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ANALYSIS OF THE GEOMETRIC PARAMETERS OF SAR INTERFEROMETRY 
FOR SPACEBORNE SYSTEMS 
Rüdiger Gens and John L. van Genderen 
ITC, 350 Boulevard 1945, P.O. Box 6, 7500 AA Enschede, The Netherlands 
Tel. ++31-53-4874444 Fax. ++31-53-4874400 E-Mail: gens@itc.nl 
Commission II, Working Group 4 
KEY WORDS: Geometry, Analysis, SAR Interferometry, Satellite 
ABSTRACT 
Since SAR interferometry was first introduced for topographic mapping by Graham in 1974, the suitability of various applications of 
this technique has been investigated. The second main issue has been the reconstruction of digital elevation models (DEMs). By 
using differential interferometry a wide field of applications such as the monitoring of change detection, ice sheet motion, seismic 
events and volcanic hazards has been opened. The three different implementations of the geometry (along-track, across-track, and 
repeat-track interferometry) has been further investigated. Along-track interferometry is used on airborne systems for the observation 
of ocean currents. Across-track interferometry is the standard geometry used for airborne systems. Repeat-track interferometry is 
realised using data acquired by spaceborne systems from slightly different orbits. This paper focuses on spaceborne systems, because 
the available systems have a standard due to their equipment. This allows a comparison of the different systems which is not always 
possible with airborne systems. Some of these systems, for example, use a number of motion compensation systems which improves 
the quality of the data significantly. A complete review of the field of SAR interferometry is given by Gens and van Genderen 
(1996). 
It has been part of the recent research in this subject to investigate suitable applications for SAR interferometry. This has especially 
included the investigation of the limitations of this technique. The quality of the results, in the form of coherence images as well as 
interferograms, depends on several parameters which have a complex influence on the geometry. These parameters are analysed in 
this paper in more detail. 
KURZFASSUNG 
Nach der ersten Anwendung von SAR Interferometrie zur Erstellung topographischer Karten durch Graham 1974 wurde die 
Eignung dieser Technik für verschiedene Anwendungen näher untersucht. Das zweite Hauptanliegen war die Rekonstruktion 
digitaler Höhenmodelle. Durch die differentielle Nutzung der Interferometrie hat sich ein weites Feld von Anwendungen, wie z.B. 
das Erfassen von Oberflächenveränderungen, die Bewegung von Eisflächen sowie die Untersuchung von Erdbeben und 
Vulkanausbrüchen, geöffnet. Es gibt drei verschiedene Umsetzungen der Geometrie (along-track, across-track und repeat-track 
Interferometrie), die grundlegend erforscht wurden. Along-track Interferometrie wird von Flugzeug getragenen Systemen zur 
Beobachtung der Meeresoberfláche benutzt. Across-track Interferometrie ist die Standardgeometrie von Flugzeugsystemen. Repeat- 
track Interferometrie wird durch Daten realisiert, die durch Satellitensysteme von zwei nebeneinander liegenden Orbits erfaDt 
worden sind. Dieser Artikel befaßt sich ausschlieBlich mit Satellitensystemen, da die verfügbaren Systeme einen gewissen Standard 
haben. Dies ermóglicht einen Vergleich dieser Systeme, was bei Flugzeugsysteme nicht móglich ist. Einige dieser Systeme nutzen 
z.B. Geräte zur Bewegungskompensation, die die Qualität der erfaBten Daten signifikant verbesssert. Ein vollstándiger Überblick des 
Bereiches der Interferometrie ist bei Gens und van Genderen (1996) zu finden. 
Ein Teil der derzeitigen Forschung in diesem Bereich konzentriert sich auf die Erforschung móglicher Anwendungen für die SAR 
Interferometrie. Dies schlieft speziell die Erforschung der Grenzen dieser Technik bezüglich der Anwendung mit ein. Die Qualität 
der Ergebnisse in Form von Kohárenzbildern und Interferogrammen hángt von verschiedenen Faktoren ab, die einen komplexen 
Einfluß auf die Geometrie haben. Diese Faktoren werden in diesem Artikel im Detail analysiert. 
1. GEOMETRIC PARAMETERS The position of a satellite is described by its orbit. The main 
problem here is the accuracy of the determination of the orbit. 
The first error source for the geometry is the satellite system The precision of tracking varies for different satellites. For 
itself. All the available spaceborne systems are well calibrated, example, with the launch of ERS-2, the Precise Range and 
but, nevertheless, they have different characteristics, e.g. Range-rate Equipment (PRARE) system is supposed to provide 
different incidence angles, different spatial resolution, etc. The additional information for determining the antenna position 
calibration of the satellites has to be very precise to reach the more precisely during the data acquisition. The fact that the 
expected accuracy, i.e., minor effects such as an internal clock orbits are often not exactly parallel leads to range migrations 
drift which can cause phase artefacts (Massonnet and Vadon, which can be removed during the data processing. Another 
1995) have to be taken into account even at this stage. The point is the time the satellite needs to repeat in a slightly 
System introduces a small amount of error caused by systematic different orbit. The accuracy needed for a specific application 
effects such as system noise, image misregistration, etc. determines whether data e.g. from RADARSAT (24 days repeat 
107 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B2. Vienna 1996