The German satellite TerraSAR-X, scheduled for launch in late 2006, will permit high-resolution ocean current measurements by along-track interferometric synthetic aperture radar (along-track InSAR) in various experimental modes of operation, using different subsections of its X-band SAR antenna array with a total length of 4.8 m as individual receive antennas. Depending on antenna and receive chain settings, various InSAR time lags can be realized in combination with different noise levels, single-look resolutions, swath widths, and incidence angles. We give an overview of the possible InSAR modes and evaluate their suitability for current measurements on the basis of simulated data products. Our results indicate that interferometric stripmap data from TerraSAR-X will be clearly superior to the existing first spaceborne along-track InSAR data from the Shuttle Radar Topography Mission (SRTM); accurate current retrievals can be expected at effective spatial resolutions on the order of 500 m and usable swath widths of 15 or 30 km. Under certain conditions, also ScanSAR data with a maximum possible swath width of 100 km will be suitable. Finally, we consider fundamental relations between along-track baseline, instrument noise, and InSAR phase noise and discuss possible InSAR performance improvements of TerraSAR-X follow-on satellites.