In principle, the along-track InSAR technique known from our  airborne InSAR experiments can be used for current measurements from spaceceborne platforms as well. A comprehensive discussion of theoretical aspects of ocean applications of spaceborne InSAR can be found in the final report of our project  KoRIOLiS. The most obvious difference between airborne and spaceborne InSARs lies in the fact that suitable spaceborne platforms are much faster than aircraft (7000 m/s vs. 100 m/s), thus much longer along-track antenna separations are required to obtain comparable time lags between the images from the two InSAR antennas.

A first demonstration of current measurements from space could be given with data from the Shuttle Radar Topography Mission (SRTM) of NASA, DLR, and ASI in early 2000. For this mission, a combined C band / X band InSAR system with a cross-track antenna separation of 60 m was installed on the Space Shuttle Endeavour with the primary objective to acquire data for the generation of a global high-resolution map of the Earth's land surface topography. For technical reasons, there was an additional along-track antenna separation of 7 m (see Fig. 1), which could be exploited for along-track interferometry with an effective time lag of 0.5 ms.

Even for X band interferometry, a time lag of 0.5 ms is very short and leads to a very low sensitivity of the InSAR phase to small current variations (2π / 38.5 m/s). Furthermore, the amount of data acquired over coastal waters with pronounced signatures of spatially varying currents is very limited, since the SRTM mission had not been optimized at all for ocean applications. However, it has been possible to retrieve useful current fields in a few cases and to validate them by comparison with numerical circulation model results. Fig. 2 shows a result obtained for the Dutch Wadden Sea. Results of a statistical analysis indicate that SRTM resolves spatial variations in the current field on length scales on the order of 1 km in this case, which is consistent with theoretical findings on the basis of  M4S model simulations.

We have evaluated the potential of several other upcoming and proposed spaceborne InSAR systems for current measurements in various recent projects, such as the national German project KoRIOLiS and two studies for ESA-ESTEC on simultaneous InSAR techniques and on TerraSAR-L Interferometric Cartwheel constellations. Theoretically, one can achieve the same current measuring accuracy and spatial resolution with spaceborne InSAR systems as with airborne ones. However, most spaceborne InSAR configurations which are currently being discussed are mainly designed for cross-track InSAR applications such as topographic measurements over land or ice; current measurements are a side issue only. This applies, for example, to the Interferometric Cartwheel constellation proposed by French scientists (see Fig. 3), which consists of three microsatellites with SAR receivers that follow a master satellite with a conventional SAR in such a way that they appear to rotate around each other and provide sufficient baselines for cross-track interferometry throughout the whole orbit. Such systems could also be used for current measurements at certain orbital phases, where appropriate along-track baselines would be available.

The most promising system for current measurements from space in the near future will be implemented with the German high-resolution X band SAR satellite TerraSAR-X, which is scheduled for launch in 2006. The programmable SAR antenna of TerraSAR-X, which will be 4.8 m long, can be split into two parts in an experimental split antenna mode, permitting along-track interferometry with an effective baseline of 1.2 m, corresponding to a time lag of 0.17 ms. According to simulation results, TerraSAR-X will permit current measurements with an accuracy and spatial resolution comparable to the SRTM result shown in Fig. 2.

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Potential applications of current measurements by spaceborne InSAR are similar to the ones mentioned on our airborne InSAR page, but there are some important differences: On the one hand, spaceborne sensors can be used almost everywhere around the world without major efforts, and they provide a good coverage in space and time and are thus well suited for cost effective long-term monitoring activities. On the other hand, airborne sensors are much more flexible in terms of flight times and flight patterns and thus better suited for applications that require snapshots of current fields at certain instants (say, at certain tidal phases). Furthermore, only airborne InSARs can offer spatial resolutions on the order of 100 m or better at present, and only airborne InSARs can provide quasi-instantaneous two-dimensional current measurements if data from two overpasses with different look directions are combined (concepts for air- or spaceborne dual-beam InSARs that permit two-dimensional measurements during a single overpass exist, but such systems are not yet operationally available). Due to the different characteristics, we think that airborne and spaceborne along-track InSARs would complement each other rather than competing with each other; low-resolution results of analyses of spaceborne InSAR data could usually be refined by performing dedicated flight experiments where necessary. To some extent, spaceborne along-track InSARs could also complement altimeter missions, since they would provide direct current measurements in coastal areas, where the retrieval of currents from altimeter-derived sea surface height anomalies is not feasible. For one particular application, the remote sensing of river runoff, one even seeks to obtain collocated InSAR-derived current and altimeter-derived water level data for estimating actual volume transports.