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Ocean Surface Topography from Space
Precise orbit determination for the Jason series of missions and TOPEX/Poseidon in support of a long-term and consistent altimeter record


Frank Lemoine - (NASA Goddard Space Flight Center)

Co-Investigator/Co-PI (non-US organization only):
  Prof. Marek Ziebart
(NASA Goddard Space Flight Center)

  Brian Beckley
Dr. Douglas Chinn
Nikita Zelensky
(NASA Goddard Space Flight Center)
(NASA Goddard Space Flight Center)
(NASA Goddard Space Flight Center)

  Jean-Paul Boy
Prof. Gary Mitchum
Guilhem Moreaux
(NASA Goddard Space Flight Center)
(NASA Goddard Space Flight Center)
(NASA Goddard Space Flight Center)

The precision, stability and quality of the satellite orbit is a critical issue for altimeter missions such as Jason-2 (OSTM) and Jason-3, since the orbits provide the central reference frame within which to analyze and interpret the altimeter data. The long-term (20-25 year) record of altimeter data can now be used alone or in combination with other data sets in studies of intraseasonal-to-interannual variability, global and regional sea level variations, ocean circulation, low-frequency tides, and other phenomena. It is imperative that the orbits used to geolocate the altimeter data be provided with the highest accuracy possible in a consistent reference frame for TOPEX, Jason-1,2, and 3. While we have made significant advances in our ability to model orbits precisely, the significant challenge we face is that we wish to observe the effects of a dynamic Earth with altimeter data but the changes in the Earth itself as manifested in the change in time-variable gravity field of the Earth and the continuing need to update the terrestrial reference frame, influence the quality of the altimeter satellite orbits that we wish to provide. Other challenges arise from the need to minimize or mitigate the effects of modeling errors, in particular those that arise from the nonconservative forces. Our objective will be to minimize or mitigate these errors through choice of modeling or analysis strategy, and prevent these errors from producing unwanted signals in the altimeter data.

For this investigation we propose to:

  1. Evaluate the new ITRF2014 reference realizations with respect to their performance on TOPEX, Jason-1 and Jason-2;
  2. Refine station position modeling to account for non-tidal atmospheric loading, and develop a fully compatible model of geocenter motion for altimeter satellite orbit computations;
  3. Define the best strategy for modeling static and time-variable gravity over the entire period from 1992 to 2016 and later, and also assess the impact of timevariable gravity on regional sea level change estimates;
  4. Evaluate and implement force model improvements, including updates to the non-conservative force modeling for Jason-2 and Jason-3;
  5. Evaluate improvements to the DORIS and SLR measurement models that will improve tracking data quality;
  6. Compute and distribute to the OSTST a full time series of orbits for TOPEX, Jason-1, Jason-2 and Jason-3 using the improvements. We will evaluate these orbits by using them in conjunction with the altimeter data (computing altimeter crossovers) as well as other POD metrics, and also by evaluating the orbit + altimeter data combination with in a global network of tide gauges;
  7. Produce a consistent set of orbits for Jason-2 and Jason-3 during the tandem phase to support the calibration of the altimeter;
  8. Test the new orbit standards on other altimeter satellite missions of interest (such as Cryosat-2 and SARAL).

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