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SAR Interferometry for DEM Generation
 Manoj K Arora and Vinod Patel Department of Civil Engg., IIT Roorkee, Roorkee M L Sharma Department of Earthquake Engg. IIT Roorkee, Roorkee mkarora@syr.edu
Remote sensing technology is gaining acceptance day by day for a variety of applications in various fields such as hydrology, geology, environment, transportation, ecology, earthquake engineering etc. The remote sensing systems may be categorised as active and passive systems. The active systems supply their own source of energy or illumination and use it to produce images of the earth surface, whereas the passive systems depend upon other source of energy (e.g., solar energy) for the purpose. There are a number of passive sensors onboard many remote sensing satellites that operate in the visible and infra red (optical) to microwave region of electromagnetic spectrum. Radar (Radio Detection and Ranging) is an active sensor operating in the microwave region. Among many radar-based systems, SAR, has commonly been used in remote sensing studies. The satellites such as ERS-1 and 2, RADARSAT and JERS carry SAR sensors.
The remote sensing data whether in optical or microwave region (active or passive) are used to prepare a variety of thematic maps at various scales with high degree of accuracy. Nevertheless, from engineering point of view, the third dimension (i.e., elevations or heights above mean sea level) of the features or objects on the earth’s surface, is of paramount importance. Various engineering activities such as environmental modelling, rainfall-runoff studies, landslide hazard zonation, seismic source modelling etc. require information about the third dimension in some form or the other.  Conventionally, height (altitude) has been determined through field surveys and to some extent from stereo-photogrammetry provided that the aerial photographs are available. The field surveys though provide elevation information to a high degree of accuracy, but are time consuming, laborious and costly, and provide information on point basis only. The point information on height may not be sufficient for conducting an engineering study on regional basis that requires spatial information. The spatial extent of height can be obtained from DEM. A DEM portrays the topographic information in the form of an array of numbers denoting location of features in terms of their x and y coordinates and the elevation. These data about the features are represented in raster form at a certain spatial resolution. Larger the resolution, cruder is the DEM representation and vice versa. Nevertheless, for plain areas where the elevations do not change much, the resolution may be kept large or coarse whereas in hilly areas, this is generally kept small or fine.  Traditionally, DEMs have been generated from contours in topographical maps produced from field surveys or aerial photogrammetry. The accuracy achieved from these DEMs may be limited to half the contour interval of the map. Thus, for example, a DEM produced from Survey of India (SOI) toposheet at 1:50,000 may be accurate up to a height accuracy of only 10m in plain areas. Due to the availability of stereo images from many remote sensing sensors operating in optical region (e.g., SPOT and IRS 1C/1D PAN), it is now possible to produce DEM at this level of accuracy and greater efficiency than before. Although, height accuracy in the range of 5 m to 10 m may be sufficient for many engineering projects, there are activities that require height information at centimeter to millimeter level accuracy. For example, monitoring of changes on the earth surface due to activities such as land subsidence, landslides, crustal deformations etc. Though, this monitoring can be achieved from Differential Global Positioning Systems (DGPS) but the information from these is again obtained on point basis only. With the advent of InSAR, it may now be possible to obtain height information on spatial basis thereby producing DEM up to millimeter level accuracy. Due to this, the technology is gaining its momentum in many application areas such as lithospheric movements in geology, crustal deformation studies in seismology, global volcano monitoring, landslide monitoring, ice and glacial studies, etc. (Massonnet et al., 1996; Aldsorf and Smith, 1999; Kimura and Yamaguchi, 2000). InSAR uses two SAR images acquired from satellites as mentioned above. Besides this, the data from a dedicated small test mission namely Shuttle Radar Topographic Mission for 15 days in February 2000 may also be used for InSAR. Moreover, with the expected launch of ENVISAT in near future, more SAR data shall be at the disposal of user community thereby strengthening the InSAR activities. Since, the SAR data are acquired in microwave region, it has an added advantage due to its capability to penetrate the atmosphere in virtually all weather conditions. The aim of this paper is to provide an overview of aspects related to InSAR such as the concepts, the data acquisition and processing steps, and the availability of software to perform InSAR for DEM generation.
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