Matsui Kai (JAXA, Japan)

Japan Aerospace Exploration Agency (JAXA) is working on a conceptual study of altimeter mission named Coastal and Ocean measurement Mission with Precise and Innovative Radar Altimeter (COMPIRA), which will carry a wide-swath altimeter named Synthetic aperture radar (SAR) Height Imaging Oceanic Sensor with Advanced Interferometry (SHIOSAI).　As a feasibility study, airborne cross-track InSAR is conducted using two types of airborne SAR systems. One is AT-SAR developed by Alouette Technology Inc., which is designed to simulate SHIOSAI by fitting geometry (baseline/slant range) and another one is Pi-SAR-X developed by National Institute of Information and Communications Technology (NICT). The AT-SAR observations were conducted along Jason-2 ground tracks to compare the derived SSHs with ones from Jason-2. Coherences and interferograms are calculated using these InSAR data. First, the coherences are compared with theoretical values that are calculated considering decorrelations due to thermal noise, baseline, co-registration, and quantization. The calculated coherence from AT-SAR is about 0.94, being a little lower than 0.98 of theoretical one, while those form Pi-SAR-X are almost consistent with the theoretical ones. Using the Pi-SAR-X result, interferograms are converted to sea surface heights and the error of the derived SSH is compared with the theoretical one due to thermal noise. The errors with respect to an incidence angle are estimated as a standard deviation of the derived along-azimuth SSHs. The results show that the estimated errors approach the theoretical ones with decreasing the look numbers used for the SSH estimation. It is suggested that SSH variation except for noise component, such as, wave height, contaminates the comparison for the large look number cases and these effect is reduced with decreasing look numbers. In fact, wave heights have a relationship with the differences between the estimated and theoretical noises at the extreme large look numbers, indicating possibility of detecting wave information by the cross-track InSAR. Considering these effects, it is evaluated that the InSAR results of Pi-SAR-X meet the theoretical equations in terms of coherence and SSH errors due to thermal noise.
From the AT-SAR data, two types of SSHs over 65km length along the Jason-2 tracks are calculated using the interferogram and the leading edges of nadir echoes and compared with the SSH of Jason-2. Although the derived SSHs capture the large-scale geoid undulation with 5m change over 65km, they have high frequency errors. The rms error of the SSHs from the nadir echoes is less than 10cm, while those from the interferogram is about 1m with periodic errors probably due to attitude oscillation of the airplane.