Abstract
The resolutions of current global altimetric gravity models and mean sea surface models are around 12 km wavelength resolving 6 km features, and for many years it has been difficult to improve the resolution further in a systematic way. For both Jason 1 and 2, a Geodetic Mission (GM) has been carried out as a part of the Extension-of-Life phase. The GM for Jason-1 lasted 406 days. The GM for Jason-2 was planned to provide ground-tracks with a systematic spacing of 4 km after 2 years and potentially 2 km after 4 years. Unfortunately, the satellite ceased operation in October 2019 after 2 years of Geodetic Mission but still provided a fantastic dataset for high resolution gravity recovery. We highlight the improvement to the gravity field which has been derived from the 2 years GM. When an Extension-of-Life phase is conducted, the satellite instruments will be old. Particularly Jason-2 suffered from several safe-holds and instrument outages during the GM. This leads to systematic gaps in the data-coverage and degrades the quality of the derived gravity field. For the first time, the Jason-2 GM was “rewound” to mitigate the effect of the outages, and we evaluate the effect of “mission rewind” on gravity. With the recent successful launch of Sentinel-6 Michael Freilich (S6-MF, formerly Jason CS), we investigate the possibility creating an altimetric dataset with 2 km track spacing as this would lead to fundamental increase in the spatial resolution of global altimetric gravity fields. We investigate the effect of bisecting the ground-tracks of existing GM to create a mesh with twice the resolution rather than starting all over with a new GM. The idea explores the unique opportunity to inject Jason-3 GM into the same orbital plane as used for Jason-2 GM but bisecting the existing Jason-2 tracks. This way, the already 2-years Jason-2 GM could be used to create a 2 km grid after only 2 years of Jason-3 GM, rather than starting all over with a new GM for Jason-3.
Highlights
In the following we focus on the improvement on high resolution gravity as we can directly evaluate these using marine gravity observations, but the investigations are important to the determination of Mea sea surfaces
Gravity from the first J2 371-day Long Repeat Orbit (LRO) compares at 4.83 mGal and the 406-day Geodetic Mission (GM) of Jason-1 compares at 4.66 mGal
Starting out with data from the first 179-day cycle of J1, we found a standard deviation of 5.35 mGal with marine gravity observations
Summary
The GM is basically a Long Repeat Orbit (LRO) where the orbital pattern is designed for mainly geodetic purposes This means that the spatial sampling it optimized to map short wavelength in the geoid or gravity field at the price of no or long temporal sampling. Exact repeat missions (ERM) are primarily designed for oceanographic purposes to map oceanographic signals optimally This requires frequent temporal sampling at the price of coarse spatial sampling. The satellite is lowered or raised a number of kilometers into the EoL orbit which eventually will become the graveyard orbit for the satellite During their EoL missions, Long Repeat Orbits (LRO) were selected for both Jason-1 and Jason-2, where for each the repeat was longer than 1 year. We derived gravity from the two 378-days LRO cycles of Jason-2 and the two 178-days sub-cycle of Jason-1 as these (sub-) cycles were affected differently by safe-holds. Such analysis is important to guide future EoL missions (most profoundly the EoL for Jason-3) and their strategies to remedy the effect of future safe-holds
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