820 IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 48, NO. 2, FEBRUARY 2010 First Analysis of TerraSAR-X Along-Track InSAR-Derived Current Fields Roland Romeiser, Member, IEEE, Steffen Suchandt, Hartmut Runge, Ulrich Steinbrecher, and Steffen Grünler Abstract—We present the first analysis of surface current fields derived from TerraSAR-X along-track interferometric synthetic aperture radar (along-track InSAR, ATI) data. The images were acquired over the mouth of the Elbe river (Germany) during six satellite overpasses in spring and summer 2008, using the experimental “aperture switching” mode of TerraSAR-X. In this mode, the phased-array synthetic aperture radar (SAR) antenna is split into two halves for receiving, but in contrast to the “dual receive antenna” mode, which uses two independent receivers in parallel, a single receiver is multiplexed to process signals from the two antenna halves in an alternating manner at a doubled pulse repetition frequency. The effective ATI baseline is on the order of 0.8 m. The SAR/ATI raw data processing is described in another paper in this issue. This paper focuses on the conversion of the basic interferograms into line-of-sight surface current fields, which includes an elimination of ship signatures, identification, and correction (as far as possible) of imaging artifacts, additional filtering and smoothing, and a subtraction of contributions of wave motions to detected velocities according to a theoretical model. We evaluate the quality of the results by comparison with current fields from a numerical flow model and with available in situ data. The ATI performance of TerraSAR-X is found to be basically con- sistent with theoretical expectations. After applying the same data processing algorithms to all six images, mean differences between TerraSAR-X-derived currents and reference currents in our main test area range from -0.11 to +0.08 m/s in five of the six cases with one outlier at +0.42 m/s. The spatial current variations within the TerraSAR-X-derived current fields are consistent with the model in three cases, but unrealistically strong variations across the images are found in the other three cases. We attribute this to shortcomings of our preliminary raw data processing algorithms, which can probably be fixed after some more detailed analysis and testing. The results obtained so far encourage us to believe that our internal performance goal of a typical current measuring accuracy of 0.1 m/s at an effective spatial resolution better than 1 km can be met. Manuscript received January 28, 2009; revised June 22, 2009. First published October 30, 2009; current version published January 20, 2010. This work was supported in part by the U.S. Office of Naval Research under Grants N00014- 06-1-0931, N00014-08-1-0581, and N00014-09-1-0366 and by the German Research Foundation (DFG) under Grant STA 410/8-1. R. Romeiser was with the Institute of Oceanography, Center for Marine and Climate Research, University of Hamburg, 20146 Hamburg, Germany. He is now with the Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL 33149 USA (e-mail: rromeiser@rsmas.miami.edu). S. Suchandt and H. Runge are with the Remote Sensing Technology In- stitute, German Aerospace Center (DLR), 82234 Wessling, Germany (e-mail: steffen.suchandt@dlr.de; hartmut.runge@dlr.de). U. Steinbrecher is with the Microwaves and Radar Institute, German Aerospace Center (DLR), 82234 Wessling, Germany (e-mail: ulrich. steinbrecher@dlr.de). S. Grünler is with the Institute of Oceanography, Center for Marine and Climate Research, University of Hamburg, 20146 Hamburg, Germany (e-mail: steffen.gruenler@zmaw.de). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TGRS.2009.2030885 Index Terms—Current measurement, interferometry, radar data processing, radar imaging, radar signal analysis, radar velocity measurement, rivers, synthetic aperture radar. I. I NTRODUCTION T HE IDEA of operating TerraSAR-X in an along-track in- terferometric synthetic aperture radar (along-track InSAR, ATI) mode for direct target velocity measurements has been discussed since 2002. The ATI technique—originally devel- oped in the 1980s at the Jet Propulsion Laboratory, described for the first time in [1], and demonstrated with an airborne instrument in [2] and [3]—requires two synthetic aperture radar (SAR) antennas separated in flight (along-track) direction by a short distance. The two antennas acquire images of the same scene with a time lag on the order of milliseconds. Phase differences between the two complex images are proportional to the Doppler shift of the backscattered signal and thus to line- of-sight target velocities. This way, an ATI system permits high- resolution imaging of surface current fields in the open ocean, coastal waters, and rivers, which is attractive for a variety of applications in the fields of oceanographic and hydrological research, operational monitoring, and offshore engineering. After a first introduction of the basic instrument design of TerraSAR-X with 384 individual transmit/receive modules in [4], the possibility of implementing a “dual receive antenna” (DRA) mode for ATI, using two halves of the antenna panel as independent receive antennas, was discussed in [5] and exam- ined in more detail in [6]. In addition to the original concept of using the main receiver and an existing backup receiver of the system in parallel, the idea of an “aperture switching” (AS) mode was presented in [7] and [8]. In AS mode, a multiplexed single receiver is used for both antenna halves at a doubled pulse repetition frequency. This is associated with an increased noise level, a reduced swath width, and problems with azimuth ambiguities in the raw data processing, but switching the SAR from conventional operation to AS mode is a less complex procedure than switching to DRA mode, thus the AS mode can be used more easily and more frequently. The requirements of both ATI modes were taken into account in the hardware and software developments for TerraSAR-X before launch. The theoretical performance of TerraSAR-X in DRA and AS modes for current measurements was analyzed in [9], based on the numerical ATI imaging model described in [10] and spaceborne ATI experiences gained with an image of the Dutch Wadden Sea from the Shuttle Radar Topography Mission (SRTM) [11]. According to the model simulations, current measurements with an accuracy of 0.1 m/s and an effective spatial resolution better than 1 km should be possible in both 0196-2892/$26.00 © 2009 IEEE