|Paper title||The Swarm “counter-rotating orbits” – Orbital dance in 2021|
|Form of presentation||Poster|
Swarm is the magnetic field mission of the ESA Earth Observation program composed of three satellites flying in a semi-controlled constellation: Swarm-A and Swarm-C flying in pairs and Swarm-B at an higher altitude. They carry a sophisticated suite of magnetometers and other in-struments: the ASM (Absolute Scalar Magnetometer) and VFM (Vector Field Magnetometer), the Electric Field Instrument (EFI) and an Accelerometer (ACC).
Since early on during the mission, the goal for the Swarm lower pair was to orbit in similar low-eccentricity orbits separated by a small Right Ascension of the Ascending Node, in very close orbital planes and separated along the orbit by a distance between 4 and 10 seconds. This interval was identified as a compromise between the need to control the constellation, ensure the proper reaction time and avoid crossovers and the need to keep them close enough to correlate the science data.
Swarm-B instead is orbiting at an higher altitude (currently 507 km average altitude compared to 432 km of the lower pair) and, due to different orbital perturbations its plane is rotating at a different speed, although being quasi-polar like Swarm-A and Swarm-C.
Due to the orbital planes’ different rotation rates, there is a periodic point in time when the planes come so close that they almost are co-planar. This exciting opportunity comes every 7.5 years and happened between Summer and Winter 2021, the closest alignment being at the be-ginning of October, 2021. In this phase, called “counter-rotating orbits phase”, Swarm-B is counter-rotating with respect to Swarm-A and Swarm-C in very close orbital planes.
That is why, in order to grasp every ounce of science data out of this orbital configuration, it was decided to investigate and tune also the lower pair along-track separation during the “coun-ter-rotating orbits” phase.
The first phase, in the Summer, was to decrease the separation from the [4;10] seconds to the lower band, i.e. closest as possible to 4 seconds.
Then, for a period of 2 weeks close in time to the closest-plane-alignment, the along-track sepa-ration was decreased to only 2 seconds, corresponding to around 15 km: this configuration was applauded by the Swarm scientific community, due to the science that will be made out of this “pearl on a string” scenario, but implied an intensive work and planning, analysis and mitigation measures undertaken by the Flight Operations Segment at ESOC, both by the Flight Control and the Flight Dynamics teams. It was paramount to ensure keeping the 2 seconds separation at all times and react quickly to any anomaly that could jeopardize it or, even worse, be a risk for the safety of the constellation.
With the third phase, also the interest of the scientists in studying the Earth co-rotating phenomena was taken into account: the lower pair separation was gradually and linearly increased from 4 to a maximum of 40 seconds until mid-December 2021, before the return to the original configuration.
The poster will describe not only the basics of the Swarms orbital configuration, but the journey of the counter-rotating orbits in particular, and the challenges of the closest 2seconds separation, showing how it was possible, from an planning and operational point of view, to play with the lower pair distance such as to achieve different scenarios that will be a diversified sensing input for the Swarm science community for years to come.