Full text: International cooperation and technology transfer

M. Crosetto ( \ B. Crippa (t) 
( ] DIIAR - Sez. Rilevamento - Politecnico di Milano, 20133 Milano, Italy 
(+) DRP - Università degli Studi di Messina, 80100 Messina, Italy 
Commission VI, Working Group 3 
KEYWORD: Interferometric SAR, DEM generation, InSAR geometry, calibration. 
The generation of digital elevation models (DEM) with interferometric techniques has become one of the major potential 
applications of SAR imagery. This paper is focused on the geometric aspects of the DEM generation with interferometric 
SAR (InSAR). The different parameters that describe the satellite orbits, the image acquisition geometry and the SAR 
processing are analysed. The accurate geolocation of the InSAR DEMs can be only obtained through the refinement of 
such parameters. A new procedure for the calibration of the InSAR geometry based on the measure of ground control 
points (GCP) is introduced. The results obtained on a test site covering South Catalonia (Spain) are discussed. 
The complex SAR images contain both the amplitude and 
the phase of the radar signal. When used in the 
interferometric way, the phase brings a valued information 
that greatly expands the potentialities of the SAR. The 
main products of InSAR include the DEMs; the Coherence 
Maps, employed in land-use classification; and the 
Displacement Maps used to monitor landslides, 
subsidence areas, etc. The SAR images can be acquired 
either by airborne or spaceborne sensors. A general 
review of the technique is given in (Gens and Van 
Genderen, 1996). In this paper only the generation of 
DEMs using data coming from spaceborne sensors 
(repeat-pass SAR interferometry) is considered. 
The InSAR DEM generation is based on the processing of 
at least two complex SAR images. In order to generate a 
DEM, the SAR data have to undergo several processing 
stages among which there are the image registration, the 
interferogram calculation and filtering, the phase 
unwrapping (i.e. the reconstruction of the full 
interferometric phase value from its principal one, which is 
only known module n) and the phase-to-height 
In the InSAR procedure we implemented (see Crippa et 
al., (1998) for more details) the first processing stages 
(image registration, interferogram generation and the 
coherence calculation) are based on the ISAR- 
Interferogram Generator software (distributed, free of 
charges, by ESA-ESRIN), an effective tool to obtain good 
filtered interferograms and the related coherence images 
(Koskinen, 1995). The filtered interferograms are 
unwrapped using the “branch cuts” approach (Goldstein et 
al., 1988). The phase-to-height transformation (calibration 
of the InSAR geometry and the generation of the DEM) is 
described in detail in the following sections. 
The generation of the InSAR DEM is performed using the 
unwrapped phases, the orbital parameters of the two 
images and few other sensor parameters (available in the 
image auxiliary data) that describe the imaging geometry. 
The transformation from unwrapped phases to heights is 
realised with a very simple procedure that works point 
wise and generates an irregular grid of 3D points (i.e. 
already geocoded). This procedure differs from those 
usually employed in InSAR, based on approximate 
transformations from phases to heights, followed by a 
geocoding (transformation from image space to object 
space). Very often this geocoding is similar to those used 
for the amplitude SAR images, i.e. it requires a known 
DEM. The proposed procedure does not require any a 
priori known DEM and it is quite flexible to allow the fusion 
of data coming from different sources (e.g. ascending and 
descending SAR, SAR and SPOT). 
For each pixel of the interferogram (i.e. for each 
unwrapped phase), starting from the azimuth coordinates 
of the two images (the master one has the same 
geometry of the interferogram, the slave azimuth can be 
easily derived from the image registration transformation), 
the relative acquisition times T are derived using: 
T =T 0 + AT (lin-1) 
where To is the acquisition time of the first image line, AT 
is the pixel size in azimuth direction and lin is the azimuth 
coordinate. The acquisition times known, positions and 
velocities of the master M and slave S satellites are 
calculated (see Figure 1). 
Assuming the master M, the slave S and the unknown 
point P to lie in the same plane (the Doppler centroid 
plane or antenna mid-plane that goes through M), the 
position of S that fulfil the equation (1) is found. Then, 
using the range equation (2), the interferometric equation 
(3) and the Doppler centroid equation (4) the position 
(xp,yp,zp) of the pixel footprint P is estimated. 
Repeating the procedure for all the pixels of the 
unwrapped interferogram, an irregular grid of 3D points is 
generated. The points are known in a geocentric 
Cartesian system (the same used for the orbits), thus a 
transformation to a cartographic projection and to 
orthometric heights is performed. Finally does the 
resampling to get the final regular geocoded grid follow.

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