Development of Algorithms for the Exploitation of ERS-ENVISAT data by using the Stable Points Network technique.

Radar interferometry is a well-established remote sensing technique. Up to now, acquisitions from compatible radar sensors were processed. During the past ten years, a huge amount of data was acquired by the SAR sensors of the satellites ERS-1 and ERS-2 offering a wide range of applications.

In March 2002, ESA launched the new radar sensor ENVISAT/ASAR.

The radar interferometry techniques, together with the unique amount of ERS data available allow for the study of ground deformation phenomena with high accuracy and without the need for ground instrumentation.

However, classical interferometry suffers from several drawbacks which can be classified into 2 groups:

•	artifacts in the phase measurements
•	geometrical and temporal constraints in the choice of the images

Recently, the Stable Points Network (SPN) technique has been proposed, allowing for the implementation of interferometric measures with high accuracy (millimetric displacements) and reliability (artifacts can be detected and compensated).

ENVISAT has onboard the advanced SAR sensor which has been very much improved with respect to ERS AMI. However, due to technical constraints the operating frequency of the ASAR had to be slightly different from the one of ERS 1 and 2, leading to difficulties – but not to impossibility- for implementing the interferometry techniques.

The interferometric combination of ENVISAT and ERS data is not as simple as it used to be with the similar constructed SAR sensors on board the ERS-1 and ERS-2 satellites. Especially, the difference of 31 MHz between both radar center frequencies prevents the simple combination of the different sensor data because the interferometric phase is obviously strongly dependent on the wavelength and thus on the radar frequency.

The radar center frequency of ENVISAT/ASAR (5.331 GHz) has been slightly changed compared to the sensors ERS-1 and ERS-2 (5.300 GHz). To compensate for this frequency change, the interferometric observation geometry needs to fulfill certain requirements, i.e. large baselines or stable points.)