Estimating orbital trajectories from fringe gradients in SAR interferograms for measuring crustal strain

We develop and assess a new orbital tuning approach by applying differential synthetic aperture radar (SAR) interferometry. We use interferograms of the same site for estimating across-track and radial orbit adjustments from fringe gradients caused by orbit uncertainties. Our approach eliminates these fringes by using the improved short-arc orbit estimates. Taking six estimates from the Delft Institute for Earth-Orientated Space Research (DEOS), the approach yields mean standard deviations of 2.4 cm for the across-track and 4.5 cm for the radial components. The interferograms calculated with our post-fit orbital estimates compare favorably with those corrected with a conventional orbital tuning approach. We can now distinguish between orbital and deformation contributions to interferometric SAR phase gradients and are able to measure surface deformation changes over an interseismic time interval longer than one year. Our new approach is limited, however, to well-correlated interferograms where it is possible to measure the fringe gradient. We have also applied Permanent Scatterers (PS) technique to 42 SAR images acquired by ERS between August 1992 and June 1998. This approach estimates the average range change rate of more than 3 million PS with a formal standard deviation of 0.3 mm/yr along the line of sight. We obtain a phase coherence factor greater than 0.8 and a standard deviation of 3 mm for a single line of sight measurement. This velocity field is much easier to interpret than the separate interferometric pairs. Interpreting these interesting features in terms of geophysical models of inter-seismic and post-seismic deformation, however, will require further research effort.