ERS-1 INTERNAL CLOCK DRIFT MEASURED BY INTERFEROMETRY

Radar interferometry opens a very wide field of new applications to synthetic aperture radar data. Although some of them could be satisfied with relative calibration of the phase signal over a short range, many of the applications linked to geophysics would require a global calibration of the signal. This calibration should ideally be valid over several hundred kilometers. Among the potential artefacts to be taken into account to fulfill this requirement are atmospheric thickness variation, improper orbit modeling, spacecraft vibrations. In order to eliminate such artefacts, the orbits must be properly tuned. This can be done by taking advantage of the limited number of the parameters which govern orbital positions. In this paper, we show evidence that the internal clock aboard the ERS-1 satellite can cause phase artefacts. The evidence is obtained through the analysis of a very long segment of ERS-1 data, in which the fringe patterns show features which can be caused only by a drift of the internal clock. A linear drift with time is sufficient to explain the phenomenon: the total frequency change over a five minutes orbital segment is of the order of 5000 Hz. The drift is characterized and its consequences are detailed, in particular on the image geometry. Some new specifications are given which should avoid such an artefact in future SAR systems: with the features of ERS-1, the clock should stay within 200 Hz of its nominal value during the largest interferometric segment envisaged for the mission.