A review of CryoSat-2/SIRAL applications for the monitoring of river water levels
Bercher, Nicolas1; Calmant, Stephane2

Three aspects of the CryoSat-2 ice mission can be highlighted regarding hydrology and particularly the monitoring of river water levels. First, CryoSat-2 flies on a drifting orbit characterised by a very long repeat period (369 days) and a really dense spatial coverage (7km inter-track distance at equator). The traditional concept of virtual station (i.e., the crossing point between a river and a satellite track) used to derive water level time series vanishes because of the very long repeat period. New approaches must be implemented, for example, to extract and filter the data. Among the new applications of this drifting orbit, CryoSat-2 allows us to build spatio-temporal time series of the river water level, map the river's topography, derive pseudo-time series and pseudo-profiles of the river water level. Second, CryoSat-2 embed the first nadir altimeter to implement SAR mode. This paves the way to assess the performance of future SAR missions such as Sentinel-3 and Jason-CS. The SAR mode refines the along-track sampling resolution up to 300 meters. Moreover, its synthetic processing allows to focus the synthetic beams anywhere along the satellite track. Hence, a new and exciting application consists in increasing the along-track sampling resolution over rivers (say by factor 10) and select, among the produced SAR echoes, the less contaminated echo. The ability to focus accurately the SAR beams in the middle of the river channels, coupled to the higher along-track resolution, definitely improves the ability to detect and monitor tighter rivers that we couldn't monitor before. Using simple processing routines, it is possible to derive pseudo-LRM (or so called "reduced-SAR") data from SAR measurements and directly assess the benefits of the SAR compared to the conventional LRM (Low Resolution Mode). Finally, CryoSat-2 is the only mission planned to implement SARin mode, still this mode may be even more interesting than SAR for inland waters applications (despite its slightly higher noise level, that is still under the requirements for inland applications). Just like the SAR mode, SARin mode allows to focus the synthetic beams anywhere along the satellite track. Moreover, its robust tracking procedure ensures that the altimeter nearly never lost track of its target. Finally, the SAR interferometric measurements (that exploits the phase difference between the two across-track antennas) make it possible to locate the source of the retracked echo. As a consequence, the SARin mode is really well suited for the monitoring of upstream sections of rivers where topography typically make it difficult for conventional LRM altimeters to track the water surface. In some ways, CryoSat-2 can be seen as the closest altimetry mission to the future Swot mission, thanks to the SARin mode and the very dense orbit. This abstract will give an overview of the major advances made thanks to the CryoSat-2 mission and products for the monitoring of river water levels. Detailed examples will be given for several basins around the world, as well as validation results (vs. in situ data) and comparisons to conventional altimeters (Envisat, Jason-2).