Validation of Jason-3 tracking modes over French rivers

Sylvain Biancamaria, Thomas Schaedele, Denis Blumstein, Frédéric Frappart, François Boy, Jean-Damien Desjonquères, Claire Pottier, Fabien Blarel, Fernando Niño
2018 Remote Sensing of Environment  
19 Satellite nadir radar altimeters have been widely used to measure river and lake surface water 20 elevations. They can now retrieve the elevations of some rivers less than 200 m wide. However, 21 as these satellite missions are primarily designed to observe ocean surface topography, they are 22 not always able to observe continental surfaces. For steep-sided rivers (i.e. in river valleys no 23 more than a few km wide and surrounded by slopes over 50 m high), altimeters tend to observe 24 the
more » ... top of the surrounding topography rather than the river itself. 25 The Jason-3 altimetry mission, launched in January 2016, has an alternative instrument operation 26 mode, the so called Open-Loop (OL) or Digital Elevation Model (DEM) tracking mode. This 27 mode is intended to help overcome this issue, by using an on-board DEM. However it was not 28 used in 2016 as the operational mode because of difficulties in defining an accurate on-board 29 global-scale DEM. Mainland France has been chosen to test the OL tracking mode, as water 30 masks and DEMs of sufficient accuracy are available. 31 Following the launch of Jason-3, Jason-2 (its predecessor) was maintained on the same nominal 32 orbit as its follow-on, for more than 6 months. During this tandem period, data from the first 10 33 Jason-3 cycles (a Jason-2/-3 cycle corresponds to 10 days) were acquired in the traditional 34 Closed-Loop (CL) tracking mode. Jason-3 data from the last 13 cycles were acquired in OL 35 tracking mode. Jason-2 was always in CL tracking mode. Compared to nearby in situ gages and 36 for river wider than 100 m, Jason-3 water elevation anomalies have a RMSE between 0.20 and 37 0.30 m for most reaches. Jason-3 performance over narrow rivers is similar to that of Jason-2. In 38 CL tracking mode, Jason-3 altimeter tends to be locked over the surrounding topography more 39 frequently than Jason-2 (due to the specific post-launch Jason-2 altimeter tuning). This study 40 shows that Jason-2 observed 60% of river reaches studied (48 of 86 reaches), whereas Jason-3 in 41 3 OL tracking mode was able to measure all river reaches for every cycle. This result clearly 42 highlights the significant advantages of the OL tracking mode for observation of steep-sided 43 rivers. However, further investigations are required to compute an accurate on-board global-scale 44 DEM and to determine those locations where the use of OL tracking mode is or is not 45 appropriate. 46 47
doi:10.1016/j.rse.2018.02.037 fatcat:3oi4bvesxng2fjywleilcwcu7u