CryoSat-2 over rivers: a present mission, an insight into the future of altimetry

Nicolas Bercher (LEGOS/CNRS, France)


Stephane Calmant (LEGOS/IRD, France); Salvatore Dinardo (ESA/ESRIN/SERCO, Italy); Sara Fleury (LEGOS/CTOH/CNRS, France); François Boy (CNES, France); Nicolas Picot (CNES, France); Jérôme Benveniste (ESA/ESRIN, Italy)

Event: 2014 Ocean Surface Topography Science Team Meeting

Session: Science Results from Satellite Altimetry: Inland waters (multi-mission and long-term monitoring)

Presentation type: Type Oral

The next ten years of satellite altimetry will be made of SAR
(Sentinel-3, Jason-CS), inSAR and Ka band (Swot) techniques.
Fortunately, SARAL and CryoSat-2 missions partially implement such
techniques and already give an important opportunity to start studying
these techniques. They give an "insight of the future".

This paper focuses on the three years old CryoSat-2 ice mission.
CryoSat-2 implements three major aspects that are in rupture with
conventional altimetry.

First, CryoSat-2 flies on a geodesic orbit characterised by a very
long repeat period (369 days) and a really dense spatial coverage (7km
inter-track distance at equator). At first glance, such an orbit
seems of poor interest for the monitoring of river water levels.
However, such a geodesic orbit provides an incredible spatial coverage
along river paths that helps to derive rivers topography. It invites
to develop approaches that deals with space and time, for example, to
derive river mean longitudinal profiles and to localise river water
level time series.

Second, CryoSat-2 SIRAL altimeter is the first to implement nadir SAR
mode and paves the way toward future SAR missions. The nadir SAR mode
refines the along-track sampling resolution up to 300 meters.
Moreover, its synthetic processing allows to focus the synthetic
Doppler beams anywhere along azimuth direction. Tweakings such as
changing, or even increasing, the along-track sampling resolution
helps to refine SAR echoes selection over the small targets rivers
are, thus reducing echoes contamination. The ability to accurately
focus the SAR beams in the middle of the river channels, coupled to
the higher along-track resolution, would definitely improve the the
quality of measurements over tighter rivers that we couldn't monitor
before. SAR mode multi-look echoes (prior to waveforms stacking) can
be used to characterise angular surface response and to help surface
detection and classification. Using simple processing routines, it is
also possible to derive pseudo-LRM (or so called "ReDuced-SAR")
waveforms, from SAR measurements, in order to assess the benefits of
SAR mode over LRM, in perfect synchronism.

Finally, CryoSat-2 implements SARin mode that inherits the SAR mode
properties described above, a more robust tracking procedure and, most
importantly, exploits the across-track echoes phase difference between
echoes reception on the two SIRAL antennas. This is used to estimate
the location of the main reflectors within the altimeter swath, i.e.,
the echo target angles are no longer arbitrarily equal to the nadir.
To some extent, this gives similar information about bright targets
as the Swot inSAR swath processing would do. It can be used to derive
3D swath backscattering maps.

This paper will give an overview of SAR and SARin modes versus LRM and
the major advances reached thanks to the CryoSat-2 mission for the
monitoring of river water levels. Detailed examples will be given for
several basins around the world, as well as validation results and
SAR/SARin future applications.


Oral presentation show times:

RoomStart DateEnd Date
Red salon Tue, Oct 28 2014,15:30 Tue, Oct 28 2014,15:45
Nicolas Bercher