Radargrammetry and Interferometry Sar for Dem Generation

Digital elevation models (DEMs) can be generate by interferometric SAR (InSAR) and radargrammetry techniques from different positions of Synthetic Aperture Radar (SAR) images. Radar imaging systems record both the phase (or time) and intensity information of the backscattered signals. InSAR utilises the phase information of the images to extract useful geodetic information, such as the height of terrain, ground deformation. However, InSAR technique is constrained by the temporal and spatial separations between the images used, as well as the various atmospheric conditions at the time of acquisitions. In comparison, radargrammetry technique utilises the intensity information in a stereo-pair of radar images. That is similar to stereogrammetry or photogrammetry which is a classic method for relief reconstruction using airborne/spaceborne optical images. In this paper, the advantages and disadvantages of both InSAR and radargrammetry for DEM generation are demonstrated using the ALOS/PALSAR and Envisat/ASAR data. Introduction There are two major techniques for DEM generation using SAR data. One is based on interferometric SAR (InSAR) and the other is based on radargrammetry. In general, higher accuracy DEM can be generated through using InSAR technique; therefore, InSAR is used when application. InSAR method is relatively cost efficient and effective, also wide-coverage on DEM generation. This technique involves interferometric phase comparison from two SAR images acquired at different positions with a separation of perpendicular baselines. It can generate DEMs at metre-level accuracy. InSAR extracts information of the terrain from the phase difference “interferogram” of two SAR data images (Ferretti et al. 2001). However, InSAR DEM generation is subject to decorrelation, atmospheric disturbance and the conditions on incidence angle and Doppler similarity are stringent (Massonnet and Souyris 2008). InSAR need the expectations of a certain range baseline because the interferometry is sensitive to the direction of sensor movement and some other factors. To overcome those limitations, the radargrammetry technique is then an important alternative for DEM generation (Chen and Dowman 2001). The main difference of two techniques is that radargrammetry calculates the image range offset using the position matching of the same ground targets in two images while InSAR calculate the phase difference of two images (Kyaruzi 2005; Sansosti 2004). Radargrammetry is based on stereogrammetry which is a classic method for relief reconstruction using optical remote sensing images. This technique can be applied to radar images for generating good quality DEMs (Paillou and Gelautz 1999). One of the advantages of radargrammetry is less affected by atmospheric influence compare with interferometry. Basically, atmospheric effect on the SAR imagery is same in the radargrammetry or in the InSAR. However, radargrammetry uses the magnitude (intensity) value while InSAR uses the phase difference in SAR imagery. Considerably, magnitude is less affected than phase by atmospheric heterogeneous. The atmospheric disturbance is undesirable for the InSAR processing but not much of a concern for the radargrammetry processing (Massonnet and Souyris 2008). But the radargrammetry uses stereoscopic pairs acquired from different incidence angles. Also, the radargrammetry is different with photogrammetry mainly in three aspects (Schanda 1985): (1) the interaction effects with the surface at radar wavelengths are different from those at optical wavelengths, (2) radar measures distance between sensor to target, therefore the parallax appears reversed when compared with optical image, (3) the long wavelengths cause poor angular resolution at a given size of ‘optics’ therefore a useful stereo base cannot be established simultaneously from one simple platform. Furthermore, stereoscopic pairs for radargrammetry should be considered that the geometry and parallax produced from a particular system configuration and the awareness of the image pairs by the interpreter. Obviously, the radargrammetry technique for terrain elevation extraction requires the conditions of have approximately 10~20 degrees incidence angle difference and overlapping of two images between input image pairs (Mercer 1995; kaupp et al. 1983). In this paper, the authors discuss and demonstrate the advantages and disadvantages of both InSAR and radargrammetry using the real satellite data. Radargrammetric DEMs generated using the stereo-image pairs with various look-angles, baselines, ascending and descending orbits are examined closely.