CryoSat-2 is the first altimeter offering SAR mode, and is being followed by the Sentinel-3 family. The improved along-track resolution is accompanied by a naturally reduced land contamination thanks to the increased along-track resolution.

The great advantages of the SAR mode with respect to the conventional (LRM) altimetry are:

  • The enhanced resolution along-track (e.g. ~300 m compared to few km) due to Delay-Doppler processing [Raney1998, Wingham2006];
  • The possibility of beam steering that enables to focus the measurement at a desired place, thus enables the determination of the height from the processing of echoes returned by a narrow band centred within the river (benefiting of water masks a priori information);
  • The accumulation of looks from the same footprint which shall allow to increase the signal to noise ratio, thus achieve better precision of the altimetric measurements and characterize surface response according to the look angles.

It is foreseen that future major investigations will focus on the optimal exploitation of L1B-S data (direction dependent masking, etc.) in order to produce decontaminated waveforms for inland waters.

An example of L1B waveforms over the river is shown below. It can be appreciated that the record located in the centre of the river is much clearer and peakier while the record near the border presents some land contamination.

mekong_2 mekong_1  mekong_profile

fig 1. Illustration of SAR waveforms over the Mekong river, left and river profile compute from SAR data, Bercher et. al.

 

 

brahmaputra_river brahmaputra_river_2
fig 2. Section of the Brahmaputra in the Assam Valley showing the Landsat river mask, the CryoSat-2 observations and their mapping to the 1D river model, all for 2013. Schneider et. at. EGU 2016 fig 3. CryoSat-2 observations along the Brahmaputra River from 2010 to 2013. The map only displays the outlier-filtered observations, the longitudinal profiles show both outliers and the outlier-filtered data. Schneider et. at. EGU 2016

References:

  • Lotus project 
  • CRUCIAL project
  • Schneider, Raphael, et al. “Application of CryoSat-2 altimetry data for river analysis and modelling.” Hydrology and Earth System Sciences 21.2 (2017): 751.
  • [Bercher2013a] – Bercher N., Dinardo S., Lucas B. M., Fleury S., Calmant S., Crétaux J.-F., Femenias P., Boy F., Picot N. et Benveniste, J. (2013a). Applications of CryoSat-2 SAR & SARin modes for the monitoring of river water levels. In Proceedings of the “CryoSat Third Users Workshop”, 12-14 March, Dresden, Germany. Oral communication and paper. Download from chronos.altihydrolab.fr.