a strategy for a new block adjustment of DEMs is set up and evaluated with simulated and real DEMs.

The approach for combining dem acquisitions for the TanDEM-X dem mosaic

For the global TanDEM-X DEM the whole world will be acquired by at least two coverages. Thereby on the one hand phase unwrapping errors are reduced by applying the dual-baseline method and on the other hand a low noise level is ensured even for difficult areas like forests and steep terrain. During DEM mosaicking, the single interferometric DEMs are merged together. This paper focuses on the combination of heights in overlapping areas with significant height differences. The challenge here is to choose the most reliable height value. The improvement applying this strategy in contrast to simple averaging and the general benefit of using more than one acquisition is shown by means of some example mosaics.