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Ocean Surface Topography from Space
SCIENCE
Inter-Calibration of Multi-mission Altimeter Systems and Time Series of Dynamic Ocean Topography (iCASiDOT)


Author:

Wolfgang Bosch - (OCA-GEOAZUR)

Co-Investigator(s):
  Denise Dettmering (Deutsches Geodätisches Forschungsinstitut (DGFI))

Abstract:

Sample image for Inter-Calibration of Multi-mission Altimeter Systems and Time Series of Dynamic Ocean Topography (iCASiDOT)
Geostrophic velocity field derived from Dynamic Ocean Topography (DOT) profiles for the area of the Agulhas counter current (animated at http://www.dgfi.badw.de/?333).
For the period from 1992 up to the present the project will conduct - on a global scale - inter- calibrations of contemporaneous altimeter systems through extended crossover analyses based on single- and dual-satellite crossover differences between nearly simultaneous measure- ments. The common analyses of two or more altimeters provide complete time series for their radial errors, capture relative range biases between the altimeter systems involved, estimate differences in centering the altimeter orbits with respect to a terrestrial reference frame (ITRF), and provide for each mission a comprehensive characterisation of radial errors in terms of auto-covariance functions and geographical correlated error pattern. The approach, already applied for TOPEX, Poseidon, ERS1/2, Jason-1, Jason-2, Envisat, GFO, ICESat and CryoSat-2 will be extended to the full life time of Jason-2 and to the transition and operation phase of Jason-3 including all other altimeter systems (in particular HY-2A, Altika/SARAL, Sentinel-3) operating at the same time. The inter-calibration result will allow merging data from all missions and thereby improving significantly the temporal and spatial sampling of the ocean surface.

The latest GOCE gravity field solutions (e.g. GOCO02S) realize another significant improvement: the actual knowledge of the Earth gravity field allows for the first time to estimate meso-scale pattern of the dynamic ocean topography (DOT) by subtracting the geoid undulations N from sea surface heights h. As geoid undulations and altimetric sea surface heights have completely different spectral content a filter is consistently applied to both, h and N. Compared to GRACE, the latest GOCE gravity field allow to shorten the low pass filter down to a half-width of about 70km, thus approaching meso-scale resolution. In order to avoid any initial gridding of h with undesirable smoothing a method has been developed to estimate the DOT directly along the altimeter profiles. Using the inter-calibrated multi- mission profiles allows to consistently estimate individual DOT profiles for all altimeter systems and subsequently to generate, for example, a gridded time series of 10-day DOT snapshots for a period of actually two decades, 1992 up to the present. This time series realizes the temporal evolution of the dynamic ocean topography and will be translated to geostrophic velocities and eddy kinetic energy complemented by a rigorous error propagation of altimeter error covariances and the full variance-covariance information of the gravity field solution. The project will generate DOT time series on a systematic basis and gradually extend the time series to the full lifetime of Jason-2 and Jason-3 as soon as new consolidated altimeter data becomes available.



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