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First Results from ERS Tandem Insar Processing on Svalbard
Abstract
This paper describes some results from a work on how INSAR tandem data can be utilized with the EETF SAR processor combined with the GEOSAT orbit determination program. It is demonstrated how an interferogram simulator can be used to detect errors in existing DEM's and how fringes caused by the terrain can be subtracted from the real interferogram. For the first time it has been possible to monitor motion of the lower part of the fast moving glacier Kronebreen with INSAR due to 1 day interval between the two SAR images. A DEM computed from interferograms (INSAR DEM) over Svalbard is assessed using the existing NP DEM. The precise orbit determination makes the fringes follow the coastline very well over parts of the scene, however, uncertainties in the baseline estimation may cause systematic errors in the INSAR DEM over larger areas in the azimuth direction.
... From the literature, it could be found that visual appearance of the fringes is widely used to indicate the quality of interferogram (Eldhuset, et al., 1996;Zou, et al., 2002). That is, clear fringes mean good quality of an interferogram. ...
It is well-known that interferometric SAR (INSAR) is a technology for the generation of high-precision digital elevation models (DEM) and the precise measurement of terrain surface deformation. The accuracy of DEM and deformation measurement is highly dependent on the quality of the interferogram generated. Such an interferogram is constructed by a point-wise complex multiplication of corresponding pixels in both datasets, which are respectively contained in master image and registered image. An exanimation of existing literature reveals that there is no, good, quantitative measure for the quality of interfero-grams, and visual inspection is still the best solutions available so far. By visual inspection, one recognizes those interfer-ograms with continuous fringes as good ones and regards those with many discontinuous interferograms and speckles as being not good. As the pixels in the interferogram represent the phase value values, it is natural to think that, if the quality is good, the phase differences between neighbor pixels should be small, and thus the sum of all phase differences will still be small. This leads to the proposal of “sum of phase differences” (SPD) as a quantitative measure for the quality of interferogram. Two simulation tests have been conducted, and the results show that the SPD is a reliable measure.
... A number of parameters can be used to measure the quality of an interferogram, such as the visual appearance of the fringes (Eldhuset et al., 1996;Zou et al., 2002) and the histogram of coherence of the interferometric SAR (Guarnieri and Prati, 1997;Gens, 1998). All of these methods have their shortcomings. ...
Interferometric Synthetic Aperture Radar (InSAR) is a new measurement technology, making use of the phase information contained in Synthetic Aperture Radar (SAR) images. InSAR has been recognised as a potential tool for the generation of digital elevation models (DEMs) and the measurement of ground surface deformation. In InSAR data processing for the generation of DEMs, the first step is image co-registration, which brings both images into the same coordinate system for the generation of an interferogram for further processing. The accuracy of the resultant DEM will certainly be affected by the quality of the co-registration, which is in turn affected by the set of tie points used and some other factors. In this study, only the interval of tie points is considered. This means that other factors are kept unchanged. Four pairs of SAR images with size of 1760×400 pixels were used for testing. The effects are assessed by a relative measure for the quality of the resultant interferogram and an absolute measure for the accuracy of the resultant DEM. Results show that the effect of tie point interval on the accuracy of the final DEM is not linear. When the tie point interval is smaller than 273×44 pixels (273 pixels in row and 44 pixels in column), the variation in the resultant DEM accuracy is not significant. It is also noticeable that an interval of 205 × 34 pixels always results in the best or very good results. Therefore, it seems that an interval of around 200 × 30 pixels (200 pixels in row and 30 pixels in column) is an appropriate choice for the selection of tie points for image co-registration in hilly areas. © 2006 The Authors. Journal Compilation 2006 The Remote Sensing and Photogrammetry Society and Blackwell Publishing Ltd.
... As part of the ESA AO-Tandem project no. 301 carried out at FFI, 18 pairs of ERS Tandem SAR raw data from descending satellite pass over Svalbard were ordered from ESA/ESRIN [7]. SAR raw data tapes were received from German and U.K. processing and archiving facilities. ...
