First demonstration of surface currents imaged by hybrid along- and cross-track interferometric SAR

This paper is concerned with the simultaneous measurement of terrain heights and currents using an airborne interferometric synthetic aperture radar (INSAR). For the first time, a hybrid two-antenna INSAR system with both along- and across-track baseline components is used to measure high-resolution digital elevation maps (DEMs) and current fields in a Wadden sea area. Coastal applications like the monitoring of sediment transport or the numerical modeling of morphodynamic processes require measurements of topography and currents. Classical in situ measurements of these parameters are both expensive and time consuming, or even impossible if higher spatial resolution is required. Pure along-track interferometers (ATIs) have demonstrated their ability to provide information on ocean currents, while across-track interferometric systems (XTIs) have been successfully used to measure DEMs. In this paper, a hybrid system with both ATI and XTI components is used to acquire synoptic measurement for the first time. An experiment with an airborne system taking data over a Wadden sea area is presented demonstrating the potential of the technique. A geometrical model is developed for the interferometric phase of hybrid INSAR systems. This model combines ATI and XTI techniques using a baseline that spans between two antennas consisting of along-track and across-track components. In this model, both the effect of topography and the radial velocity of the water surface enter into the resulting interferometric phase. To separate both components, the system takes data of the respective scene flying two or more tracks with different flight directions, e.g., antiparailel tracks. This approach leads to a set of linear equations that has a unique solution for the along- and cross-track phase. Finally, an additional phase bias has to be considered due to the radial velocity and the influence of squint. This is caused by a misregistration effect between both antennas, which is related to cross-track imaging of surface motion. The new INSAR technique is tested with data acquired during a campaign in February 1997 over the Weser Estuary at the German coast. The airborne INSAR system AeS-1 was used. The interferometer configuration consists of two SAR antennas separated by a mixed along-track and cross-track baseline. Two datasets acquired on antiparallel tracks are used. The calculated velocities were compared with a hydrodynamic model operated by the Federal Waterway Engineering and Research Establishment. The experimental results agree well with the numerical model. In particular, the mean velocity of 0.7 [ms.sup.-1] matches in both datasets. Deviations in the fine-scale structure of the current field are discussed. Topographic analysis and validation are performed in a separate investigation. The impact of surface gravity waves and wind drift, which are known to cause significant artifacts in the ATI phase under certain circumstances, is discussed. Index Terms--Airborne radar, along/cross-track interferometry, coastal application, mixed baseline, ocean surface currents.