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Ocean Surface Topography from Space
SCIENCE
Validation, cross-calibration and performances of altimetry missions over ocean for mesoscale, coastal and climate applications


Author:

Michaël Ablain - (CLS)

Co-Investigator(s):
  Sylvie Labroue
Matthias Raynal
Sabine Philipps
Lionel Zawadzki
Annabelle Ollivier
Olivier Lauret
Prandi Pierre
Joël Dorandeu
Nicolas Picot
(CLS)
(CLS)
(CLS)
(CLS)
(CLS)
(CLS)
(CLS)
(CLS)
(CNES)


Abstract:
Ablain browse image
Long-term evolution of global Mean Sea Level measured by all the altimeter missions
The main objective of the investigation is to take benefit of multi-mission altimetry and other sources of independent data to improve the altimetry performance assessment methods.

Satellite altimetry is now an essential source of data for oceanic signal studies. From the ERS and TOPEX/Poseidon (T/P) missions, it has been demonstrated that a continuous precise multi-mission altimeter dataset is mandatory for many scientific applications: mesoscale variability description (Le Traon and Morrow, 2001), seasonal / interannual signals determination (e.g. Chambers et al., 2000, Mc Carthy et al., 2000, Roemmich et al., 2007) and estimation of the Mean Sea Level (MSL) variations (e.g. Nerem et al., 1997, Cazenave et al., 1999, Cabannes et al., 2001, Lombard et al., 2006 among others).

Jason-1, Jason-2, ENVISAT, SARAL/Altika still provide reliable high-quality data albeit not on their original ground track. If we add older datasets such as T/P, ERS-1&2 and GFO missions, complemented with on-going Cryosat-2 and HY-2 an unprecedented ocean sampling of 80 years of cumulated data with up to 4 concurrent altimeters is available. The recent altimetry missions launched in 2016 (Jason-3/OSTST, Sentinel-3a) and future altimetry missions (Sentinel-3b, Sentinel-6/Jason-CS, SWOT) will provide a precious continuity of the sea-level observations for mesoscale, coastal or climate applications.

In order to continue to build an accurate altimetry data record, the validation and cross-calibration of these new altimetry missions is a crucial step. It is also a prerequisite for the use of multi-mission datasets in both delayed mode and near real time and for studies of long-term, low-frequency oceanic signals, or signal and error aliasing.

This analysis includes but is not limited to:

  • Improvements of existing calibration and cross-calibration methods for present and future altimeter missions. Dedicated studies will be carried out to better estimate the altimeter system error budget, depending on the different contributions (instrument, orbit precision, geophysical corrections) and for different applications (e.g.: mesoscale studies, mean Sea Level estimations). Significant changes in cross-calibration algorithms will be able to use multiple reference missions in an operational framework, thanks to the POD accuracy provided by altimetry missions.
  • Altimetry / In Situ systematic comparisons to validate the altimeter measurements. Tide gauge measurements but also temperature/salinity profiles and surface velocity data (drifters, moorings) will be used in inter-comparison methods to validate the signals derived from altimeter missions.
  • Studies of low-frequency oceanic signals from altimeter long time series. Currently, 25 years of altimeter data are available, after precise assessment and cross-calibration, for studying decadal and low frequency signals. Comparisons between signals deduced from altimetry and signals from other sources of independent data (SST, In Situ, GOCE) will be carried out.
  • Studies of altimeter specificities on error budget analyses and performance monitoring. The list includes but is not limited to the analysis of: reduced high-frequency noise with and without improved ground processing (e.g.: CNES PEACHI project) and the associated impact on oceanic spectrum observability and the monitoring of coastal currents, different ionosphere error and aliasing and long-term stability.



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