The measurement of geocentric Mean Sea Level (MSL) change from satellite altimetry requires an extreme stability of the altimeter measurement system since the signal being measured is at the level of a few mm/yr. This means that the orbit and reference frame within which the altimeter measurements are situated, and the associated altimeter corrections, must be stable and accurate enough to permit a robust MSL estimate. Foremost, orbit quality and consistency are critical not only to satellite altimeter measurement accuracy across one mission, but also for the seamless transition between missions (Beckley, et. al, 2004). The analysis of altimeter data for TOPEX/Poseidon, Jason-1, and Jason-2/OSTM (Ocean Surface Topography Mission) requires that the orbits for all three missions be in a consistent reference frame, and calculated with the best possible standards to minimize error and maximize the data return from the time series, particularly with respect to the demanding application of measuring global and regional mean sea level trends.
In an effort to generate a Sea Surface Height Climate Data Record (SSH CDR) from the three missions spanning nearly two decades, a number of revisions/re-calibrations to the respective mission’s heritage Geophysical Data Record (GDR) correction algorithms and models were implemented, including a consistent Precise Orbit Determination (POD) strategy that would geodetically tie the multiple missions. The orbits for this time series of altimeter data were computed at NASA GSFC using Satellite Laser Ranging (SLR) and DORIS data, using the standards described in Lemoine et al. (2010, 2012, 2015), updated to use the ITRF2008 reference frame (Altamimi et al., 2011). The SLR and DORIS tracking data for TOPEX, Jason-1 and Jason-2 were provided with the support of the International Laser Ranging Service (ILRS) (Pearlman et al. 2002), and the International DORIS Service (IDS) (Willis et al. 2010). Details of the development, construction and quality assessment of the TOPEX/Poseidon, Jason-1, and Jason-2/OSTM (TPJAOS) SSH CDR can be found in the documentation at ftp://podaac.jpl.nasa.gov/allData/merged_alt/L2/docs/.
A few notable advancements towards the development of the SSH CDR are listed here with links to additional information from relevant publications and presentations.
- Apply consistent, across missions POD strategy to GSFC std1504 replacement orbit (Lemoine et al., 2015), based on most current terrestrial reference frame realization ITRF2008 (Altamimi et al., 2010), and Time Variable Gravity (TVG) realizations (Lemoine et al., 2015).
- Current stability assessment and re-calibration of Jason-2 AMR (Brown, 2012).
- Improved wet troposphere correction algorithms providing enhanced coastal radiometer measurements, rain and ice detection (Brown, 2010, Misra, 2012).
- Revisions to ocean tide model (GOT4.10) to better account for S2 tidal constituent (Ray, 2011); improved ground truth dataset based on 137 bottom-pressure gauge network to assess ocean tide models (Ray, 2011).
- Revised pole tide correction (Desai et al., 2015).
- Consistent non-parametric sea state bias algorithm (Tran et. al, 2010, 2012).
- Improved dry troposphere correction through recovery of atmospheric tides, resulting in a reduction of the 59-day signal (Ponte and Ray, 2002).
- Evaluation and application of inter-mission biases derived from validation phases and global tide gauge network (update of Beckley et al., 2010).
- Altimeter stability monitored from a well-maintained 64-site global tide gauge network (Mitchum, 2000, update of Beckley et al., 2010).
Maintenance and improvements to the fidelity of the SSH CDR is continuous through the research activities of the Ocean Surface Topography Science Team (OSTST). As further advancements and/or re-calibrations are made to any of the correction parameters or models, the TPJAOS is recalculated with the most accurate algorithms sanctioned by the OSTST. Notification and details of revisions to the TPJAOS will be provided at this site.