Interferometric satellite synthetic aperture radar (SAR) data was
acquired from an area on Spitsbergen, Svalbard, during the European
remote sensing satellite (ERS) Tandem mission in 1995-1996. Analyzing
these data sets shows that the estimated SAR coherence is highly
dependent on the satellite baseline length, and that corrections for
this decorrelation effect is necessary if the baseline is a few hundred
meters or more. Meteorological recordings are compared to SAR coherence
estimates made at different seasons and surface categories: glaciers in
motion, glacial forefields dominated by ice-cored moraines, lakes,
rivers, and flat valleys with fine moraine materials like gravel and
sand. It was found that temporal decorrelation effects are mainly due to
changing surface conditions caused by precipitation and temperature
variations around freezing, but that wind redistribution of snow also
may play a role. Structures and cracks in the fjord ice as well as
boundaries of lakes, rivers, and coastlines can be detected in SAR
coherence images because of the contrast between high and low coherence
areas. Low coherence is observed from those parts of moving glaciers
that experience deformations shear, or zones of relative high velocity.
The usefulness of 35-days interferometric SAR (e.g., the foreseen
ENVISAT configuration) will be limited, even in sparsely vegetated areas
like Svalbard, as compared to the ERS Tandem configuration
Synthetic Aperture Radar (SAR) coherence images were analysed over vegetated areas and urban features. Coherence images were formed from interferometric SAR data acquired 1 day or 35 days apart by two European Remote sensing Satellites (ERS). Forested areas are discriminated very well from cultivated fields using 1-day SAR coherence data taken in the winter when the temperatures were below freezing. This is because fields under these conditions decorrelate much less than forest. Open sandy fields gave high coherence for both winter and summer acquisitions. All vegetated areas experienced a strong temporal decorrelation over a 35-day period. This is mainly due to changing wind, precipitation and temperature conditions, but could also be due to vegetation growth or man-made changes. Many urban objects were found to decorrelate slowly with time, regardless of changing weather conditions.
In this paper, we present the results achieved in the long-term subsidence project in Norway, ESA AO-1104, which have made use of ERS SAR data. By applying the differential SAR interferometry technique referred to as Small Baseline Subset (SBAS) approach to a test area covering the city of Oslo, Norway, we show significant subsidence in several areas of more than 5 mm/year in the time period 1992–1999. The results are compared with a similar analysis making use of the Permanent Scatterers (PS) technique. The presented results demonstrate the capability of the SBAS method to detect and monitor urban land subsidence at Nordic latitudes.
Interferometric Synthetic Aperture Radar (InSAR) is a new measurement technology, making use of the phase information contained in the Synthetic Aperture Radar (SAR) images. InSAR has been recognized as a potential tool for the generation of digital elevation models (DEMs) and the measurement of ground surface deformations. However, many critical factors affect the quality of InSAR data and limit its applications. One of the factors is InSAR data processing, which consists of image co-registration, interferogram generation, phase unwrapping and geocoding. The co-registration of InSAR images is the first step and dramatically influences the accuracy of InSAR products. In this paper, the principle and processing procedures of InSAR techniques are reviewed. One of important factors, tie points, to be considered in the improvement of the accuracy of InSAR image co-registration are emphatically reviewed, such as interval of tie points, extraction of feature points, window size for tie point matching and the measurement for the quality of an interferogram.
The results presented show how glacier velocities can be measured
and calculated from ERS tandem INSAR data. A semi-automatic algorithm
using existing and new implemented moduls in the GIS package Arc/INFO
has been developed for the calculation of glacier velocity. The
velocities are decomposed into the flow direction of the glacier using
an external DEM. Velocity fields for the glaciers Isachsenfonna,
Akademikerbreen and Nordbreen at Spitsbergen have been calculated
This analysis was performed with the GEOSAT software developed at NDRE for high-precision analysis of satellite tracking and
VLBI data for geodetic and geodynamic applications.
For applications to ERS-1, a realistic surface force model is used together with the Jacchia 77 atmospheric model, semi-daily
drag coefficients, a 1-cpr sinusoidal along-track acceleration, and the GSFC JGM-2 gravity model. ERS-1 orbits have been derived
for 5.5-day arcs of laser tracking data between July 6 and August 12, 1992. Results from overlapping orbits and comparison
with precise D-PAF orbits indicate an orbital accuracy of 10–15 cm in the radial direction, ~ 60 cm in the along-track direction
and ~ 15 cm in the cross-track direction.