In August 2020 the CryoSat-2 satellite started a new mission chapter, raising its operational altitude by 887 meters. This change, however small compared to the total height (719 km), enabled an orbital resonance with NASA’s ICESat-2 spacecraft. As a consequence of this the instruments of these two satellites undergo recurring coincidences over the areas scientific interest every 1.3 days.
The main instrument of CryoSat 2 is the SAR/Interferometric Radar Altimeter (SIRAL), a single frequency Ku-band radar; whereas ICESat-2 is equipped with the Advanced Topographic Laser Altimeter System (ATLAS), which consists of six laser beams organized in three pairs: central, right, and left, as seen from the flight direction. The combined operation of these two instruments offer numerous opportunities of high scientific importance, especially because of their differences in snow penetration. Combined data of this kind has a scientific value which was already acknowledged in the past by the numerous airborne campaigns of NASA’s IceBridge mission. The current CryoSat - ICEsat configuration allows exploiting the benefits of this, while at the same time paving the way to future dual-frequency missions, such as CRISTAL.
Long coincidences occur every 19 CryoSat-2 orbital revolutions, with ICESat-2 completing 20 revolutions in the same time. But even under conditions of orbital resonance the overlap of both instruments cannot be maintained through a long geographical area. The reason for this is the large difference in orbital altitude, and therefore orbital period, between the two satellites. By tuning the target in orbit position for CryoSat 2, i.e. its orbital phase, the length of the coincidences can be maximized in the areas close to the polar regions. This leads to instrument overlap tracks that, in the best cases, can be of the order of 1000 km in length. Unfortunately, optimizing the conditions for one of the poles comes at the expense of reducing them for the opposite one.
The orbital change achieved in August 2020 was performed by means of a manoeuvre campaign with the following objectives: a) to raise the orbit up to the resonance altitude with ICESat-2; and b) to leave the spacecraft in an orbital phase such that coincidences with the ICESat-2 instrument are maximized in the Arctic. The operational concept, however, was already foreseeing potential re phasing campaigns in the future, which would optimize the conditions for the Antarctic region. The manoeuvre campaign lasted 16 days, in which a total of 14 manoeuvres were executed. This left enough time during the inter-manoeuvre periods to keep the SIRAL instrument in operation. For this purpose a mission planning orbit was created to support the transitory period between the former mission’s nominal orbit and the new one. After the orbital change CryoSat 2 has been operated in its new altitude and position, which yield periodic coincidences with the ICESat-2 instrument in the Northern hemisphere.
The CryoSat-2 mission foresees a major Antarctic campaign in November 2022. This presents an excellent opportunity to change the orbital phase, such that coincidences with the ICESat-2 instrument are switched over to the Southern hemisphere. This poster describes the details of the manoeuvre campaign necessary to achieve this, which is currently scheduled for the second quarter of 2022. The target phase is located approximately 6 degrees ahead of the current orbital position; this is equivalent to a node crossing time 100 seconds earlier than the current one. All other orbital parameters remain the same; in particular, the altitude (orbital period) that enables the resonance with ICESat 2. The orbital change consists, firstly, of a decrease in semi-major axis (altitude), which introduces an along track drift towards orbital positions ahead of the current one. This is then followed by another set of manoeuvres to recover the original altitude, stopping the along track drift, and reaching its final position. The duration of the manoeuvre campaign is a parameter inversely linked to the cost of the orbital change in terms of propellant mass, which implies a trade off between these two variables. Following the same approach as in the latest orbital change, a reference orbit dedicated to mission planning will be generated for the transitory period. This will allow keeping the SIRAL instrument active during most of the manoeuvre campaign.