Current measurements by airborne along-track InSAR: measuring technique and experimental results

Since the first demonstration of high-resolution mapping of surface currents by airborne along-track interferometric synthetic aperture radar (along-track InSAR) in the late 1980s, theoretical models of the along-track InSAR imaging mechanism and recommendations for ideal instrument parameters, measuring strategies, and data processing and interpretation techniques have been discussed in a number of publications. However, due to the experimental nature of existing instruments and algorithms and a very limited reference database from actual experiments, potential users have not recognized the along-track InSAR as a readily available tool for current measurements until now. In order to promote the use of InSAR and to validate and demonstrate current measurements on the basis of instrument parameters and data processing techniques proposed earlier, the authors have carried out experiments with an airborne X-band along-track InSAR over spatially varying current fields at two test sites in the German Bight of the North Sea. In this paper, an overview of the experimental scenarios and the acquired data is given, and the newly implemented algorithms for the retrieval of two-dimensional (2-D) surface current fields from the InSAR raw data are described. Using acoustic Doppler current profiler (ADCP) data and predictions of a numerical circulation model as reference, a root mean square (rms) error of spatial variations in the InSAR-derived current fields on the order of 0.1 m/s at an effective resolution of about 100 m is obtained, which is consistent with theoretical expectations. Furthermore, it is shown that the proposed iterative correction scheme for nonlinearities of the InSAR imaging mechanism on the basis of numerical simulations works well and leads to a significant improvement. Altogether, it is concluded that the proposed technique for current measurements by along-track InSAR is efficient, robust, and sufficiently mature for applications that require high-resolut- ion snapshots of surface current fields within areas of some square kilometers, such as the monitoring of bathymetric changes in coastal waters.