Measuring Surface Water Topography in the Mississippi Delta, Louisiana, Using Airborne Ka-Band Radar Interferometry

AirSWOT is an airborne Ka-band radar operated by the NASA Jet Propulsion Laboratory/California Institute of Technology. It was designed to be a calibration and validation instrument for the forthcoming Surface Water and Ocean Topography (SWOT) mission, and is capable of both single-pass cross-track and along-track interferometry, allowing it to measure surface water elevation. During a multiple day campaign in May 2015, AirSWOT collected data in the Mississippi Delta, Louisiana area, covering the Wax Lake and Atchafalaya River deltas. To obtain accurate measurements of water surface elevation, the data is calibrated to compensate for aircraft motion and radar hardware artefacts. A time-varying interferometric phase drift is estimated for each flight line. The phase drift is estimated using the radar elevation measurements over the imaged region compared to in situ water level stations, other overlapping images, and by flattening the water surface with respect to the assumed geoid over the ocean. We have produced water surface elevation maps for the data collected during three different days using a standardized data processing flow, demonstrating the repeatability and robustness of this new measurement technology and methodology. The resulting AirSWOT data provide maps of water surface elevation, revealing its surface topography at a spatial resolution of 3.6 m, and covering over 2500 square kilometers. The water surface elevation measurements were validated using in situ data from water level stations spread across the study area (which were excluded from the calibration procedure). The AirSWOT data was also used to estimate the water surface slope along the Wax Lake Outlet, yielding results within 10% of in situ measurements on all three days of the campaign. While SWOT data acquisition frequency will not provide fast-repeat measurements of elevation in these highly dynamic coastal systems, AirSWOT enables characterization of water surface dynamics at the time scale of tides. We plan to use this novel, spatially explicit measurement of water surface topography to constrain hydrodynamic models in rivers and deltas.