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Jason-1 Mission Capabilities Statement Geodetic Mission Phase

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Jason-1 Mission Capabilities Statement Geodetic Mission Phase JASON-1 MISSION CAPABILITIES STATEMENT GEODETIC MISSION PHASE Background The option to move the Jason-1 mission to a long-repeat geodetic orbit was discussed in the 2011 Sr. Review proposal and the Review Panel concluded that: The plan to move to a long-repeat orbit will result in a substantially improved resolution of the marine geoid. This would result in significant improvements in estimates of deep ocean topography, resolving many presently unknown seamounts and other geologic features on the ocean bottom. This would be a new and important contribution from Jason-1. Improvements in bottom topography will be of value in ocean modeling (e.g., allowing improved representation of topographically induced mixing), in naval operations, and in solid Earth dynamics. Improvements in the geoid would also increase the value of historical altimetry data such as from GEOSAT, and be useful (at least initially) for interpretation of the planned SWOT mission. Improving the Marine Geoid Moving the Jason-1 satellite to a new, long-repeat orbit will provide valuable new information about the marine gravity field. However, Dibarboure et al. (2011) showed that any orbit change would break the synchronization between Jason-1 and OSTM/Jason-2, causing at least a 30% increase in sampling error. Increased errors of approximately 3 cm would also result from the inability to accurately remove the geodetic signals using the well-measured historical ground tracks. Nonetheless, data from the new orbit will still be used for oceanographic purposes, despite the increase in error. However, the primary objective going forward will be to improve the accuracy and resolution of the marine geoid in order to serve the following scientific goals: a) Identify seamounts greater than 1 km in height. As noted recently by Wessel et al. (2010), as many as 90,000 additional seamounts could be identified by improving the resolution and accuracy of the marine gravity field. For a comprehensive assessment of the geophysical importance of seamounts, see the special issue of Oceanography Magazine on “Mountains in the Sea.” See: http://www.tos.org/oceanography/issues/issue_archive/23_1.html||http://www.tos.org/o ceanography/issues/issue_archive/23_1.html b) Improve global estimates of ocean bottom topography and sea floor roughness. Tidal dissipation, vertical mixing and mesoscale ocean circulation are all strongly affected by rough bottom topography. These improved estimates of ocean bottom topography and mixing will aid in understanding the ocean’s role in climate and climate change (Jayne et al., 2004). If the full repeat cycle is completed, the geodetic mission could improve the resolution of global ocean bottom topography by a factor of two. c) Improve the accuracy and resolution of gridded mean sea surface estimates. Gridded estimates of the mean sea surface are used to remove the geoid and time-invariant parts of the sea surface height field from altimeter observations that do not occur on exactly repeating tracks. Improved mean sea surface estimates will be needed in order to prepare for the Decadal Survey mission SWOT (Surface Water Ocean Topography), which will launch in 2019. Mission Capabilities Given the current state of health and lack of redundancies on the Jason-1 spacecraft, prudent mission continuation now dictates a change in orbit to remove the satellite from the 1336-km altimetry reference orbit occupied by TOPEX/Poseidon (non-operational) and OSTM/ Jason-2. Jason-3 and Jason-CS will also launch into the same altimetry reference orbit in the years ahead. Removing Jason-1 from the altimetry reference orbit accomplishes the primary goal of the Jason-1 End-of-Mission decommissioning plan: that of safeguarding the 1336-km altimetry reference orbit by minimizing the collision and debris risks to that orbit. Continuation of Existing Oceanography Requirements in the Geodetic Mission No significant modifications to the existing Jason-1 Level-1 requirements, as detailed in the Jason System Requirements document (TP2-SB-J0-100-CNES), are proposed as a result of the transition to the new geodetic mission phase. Specifically, there will be no change to the latency or accuracy requirements for the OSDR, IGDR, S-GDR and GDR science data products. It is fully expected that Jason-1 will continue to provide valuable data for mesoscale and operational oceanography in this new geodetic orbit. Production of near-real-time SSHA operational data products will also continue in the geodetic orbit. As discussed below, the sub-cycles of the new geodetic orbit will provide “near repeats,” meaning that the spacecraft will occupy points in space and time that are within the correlation space and time scales of the ocean mesoscale eddy field. This new orbit will provide a network of ground tracks for geodetic studies while also providing these pseudo sub-cycles for augmenting ocean mesoscale studies. All existing Level-1 requirements for operational data latency and product availability will be maintained for the geodetic mission phase. The operations, ILRS, ground station and network support will be continued for the geodetic mission phase at the existing levels of support previously required for the tandem mission phase. Geodetic Mission Requirements The minimum requirement of the geodetic mission is to have high spatial resolution (8 km or less) data collected over at least 95% of all available oceans coverage to ±66 will require an orbit whose ascending equator crossings are less than 8 km apart, i.e. an orbit that does not exactly repeat for at least 5000 revolutions, and not before about 388 calendar˚ latitude. days. This The geodetic mission phase planned for Jason-1 requires that the spacecraft achieve a new circular, non-sun-synchronous orbit at an altitude of 1323.4 km, (12.6 km below the current reference orbit.) In this new orbit, Jason-1 will be on a long-repeat cycle of 406 days, with sub-cycles of 4½, 15, 40 and 150 days and equatorial sampling approximately 7.5 km apart. An additional science requirement for the high spatial resolution orbit will be sea surface slope, in addition to sea surface height. The geodetic preference for slope over height was first shown by David Sandwell [J. Geophys. Res., 1984] and later independently confirmed by the French [Olgiati et al., Bulletin Geodesique, 1995]. From the preference for slope vs. height, it follows that accurate range precision is needed. These measurements and waveform data will be provided at 20 Hz, because the best slopes are obtained by specialized retracking schemes optimized for geodesy [Sandwell and Smith, Geophys. J. Int., 2005; J. Geophys. Res., 2009]. The 20-Hz sampling rate enables customization of filters to optimize slope resolution with high spatial resolution. This range precision and frequency can be calculated using the existing Jason-1 S-GDR science data product. No additional science data products will be required to support the geodetic mission. Modifications and revalidation of current data products will be covered under existing project DA funding. Unlike the full suite of payload instruments that are required for successful sea surface height data recovery, only the Ku-band altimeter is required to obtain slope and waveform data. Geodetic science (and near-real-time wind and wave applications) do not require C-band altimetry, JMR radiometry, or precise orbits from DORIS. Thus, Jason-1 could continue to provide useful science data for these applications even if several other payload systems were to fail. The temporal sizing of the S-GDR and GDR science data products will be modified from the current 10-day repeat cycle to a new “near-repeat” sub-cycle duration, after further consultation with the Ocean Surface Topography Science Team. Approximately one to two months of re-characterization and recalibration of the Jason-1 payload instruments (JMR, most notably) and algorithms will be required after Jason-1 reaches the geodetic orbit. Accordingly, the Project proposes a 60-day instrument and science validation period after reaching the geodetic orbit, during which time the Level-1 latency and delivery requirements would be temporarily relaxed. The optimal goal for the geodetic mission phase is to fully complete at least one high-resolution 406-day repeat geodetic cycle. The geodetic mission would be considered minimally successful if at least one 150-day sub-cycle were to be completed, providing a low-resolution geodetic grid. Scientific investigations related to the new geodetic mission phase would continue to be supported under the auspices of the OSTST. Education and Public Outreach (E/PO) activities for Jason-1 will also be maintained at the current levels of support during the geodetic mission phase. .
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