1. The WatER Mission in Europe: how can it help science? By Vivien ENJOLRAS (1,2 )*, Ernesto RODRIGUEZ (2), Paul BATES (3), Nelly MOGNARD (1,2), Anny CAZENAVE (1,2) (1) Centre National d’Etudes Spatiales, 18, av. Edouard Belin, 31400 Toulouse, France (2) Laboratoire d’Etudes en Geophysique et Oceanographie Spatiales, 14 av. Edouard Belin, 31400 Toulouse, France (2) Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena CA 91109, United States (3) University of Bristol, University Road, Bristol BS81SS, United Kingdom *Author to whom correspondence should be addressed - Email: vivien.enjolras@gmail.com The Water Elevation Recovery (WatER) mission, which was recently submitted by an international team to the Earth Explorer Core Mission of ESA, is dedicated to the determination of surface water extent, height, and slope. The WatER instrument consists of a Ka band Radar Interferometer (KaRIN) coupled with a nadir Ka altimeter (AltiKa) for filling the nadir gap and for risk reduction in the KaRIN calibration. There is a strong need to determine the value added science that can be attained from various spatial and temporal samplings of surface water storage and movement. A Virtual Mission has been implemented for a while and helps to answer this question. Recent large floods that occurred in 1995 on the river Meuse in Northern Western Europe have led to heightened interest in flood forecasting systems in this region. The VM concept has been applied to this river. Simulated WatER data have been generated and ground processing techniques have been tested in some specific cases. Flood forecasting improvement by assimilating generated WatER data in models is currently under study. Keywords: interferometry, hydrology, Europe Abstract WatER Orbit over Europe  A WatER Virtual Mission has been implemented and first tested through a very critical observation scenario on a narrow river such as the Meuse in Europe  The single-pixel performances of KarIN between 30 centimetres and 2 meters can be really improved thanks to the great quantity of data, even on a narrow river such as the Meuse, by doing some ground post-processing across river and along river  Having a strong reference topography such as SRTM (Shuttle Radar Topography Mission) helps to retrieve the effects of external errors, especially roll and wet troposphere errors, leading to centimetric residual errors  One year WatER data are currently generated over the Meuse and are delivered to flood forecasting specialists to look at the improvement brought by the assimilation of WatER data to their models. The important in situ networks, especially in the Netherlands, is used (from Remco Dost, ITC, Enschede, The Netherlands). Conclusion and Perspectives 15 Years of Altimetry, Venice, March 2006 Preliminary Scene File Creation WatER KarIN Theoretical Performances Developed Program Chart Alsdorf, D.E.; Lettenmaier, D.P. Tracking fresh water from space. Science 2003, 301, 1485-1488. Bates, P.D.; De Roo, A.P.J. A simple raster-based model for floodplain inundation. Journal of Hydrology 2000, 236, 54-77. De Roo, A.P.J.; Gouweleuw, B.; Thielen, J.; Bartholomes, J.; Bongioannini-Cerlini, P.; Todini, E.; Bates, P.D.; Horritt, M.; Hunter, N.; Beven, K.; Pappenberger, F.; Heise, E.; Rivin, G.; Hils, M.; Hollingsworth, A.; Holst, B.; Kwadijk, J.; Reggiani, P.; Van dijk, M.; Sattler, K.; Sprokkereef, E. Development of a European flood forecasting system. Int. J. River Basin Management 2003, 1, 49-59. Rodriguez, E.; Moller, D. Measuring surface water from space. AGU Fall Meeting 2004, H22C-08. Vandemark D.; Chapron, B.; Sun, J.; Crescenti, G.H.; Graber, H.C. Ocean Wave Slopes Observations using Radar Backscatter and Laser Altimeters. J. Phys. Oceanography 2004, 34, 2825-2842. References Performances over Meuse  The main contribution to the global height error budget comes from the instrumental error related to the instrument signal to noise ratio (SNR). The amount of power returning to the instrument is mainly dependent on the roughness of the surface (characterized by the standard deviation of small gravity-capillary waves)  The instrumental error budget is a function of the SNR, the signal wavelength, the interferometric baseline and the geometry of observation. The bigger the resolution pixel, the better the budget. However, a too big pixel will cause phase wrapping; water and land will also mix more easily, impacting the height error budget as well. It is presented for a multi looking process of 4 looks WatER Main Parameters  The orbit is generated with a time step related to the azimuth resolution intended to be reached (basis: 4 Looks)  Instrument thermal noise (see KarIN theoretical performances) is brought in the process relying on the desired multi-looking process, directly impacting the ground pixel azimuth resolution  External sources of errors: wet troposphere, attitude (roll angle), baseline length knowledge, onboard phase calibration.  A way to cope with these errors is to use as a reference interferometric phase map the one generated with the best ancillary information of topography (SRTM,…). Looking at the difference between the real maps and the simulated reference maps (averaged over hundreds of meters pixels to smooth the data) enables to estimate the tilts caused by roll and wet troposphere  The extraction of the river data is based on an amplitude threshold in the amplitude map and the a priori knowledge of the position of the observed river (see scene file creation)  The phase unwrapping (going from interferometric phase to absolute height above the ellipsoid) begins in the Far Range, where the altitude of ambiguity is the greatest (an accurate altitude (DORIS) is used through the process)  To improve the height error budget, two independent processes are computed on the raw elevation data: Cross River Average: considering the same curvilign absciss gets the same elevation Along River Moving Fit, characterized by its river length computation and the a priori elevation basis from the a priori scene file creation Precision depends on the water brightness and the length and width of the water body  Main instrument: Ka-band radar interferometer on Prima platform  Two 50 km swaths on either side of the nadir track  Spatial resolution varying continuously from 70 m x 5 m (near range) to 10 m x 5 m (far range).  First product: amplitude maps of the co- registered returns  Identification of Water Bodies  Second Product: interferometric phase map  Estimate of the topography using timing and phase difference measurements (relative and absolute elevation)  Nadir gap filled with a Ka-band nadir altimeter (AltiKa), also used in the calibration of KarIN  6 am-6 pm Sun-Synchronous Orbit of 16 days (no yaw steering impact and power consumption optimization)