Scientific Justification
Exact information about the Earth’s surface is of fundamental importance in all geoscience areas. For example, ecology investigates the dependencies between all life forms and their environment such as soil, water, climate and landscape – all are influenced by topography.
DEM’s can be derived from a variety of missions. However the resulting mosaic of data from different sources with a multitude of horizontal and vertical data, accuracies, formats, map projections, time differences and resolutions is hardly a uniform and reliable data set. The SRTM (Shuttle Radar Topography Mission) was the first step to meet the requirements of the scientific community for a homogeneous, highly reliable DEM fulfilling the DTED 2 specifications. However, many scientific and commercial applications require improved accuracy, corresponding to the DTED 3 standard (12 m posting and < 2 m height accuracy), comparable to DEM’s generated by high resolution airborne SAR systems.
TanDEM-X will generate a consistent and reliable DEM data set with global coverage, having a substantially improved spatial and height resolution. The emerging applications research covers numerous areas, such as hydrology (ice and snow, wetlands, morphology and flooding), geology (geological mapping, tectonics, volcanoes and land-slides), land environment (cartography, urban areas, disaster and crisis management, navigation, archaeology and change detection) as well as renewable resources (land use mapping, agriculture, forestry and grassland).
TanDEM-X will be able to measure the velocity of moving objects with high accuracy, e.g. for monitoring ocean currents and sea ice drifts on a large scale.TanDEM-X will exploit the potentials and challenges associated with new SAR techniques, e.g. bi-static multi angle imaging, digital beamforming, polarimetric SAR interferometry and super resolution.
The scientific merit has been endorsed with a questionnaire distributed to a large number of scientists. Many of the scientists represent end-users and have a long experience with the SRTM and SIR-C/X-SAR data evaluation. In the following a summary of the selected scientific applications concerning their needs and added values is given.
Hydrology: Accurate maps of surface topography are a pre-condition for monitoring and modeling glacier mass balance, glacier climate interactions and run-off from glacier basins. In addition, temporal changes of glacier topography over several years would provide detailed and unique information of multi-year glacier mass balance with near global coverage, which are up to now unknown quantities. Surface lowering of retreating glaciers may amount to meters per year, thus being well detectable by time series of surface topography. Due to the low X-Band penetration, TanDEM-X will provide a unique data set on changes of glacier mass and on the sea level contribution due to glacier retreat.
Forestry: High resolution radar interferometry allows the assessment of three dimensional canopy architectural descriptors. These descriptors, such as crown size statistics, vertical distribution of the crown layer, gap occurrence intensity, etc., allow forest degradation/regeneration stage mapping or natural regeneration mosaic mapping. Information on the forest state is needed for land use planning, forest rehabilitation planning, forest protection, fire prevention and nature conservation. Better knowledge of the forest state, and the possibility to monitor this state, or to monitor the natural succession stages, are of importance for climate studies related to global carbon cycle science and ecology (biodiversity). The TanDEM-X mission would provide a unique technology in terms of resolution and temporal consistency to satisfy these requirements.
Navigation: There is a need for the development of world wide terrain databases and related system processes for use in safety critical aviation applications like Enhanced Ground Proximity Warning Systems or Synthetic Vision Systems. The database will be based on multiple sources contributing to a consistent terrain database to support aviation applications. The processing supported by the system will also include reliable, verified, and standardized quality parameters supporting accuracy and statistical needs for a consistent safety critical terrain database. The proposed project and its technical outcome will be a highly desirable contribution to improve the overall system and the quality of future products in an ongoing development effort for safety critical aviation terrain databases.
As far as the secondary scientific applications are concerned, along-track interferometry will allow innovative applications to be explored. Along-track interferometry can be performed by the so-called dual-receive antenna mode and/or by adjusting the along-track distance to the desired value. By means of newly developed orbit concepts, the along-track component can be adjusted from 0 to several kilometers.
Oceanography: Up to now space-borne SAR is the only instrument, which is able to provide information on wave height, wave propagation direction and wavelength on a global and continuous basis. SAR is able to provide information on high resolution wind fields, which are of particular value for coastal applications like offshore wind farming. Along-track interferometric SAR data contain information on ocean currents. Currents are e.g. an important factor in the generation of extreme waves, which can be dangerous for ships. The additional current information will open up a wide range of new applications, like e.g. the investigation of current wave interaction, which plays a key role in the generation of dangerous sea states.
Moving Target Indication: There is a need in several applications for the detection of ground moving targets and the estimation of their velocity. This outstanding capability of a SAR system with along-track interferometry can be used e. g. for a wide area traffic monitoring mode. Signal processing for moving target indication using a tandem configuration shows great potential for more accurate estimation of the moving target velocity, especially due to the combination with the dual-receiving antenna mode of both TSX-1 and TanDEM-X satellites. With such a configuration, new and more algorithms can be developed for estimation of the velocity component of the moving target in all directions.
Within the scope of the secondary objectives, new SAR techniques will be explored which open new perspectives for future SAR systems:
Multi-static SAR mode: The main goal of the planned investigations is to gain further insight into the potentials and challenges associated with multi-static SAR systems. In the future, co-operating and distributed radar systems will provide a wealth of innovative imaging modes. The distributed functionality in multi-static radar will enable affordable and cost-efficient SAR sensors that outperform today’s mono-static SAR systems by far. The development of a fully operational multi-static SAR requires further investigations in close formation flight, autonomous orbit control, instrument synchronization, relative orbit determination and new strategies for mission planning. Furthermore, innovative processing algorithms are required to exploit the full potentials associated with a multi-static system configuration.
Super Resolution: The signals received by two satellites show different aspect angles for each scattering point at ground. This property can be used to achieve a resolution finer than nominal by combining the received signals coherently. The product of such a mode would be SAR images with increased resolution – super resolution. There is a need on developing adapted signal processing tools to separate or extract individual reflectors or clusters that are closed to each other (compared to the image resolution cell), while preserving their phase as much as possible. These algorithms could then be applied on structures such as buildings, electric poles, bridges etc. (also called “micro-reliefs”) to get a better understanding of their contribution (especially 3D and multi-paths) and of their interaction with the background (for clutter height measurement for instance).
Polarimetric SAR Interferometry: The DEM optimization using polarization diversity helps to minimize interferometric phase variance and hence helps to achieve high quality interferograms as well as high resolution DEM’s over land surfaces. Estimating the vegetation bias in X-Band SAR interferometry and integrating them with data from other sensors (L-band air and space borne SAR) is useful to extract maps of vegetation structure parameters such as height, canopy layering etc. These application products require a sensor capable of quasi single pass polarimetric SAR interferometry. The improved performance of DEM quality using polarimetry will be central to the achievement of the high resolution DEM and the short time baseline between acquisitions will minimize coherence loss due to temporal effects.
TanDEM-X allows maturing the understanding and the algorithms for several scientific areas and will contribute to the generation of future full operational products.