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Operational Interferometric Process for the Generation of a Digital Terrain Model

Youcef Smara(1)



Operational interferometric process for the generation of a digital terrain model applied to the couple of images ERS-1 ERS-2 to the area of Algiers

A. Belhadj-Aissa, F. Hocine, M. Belhadj-Aissa, M.Ouarzeddine, Y. Smara

Faculty of Electronics and Computer Science

Houari Boumediene University of Sciences and Technology,

Image Processing and Radiation Laboratory, Algiers


Interferometry radar is a recent technique of image processing based  on the use of two images radar of the same scene acquired by two radar antennas separated by a minimal distance known as baseline. These two images are better acquired in a reduced time interval.

One of the principal applications of interferometry radar is the generation of ground digital models. This interferometric technique exploit the information of phase signal backscattered by the radar.

We present in this communication the interferometric procedure which we are developing. as well  as the practical problems we are facing. The data used is a couple of SLC (Single Look Complexes) images acquired in the Tandem mission of the ERS-1( 3 January 1996 ), ERS-2( 4 January 1996 )) satellites of a part of  Algiers area.

Three basic steps give the interferometric procedure.

1/ The image processing are the coo-registration:  5 looks images are generated giving  20*20m resolution images. The 5 looks transformation allowed to find the objects shape and to reduce the speckle effect. The images coo-registration quality  for the interferometric process is very important for a good result. (complex interferogram,  coherence, interferogram).

We used in a first step acquisition parameters

2/ In this step we generate the interferometric product and the flat earth file. The complex interferometric is given by

Where I C1 , I C2   are the master and slave images. In the formula above the window is 3*3.

As a result we give in " figure 1 " the intensity image of the Blidean Atlas (Algiers)extracted from the global scene. The image size is 500x500 pixels.

In " figure 2 " we present the correlation of the two images given by the coherence map. This image gives an idea about the interferogram quality and consequently the precision of the digital terrain model.

The interferogram which is the phase difference between the two images is given in " figure 3 ". In fact this phase difference which is between 0 and 2 p is related directly to the elevation and must be corrected from the flat earth effect. This later is cased by the baseline.

In " figure 4 " the flat earth image is given. In the ideal case this image is presented by fringes parallel to the azimuth axis.

The interferogram corrected from flat earth is presented in " figure 5 ".

3/ Phase unwrapping step has been tested using the least squares method where we minimize the quadratic difference between the unwrapped phase and the gradient of the wrapped phase. which is a global one.  The markovian unwrapping method which is a mixed one in being implemented.

figure 1: Intensity image (500x500)

figure 3: Phase difference image (500x500)

figure 5: interferogram corrected image(500x500)

figure 2:Coherence image (500x500)

figure 4: Flat earth image(500x500)

figure 6: Phase unwrapping image(500x500


Workshop presentation

Full paper

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry