Improvements in ocean altimetry performances

Since the ERS-1 and Topex/Poseidon launches in the early nineties, a tremendous number of improvements impacting altimetry data in a wide variety of domains have been brought by extensive studies in different teams all over the world. Instrument processing, precise orbit determination, geophysical algorithms and corrections, data availability and coverage, but also calibration, cross-calibration and merging techniques, have all benefited from those studies.

A group work

: Typical SSH crossover deviation for the different T/P and Jason-1 GDR versions

Altimetry validation and performance assessement is a collaborative work between operation engineers, project managers, specialists in each domain, scientists and users, each doing his utmost for the same goal in an integrated concept, in the frame of quality forums such as the Ocean Surface Topography Science Team (OST/ST), or of the Envisat Quality Working Group (QWG).

Each component of the altimetry system is interacting with each other through the overall system performance. Thus the need of considering the system as a whole -- all the more since users are interested in the overall error budget for their particular application.
Sea surface height variances (at crossover points, colinear differences) are used as metrics to estimate the error beudget. Comparison to external data (other altimeter, in situ,...) are also a way of evaluating the errors.

The case of Topex/Poseidon

Topex/Poseidon cross-over variance on 1992-1993, 1995-1996, 2000-2001 and 2005-2006

T/P provides the longest series of continuous altimeter measurement (1992 ? 2006). There were four major improvement steps identified along the mission lifetime
- 1rst step: after the in-flight verification: GDR-M version B
- 2nd step: 1rst reprocessing of T/P data: GDR-M version C
- 3rd step: Jason-1 launch. New algorithms and corrections
- 4th step: reprocessing of T/P (and Jason-1 and Envisat) in 2008-2009

Significant examples of improvements

Improvements in orbit calculation:

reference frames and new standards
gravity fields: JGM-2, JGM-3, Grace models

Improvements in sensor processing:

Altimeter
- Precise monitoring of altimeter measurement:
  - T/P USO drift detection
  - Wallops calibrations
  - TOPEX/Poseidon relative bias, TOPEX-A / TOPEX-B relative bias
  - Sea State Bias models: Walsh et al., Fu & Glazman, BM4 Gaspar et al., Non Parametric Labroue et al.
Radiometer:
- Drift corrections: Ruf et al., Scharoo et al.
- Detection of yaw effects

Improvements in geophysical corrections

Ocean tides: Cartwright & Ray, CSR3.0/FES95, GOT99/FES2000, GOT00/FES2004
Atmospheric corrections:
- Improvements in ECMWF model: IB and dry troposhere correction, wet troposphere correction
- Time varying reference pressure for IB correction
- Using MOG2D barotropic model for resolving High Frequency aliasing
Handling S1/S2 non gravitational tides

Improvements in reference surfaces

Mean Sea Surfaces: OSU91, OSU95, GSFC, CLS01
Geoids: not yet directly used for altimeter SSH, but large improvements for orbit accuracy and Mean Dynamic Topography

A typical example of the validation process is the aliasing of High Frequency signals in altimetry. The problem was raised by, among others, Stammer et al. (1999), Tierney at al. (2000), Hirose et al. (2001), Carrere and Lyard (2003). The OST/ST recommended the testing of barotropic models (PPHA, Mog2D). The tests were done by CalVal for model corrections. Following the results, the OST/ST recommended the use of Mog2D-HF plus an Inverse Barometer correction. This was implemented in the CMA (Jason-1 - Envisat altimetric ground segment). An assessment of the performances was done. This lead to progress in ocean circulation studies. And now that the high frequency signals aliasing are better corrected, new user needs are arising.

Including validation and performance assessment at each step has proven its efficiency. The impact on the altimetry applications is of course essential, since it drives the loop between improvements in altimetry, development of new applications and new requirements for altimeter systems.
To keep scientists and users with the engineers to gather all people, all skills, all needs is still the key of success. Even if things have become more operational and trustworthy, engineers need scientists to push : there are still so many fields of improvement; new applications (regional, coastal, ?) will require higher resolution, higher frequency.