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26. First ERS-1/ERS-2 tandem interferogram
This image shows the interferometric phase obtained from the combination of an ERS-1 image taken on 1 May and the ERS-2 image taken exactly one day after, on 2 May. It demonstrates the feasibility of the tandem mission.
An extract of the first ERS-2 image over the gulf of Gaeta in Central Italy was used in the generation of the coherence image. It contains an area of approximately 15 km by 20 km located in the far range of the original SAR image, just North of Gaeta.
The town of Terracina is located on the coast at the left of the interferogramme, while the village of Sperlonga can be seen on the coast on the right hand-side of the image.
In the plain in the middle of the image, one can see the lake of Fondi, which has taken its name from a town situated further into the plain.
The mountains surrounding the plain rise quite rapidly to heights between 800 metres and 1000 metres.
An interferogram illustrates a technical complication associated with INSAR. The phase value (and hence phase differences) is not known absolutely, but is given in the range of 0-360º; i.e., the phase is "wrapped" on to a fixed range of angle. The colour contours cycle through a colour wheel corresponds to a complete rotation of 360º in phase.
In order to compute terrain height and generate a DEM, the interferogram has to be "unwrapped", i.e., the correct multiple of 360º added to phase value of each pixel. A number of algorithms exist to perform this task.
(Rocca, POLIMI, Milano)