The CryoSat-2 satellite is currently being aligned with ICESat-2 over Antarctica, unlocking fresh possibilities in the wealth of new information being delivered by the near-synchronous orbit of the two satellites.

Between 16 and 31 July 2020, a campaign dubbed CRYO2ICE took off, where ESA raised the orbit of its Earth Explorer CryoSat-2, to periodically align with NASA's ICESat-2 satellite.

The campaign’s aim was to achieve a partial parallel ground-track every 19th revolution of CryoSat-2 and every 20th revolution of ICESat-2—allowing data in the polar areas to be collected by the two satellites within a few hours of each other.

To reach this goal, CryoSat-2's semi-major axis had to be increased by 887 metres through a series of 14 manoeuvres.

The scientific benefits that such a small orbit change for CryoSat-2 brought were impressive. By using the combined data from CryoSat-2's radar and ICESat-2's laser instruments, inaccuracies in sea ice thickness and land ice measurements were reduced for the first time. In addition, it enabled to map snow at the poles and improve climate models, among other aspects.

On top of that, the quality of coincidences is increasing as time goes by because the orbital planes of CryoSat-2 and ICESat-2 are gently coming closer to each other. This is the result of the natural perturbations affecting the orbits, and there is a direct link between the orbital plane separation and time difference between ICESat-2/CryoSat-2 observations, when we have a coincident track. This natural evolution in the satellites’ orbit offers a benefit to scientists in obtaining the combined data.

In July this year, the orbital plane separation will reach 25 degrees, and decreasing. This translates into a time difference of approximately 1 hour and 40 minutes between observations at that time (it was three hours when the campaign first started), decreasing at a rate of approximately 40 minutes per year. This means that, in about 2.5 years, we will have quasi-synchronous, co-spatial coincidences.

The orbit in which CryoSat-2 is currently flying has a 19/20 revolution resonance with ICESat-2. This enables recurring coincidences with the ICESat-2 instrument every 1.33 days. That is, every 1.33 days there is a coincident track (spatial). A coincidence, however, cannot be maintained for a very long fraction of the orbital period; after a while the ground tracks start diverging from each other.

This is why experts keep them at the areas of interest for these two missions: the poles. The parameter to control this is the orbital phase. Since CryoSat-2 acquired the resonant orbit with ICESat-2 in 2020, it has been flying at an orbital phase such that the coincidences occur in the areas close to the North Pole.

The purpose of the current campaign is to change the orbital phase of the CryoSat-2 satellites, in order to maximise coincidences in the Antarctic region (at the expense of reducing them at the North Pole).

This idea concerns the geographic (spatial) collocation between observations taken by each of the spacecraft. But the observations are not taken simultaneously. In fact, what happens for those coincident tracks is that ICESat-2 first passes over a certain area, and then CryoSat-2 follows over the same location some time later. This time difference is a very important parameter for the quality of a coincidence: ice—which constantly moves—is being observed, and the conditions change. Therefore, the lower the time difference, the better.

Overall, this translates into huge results for scientists characterising snow loads, improving the retrieval of sea ice thickness, and ultimately, the understanding of the ongoing changes in the cryosphere due to climate change.

Javier Sanchez, Flight Dynamics Engineer at ESA, states, "The satellites operate at completely different altitudes. But with the satellites in this configuration we have a mechanism put in place in space, which delivers recurring coincidences between their instruments every 1.3 days. And this is done in a passive manner, meaning that each one of the satellites is able to continue with their own mission autonomously."

Tommaso Parrinello, ESA's CryoSat Mission Manager, concludes, "Since we started CRYO2ICE, the scientific community have been very excited about this programme. It is delivering a unique and probably unrepeatable data record, which will help us to improve our understanding on how the cryosphere is responding to global warming and, at the same time, improve also the past climate records, especially now over Antarctica."