ESA have developed a plan to manouevre CryoSat-2 into a new orbit that is synchronized with ICESat-2. The campaign is tentatively known as Cryo2Ice. Although the Cryo2Ice campaign has been planned for some time, the implementation was paused due to COVID-19. However, there is a favourable opportunity to begin this manouevre on 16th July which would achieve the new orbit by early August, in plenty of time for the Arctic sea ice season.
The new CryoSat-2 orbit provides ground tracks that coincide with those of ICESat-2 for 500-1000 km every 1.3 days. It involves only a small (<1 km) move for CryoSat-2, and can be performed with no significant disruption to the mission – i.e. with no unusual risk, and with no significant consumption of fuel or sensor downtime – as it is comparable to orbit maintenance. The ground track coincidence can be targeted at specific geographical regions, and can be alternated with about half the initial orbital adjustment. The new orbit will provide identical spatial and temporal sampling of land and sea ice, with a slightly shifted ground track that will benefit applications seeking new coverage. Further details can be found below.
We are excited to inform you of this new opportunity, as it will allow everyone to explore the continuity and complementarity of the two missions. If you would like to lend your support, please add your name to the community letter which is being assembled:
-Andy Shepherd, Ole Baltazar Andersen, Sara Fleury, Christian Haas, Malcolm McMillan, Louise Sandberg Sørensen
----------------- Further Details ------------------------
- The Proposed Manouevre
The proposed synchronisation involves a modest (less than 1 km) adjustment to the CryoSat-2 orbit semi-major axis and will provide coincident ground tracks for considerable distances (up to 1000 km) every 19 orbit revolutions (1.3 days). The orbital adjustment can be achieved in a succession of thruster burns that are comparable in magnitude to orbit maintenance manoeuvres and are therefore considered nominal (normal) risk. The manoeuvre can also be completed within a relatively short (two week) period and will involve negligible (minutes of) data loss, and negligible (0.5 kg of) fuel consumption. The synchronisation can be optimized to favour regional ground track coincidence in either hemisphere, and the proposed start date of July 16th 2020 will allow synchronisation over the Arctic in time to capture the winter sea ice season. Alternating the geographical coincidence between hemispheres can be completed with a smaller (and shorter duration) adjustment to the orbital phasing. In our view, the proposed manoeuvre is effective (in terms of the synchronisation it achieves), low cost (in terms of consumption of fuel and loss of data), and low risk (in terms of the departure from nominal activities).
- The Impact
The main concern relating to the manoeuvre relates to the potential impact on the mission primary data streams, namely measurements of sea ice and land ice thickness, change. However, since the synchronisation can be achieved with only a minor adjustment to the nominal orbit, the spatial and temporal sampling of the ground track is not significantly modified, and overall the synchronised orbit provides slightly improved capability to sample trends in land and sea ice thickness. In our view, the proposed manoeuvre will not disrupt the primary measurement capability of CryoSat-2.
- The Scientific Opportunity
The scientific value of the proposed synchronisation is considerable. It will provide a unique opportunity to obtain near-coincident measurements of sea ice, land ice, and ocean surface elevation acquired at laser and radar frequencies over large (mesoscale) spatial scales and systematically. With such measurements, we can assess for the first time the continuity and consistency between radar and laser measurements of surface elevation changes, and we can then explore their complementarity for monitoring differences in the scattering properties of snow and ice. For example, the depth of snow on sea ice floes is a primary source of uncertainty in both radar and laser altimeter retrievals of sea ice thickness, and the synchronised orbits will allow us to develop a retrieval for this parameter from dual-frequency satellite observations alone. These measurements will pave the way for the next generation of satellite altimeter systems, such as the CRISTAL High Priority Candidate Mission in development as part of the Copernicus Expansion Program. Over land ice, a synchronised campaign will allow us to quantify spatial and temporal variations in radar penetration into the snowpack and laser scattering within the near surface spindrift. Over ocean surfaces, the altered sampling of the synchronised orbit will lead to improved estimates of the mean sea surface which will in turn lead to more precise estimates of marine gravity, seafloor bathymetry, dynamic ocean topography, and sea ice freeboard. We conclude, therefore, that the scientific potential of the synchronised orbit is huge.