TerraSAR-X 1m Spaceborne Radar

Data from high-resolution radar space sensors is increasingly used for commercial applications. Introduced here are specially designed commercial features and products from the new TerraSAR-X satellite. Also discussed is its forthcoming ‘twin' TanDEM-X.

TerraSAR-X has been operational since early 2008. The high-resolution radar satellite launched on 15th June 2007 from the Russian Cosmodrome in Baikonur (Kazakhstan) is the first such to be built under Public Private Partnership (PPP) in Germany, jointly developed and financed by the German Aerospace Centre (DLR). An Astrium subsidiary, Infoterra GmbH, exclusively markets data and services.

Challenging Fields
Synthetic Aperture Radar (SAR) produces images day and night, independent of weather conditions. Parameters and data products are designed to satisfy commercial applications around the globe. The many users include mapping agencies, defence and security institutions, planning and consulting offices, food and natural resource industries, insurance companies, and environmental agencies. The data forms a basis for services such as client-specific image interpretation, topographic mapping, geo-spatial databases, land-cover assessment, terrain analysis and monitoring services. Further challenging fields of application are in detection of the smallest movements of the Earth's surface due to tectonics, volcanism, earthquake and landslide.

Imaging Modes
Data can be acquired in SpotLight, StripMap and ScanSAR modes, with single-, dual- or full polarisation. With a 300MHz bandwidth, ‘SpotLight Mode' achieves 1m resolution across flight direction. The radar beam can be steered like a spotlight in the direction of flight, illuminating a ground scene of 5km x 10km for the longest possible period and achieving 1m resolution. An area of 10km x 10km can be covered in 2m resolution. For acquisition in ‘StripMap Mode' the ground swath is illuminated by a continual sequence of pulses while the antenna beam is fixed in both directions of sight. This results in an image strip of 30km x 50km (standard scene size, length extendable to 1,650km) with continuous image quality (in flight direction) at 3m resolution. Currently available as dual-polarisation images, StripMap acquisitions in quadruple polarisation are being investigated. In ‘ScanSAR Mode', areas of 100km x 150km (standard scene size, length extendable to 1,650km) are covered with 18m resolution. The swath width of 100km is achieved by scanning four adjacent sub-swaths of ground with quasi-simultaneous beams, each at a different angle of incidence.

Processing Levels
Basic Image (L1B) products are the direct output of the Multi Mode SAR Processor developed by DLR and correspond to the Committee on Earth Observation Satellites (CEOS) Level 1b quality. The four processing levels include:
- SSC (Single Look Slant Range Complex): product of the focused radar signal containing amplitude and phase that provides full bandwidth and is equivalent to the standard Slant Range Complex (SLC) products available from ERS 1/2, Envisat ASAR, Radarsat-1 and
X-SAR/SIR-C; side-looking geometry makes the pixels rectangular
- MGD (Multi Look Ground Range Detected): product with reduced speckle and square pixels, the latter obtained by a simple polynomial projection in range; interpolation artefacts are avoided, as no image rotation to a map coordinate system is performed
- GEC (Geocoded Ellipsoid Corrected): a multi-look detected product, resampled and projected to WGS84 assuming one average terrain height, recommended for marine/intel¬ligence applications where topography does not affect location accuracy (a Digital Elevation Model (DEM) is not applied, thus pixel location accuracy varies with terrain)
- EEC (Enhanced Ellipsoid Corrected): a DEM is used to compensate for dis¬tortions in GEC products caused by varying terrain height; geometric quality depends on height accuracy and DEM resolution, terrain type, and incidence angle; DEMs from Shuttle Radar Topography Mission (SRTM) and ERS provide a global basis.

Ortho-imagery
Value-adding steps create ‘Enhanced Image Products', all in standard map projections UTM with WGS84 ellipsoid. By integrating customer-provided DEMs, ‘Orthorectified Images' can be created featuring improved pixel location accuracy. These are quickly interpreted and thus well suited for combining with other information. Interpretation can be further improved to give ‘Radiometrically Corrected Images'. Radiometric corrections are feasible for classifications that do not take into account angular dependencies of the SAR data. Neighbouring orthorectified images can be seamlessly assembled into a ‘Radar Mosaic' to cover an area larger than a standard scene, typically used for map-sheet generation. Merging orthorectified images acquired in ascending and descending orbits further enhances the product, enabling replacement of no-information patches in the one image by information patches in the other. Such an ‘Ascending-Descending Merge' significantly reduces impediments typical of SAR images, such as radar layover, foreshortening and shadows, and yields image products useful for areas with steep mountainous terrain.

Twinning Satellites
TerraSAR-X is to be accompanied by an almost identical satellite, currently under construction, to constitute the TerraSAR-X/TanDEM-X constellation (Figure 3). The aim here is to collect data for a homogeneous global DEM obeying High-Resolution Terrain Information (HRTI)-3 specifications (z-accuracy better than 2m), as defined by the US National Geospatial-Intelligence Agency (NGA). Today there is no system or process providing HRTI-3 DEMs with short response times and global availability. While optical spaceborne systems require cloud-free weather conditions, airborne systems lack large-scale/global coverage capability. TanDEM-X will resolve these bottlenecks.

Further Reading

Adam, N., et al, First TerraSAR-X Interferometry Evaluation, FRINGE 2007, Frascati, Italy.

Roth, A. et al, Geocoding of TerraSAR-X data, Proceedings 20th ISPRS Congress, Istanbul, Comm. 7, pp 840-844, 2004.