Summary: The German radar satellite, TanDEM-X, is in the process of generating a global Digital Elevation Model - WorldDEM - of unprecedented quality, accuracy and coverage. In this article, William Van Rensalier Parker, associate at PACE Government Services, explains how this sensor, in conjunction with the TerraSAR-X satellite, will provide a seamless world height model.
With the successful launch of the German radar satellite TanDEM-X on June 21, 2010, a new era of global digital elevation information provision has commenced. Together with TerraSAR-X, TanDEM-X forms a high-precision radar interferometer acquiring the data basis for a global homogeneous digital elevation model (DEM). The two satellites form a unique satellite formation flying in a closely controlled helix formation with distances down to only a few hundred meters.
Accuracy of a New Dimension – Pole to Pole
The TanDEM-X mission is to generate a global digital elevation model - WorldDEM - of unprecedented quality, accuracy and coverage. This DEM is intended to be the replacement dataset for SRTM and will be available in 2014 for the Earth’s entire land surface - pole to pole. The accuracy of the WorldDEM will surpass that of any satellite-based elevation model available today and will have the following unique features:
- Vertical accuracy of 2m (relative) and 10m (absolute)
- 12m x 12m raster
- Global homogeneity
- Highly consistent dataset as a result of an initial global data collection window of 2.5 years and the opportunity to continue to collect locally beyond the initial collection period
- No need for ground control information due to the high geometric precision of German sensors
The WorldDEM is a game-changing, disruptive technology providing a seamless world height model that can be refreshed using the same data source. The two sensors acquire data absolutely reliably, as they operate independent of cloud coverage and lighting conditions.The worldwide homogeneous acquisition guarantees a global DEM with no break lines at regional or national borders and no heterogeneities caused by differing measurement procedures or data collection campaigns staggered in time.
Milestones of a Success Story
Following the picture-perfect liftoff on June 21, 2010 from Baikonur Cosmodrome in Kazakhstan, TanDEM-X recorded the first images in record time: just three days after the launch. A mere month later (and a week ahead of schedule) scientists at the DLR processed the first examples of DEMs created from data collected by the TerraSAR-X / TanDEM-X satellite formation, even though the satellites were not yet flying in their final close formation. On October 19, 2010, the world’s first DEM data using a free-flying bi-static SAR satellite formation was recorded and confirmed all expectations in regard to accuracy and quality of the data. On December 14, 2010, the operational part of its mission commenced. http://www.ilslaunch.com/launch-services/baikonur-cosmodrome
In early 2012, the TanDEM-X mission completed the next important milestone: the first complete coverage of the Earth’s landmass. This is an impressive achievement in this highly complex mission considering that the 150 km2 of landmass had to be seamlessly covered. This achievement proves that the highly sophisticated acquisition plan devised by the experts at DLR and Astrium over countless hours of computing, simulations and discussions is working. And in addition, the programming specialists were able to fully secure the continued smooth acquisition and delivery of TerraSAR-X data to customers and scientists around the world. This meant assuring the uninterrupted acquisition of time-series data, integrating short-term requests in case of unexpected events, as well as executing large-scale regional monitoring and mapping projects on schedule together with all other routine data orders.
Following the first coverage, the two satellites are already continuing with the second coverage, which again will require approximately one year for completion. More highly complex terrain areas will then be selected and covered with a third acquisition campaign to ensure the consistently high quality and accuracy of the final product.
Solutions for Challenging Production and Processing
Astrium GEO-Information Services’s German division, Infoterra GmbH, holds the exclusive commercial marketing rights and is responsible for the adaptation of the elevation model to the needs of commercial users world-wide. Astrium will refine the DEM according to customer requirements, for example, editing of water surfaces or processing to a digital terrain model (representing the bare Earth’s terrain). This refinement is being coordinated with the Geoinformation Office of the German Federal Armed Forces (BGIO) and National Geospatial Intelligence Agency (NGA). This coordination includes the establishment of requirements and procedures related to editing of water surfaces or terrain. http://www.astrium-geo.com/en/
Astrium is in the process of developing software tools for the DEM editing process. The tools are being tested on radargrammetric DEMs derived from multiple TerraSAR-X data takes. The end product is known as Elevation10 and is slightly better then the current SRTM DTED 2.
From the first global acquisition DLR will process the single baseline data (IDEM) above 60 degrees-north that will be utilized as a reference tool in building the final WorldDEM. This DEM can be made available on a retail level to interested customers. It will be an unedited level three single interferometric coverage with voids, as it is a single look, but good quality with a HRTe level of detail assessed and confirmed by NGA and BGIO. http://www.scribd.com/doc/1826785/US-Army-
Under the leadership of Infoterra / Astrium Services, BGIO and NGA, an endeavour is under way to develop a small group of nations into an MGCP line WorldDEM Alliance. The purpose of the alliance is to share the editing effort and cost of the WorldDEM, while ensuring uniformed DEM quality across the different partners sharing the editing effort. Different models are currently under discussion focusing on the work share division and hosting options for the final WorldDEM.
Digital Elevation Data for Versatile Applications
Precise elevation data is the initial foundation of any accurate geospatial product, particularly when the integration of multi-source imagery and data is performed based upon it.
A multitude of applications rely on dependable and accurate data, such as thematic mapping, change detection analysis, surface movement monitoring, as well as a more targeted preparation of defense and security related missions. In the future, operators of civil and military Earth observation satellites can be confident: No matter where their acquisition area is located on the planet, thanks to WorldDEM a standardized elevation model will be available for the orthorectification process. Cartographic authorities around the world will be able to improve or update their standard cartographic maps thanks to this more accurate and up-to-date data source. In addition to the update of the height information, the radar data used for generating the DEM can also serve as input data for change detection analyses (development of urban areas, infrastructures, etc.) and thus supports the updating of existing map material.
The global availability of the dataset without seam lines and heterogeneities along national or regional borders will also support international cooperation and cross-border mission planning. Particularly when the rapid provision of accurate information is of utmost importance (for instance in case of emergency situations like natural or environmental disasters), the availability of a standardized, highly accurate DEM will be a major advantage as it will enable the provision of reliable information to rescue teams on the ground.