Constellation and Orbit Evolution

There are three main Swarm spacecraft: Swarm A(lpha), B(ravo), and C(harlie). Their orbital configuration is designed to maximise both the measurement capabilities and the lifetime of the mission, and is occasionally adjusted to target different opportunities. Swarm A and C fly side-by-side in order to measure gradients in the magnetic field, while Swarm B flies at a higher altitude sampling the field at a different local time.

Below is a timeline with key information on the orbit evolution. For more information, see the Mission Analysis Status presentation slides from the Flight Dynamics Team .

22 November 2013 - 17 April 2014

The orbit acquisition phase where the satellites were manoeuvred into the desired configuration. At the end of this period, all three were in close to polar circular orbits (Swarm A and C having an inclination of 87.3° and Swarm B 87.7°), with Swarm A and C forming the lower pair at an initial altitude of 462 km and Swarm B at 511 km. Swarm A and C were flying side-by-side with a 1.4° longitudinal separation at the equator (DRAAN).

17 April 2014 onward

Key parameters:

  • The orbital plane of Swarm B relative to A and C drifts at a rate of about 23° per year (the difference in the RAAN, the longitude of the ascending node). This is mainly due to their inclination difference of 0.4°. This drift gives rise to the following conditions of the orbital plane of Swarm B relative to A and C:
    • Early 2014: co-planar (0°)
    • 2017/18: nearly perpendicular (90°)
    • October 2021: counter-rotating (180°)
    • 2025: perpendicular again (270°)
    • 2029: return to co-planar (0°)
  • The LTAN (local time of ascending node) coverage (0-24 hours) repeats about every 9 months. This is due to the rotation of the orbital planes (the change in the RAAN) and the orbit of Earth around the Sun.
  • While Swarm B always flew higher than A and C, their altitudinal separation has changed: 50km in 2014 to 70km in 2021, with A and C dropping from an altitude of 470km to 440km.
  • The orbits of Swarm A and C are maintained with an along-track separation between 4 and 10 seconds. Due to the inclination the maximal equator crossing time difference is then 11 seconds at a longitudinal separation of 1.4°.
17 April 2014 onward

October 2019 - October 2021: Adjusting Swarm A and C

To maximise the scientific return of the counter-rotating period in October 2021, Swarm A and C were manoeuvred so that they would be brought back into the same orbital plane (0° DRAAN). This procedure began in October 2019, slowly reducing their longitudinal separation from 1.4° to 0°, achieving perfect alignment at the exact time of the conjunction with Swarm B in October 2021. This was done by increasing the inclination of Swarm A by 0.015°. If this change is not reversed, the Swarm A and C separation will reach -1.4° in October 2023.

July - December 2021: Counter-rotating orbit phase

The DRAAN of Swarm B relative to A and C reached 180° at the same time as A and C reached 0° so that Swarm B was flying through the same orbital plane of the lower pair flying in the opposite direction. Due to the higher altitude of B, the orbital period is about 90s longer. This means that the latitudinal meeting points of the three satellites distribute all around the orbit after four days. The along-track separation of A and C was maintained as follows:

  • Mid July to mid September: ~4s
  • Two weeks until 4th October: ~2s
  • October to mid December: variable, increasing from 2s, in line with the DRAAN evolution such that the same phenomena co-rotating with the Earth are observed by both A and C
  • Mid December 2021 to mid January 2022: Return of the A and C separation to the standard maintenance band between 4s and 10s

 

In the counter-rotating configuration the inclination difference from 90° (Swarm A and C 2.7° and Swarm B 2.3°) acts in opposite directions. Thus although having common nodal points a minimum angle of 5° remained between the orbital planes leading to 600km cross track separation close to the poles.

2022 onward

 Raising Swarm A and C

With the increasing solar activity of solar cycle 25, the Earth’s atmosphere is expanding leading to higher density and stronger decay of the orbits. Without an orbit raise of Swarm A and C they would re-enter in 2024, so a first sequence of manoeuvres is planned between 3rd May and 8th July 2022, raising their altitude by about 45 km. There is a potential impact on data quality due to attitude errors during this period, with up to two manoeuvres per spacecraft per week, each lasting for up to two full revolutions. There will also be a substantial increase of the along-track separation of A and C, from the usual 4-10 seconds to several minutes, degrading the quality of a number of data products.

Further raises will be planned in the following years according to the evolution of the solar activity, so that the subsequent natural decay will put A and C at a low altitude at the next solar minimum in 2030/2031. Achieving this low altitude at solar minimum will allow a better resolution for mapping the Earth’s lithospheric magnetic field. This raise is only feasible if solar cycle 25 is not significantly stronger than the previous one as otherwise there will not be enough fuel available to raise the orbit high enough. The raise will be done in multiple steps so as to accommodate the evolution of solar cycle 25 as long as possible. However the raise needs to be completed before the solar maximum is passed such that significant uncertainties about the future orbit evolution remain.

Avoiding co-planar orbit of Swarm B in 2030

If not adjusted the orbital plane of Swarm B will be coplanar with A and C, which is undesirable for mapping the lithosphere. To avoid this it is necessary to reduce or increase the relative rotation after they are perpendicular in 2025. While there is not enough fuel available to stop the drift completely, it can be slowed or accelerated. Manoeuvres will be planned soon to begin this process so as to efficiently accumulate the effect over many years.

Stopping DRAAN drift of Swarm A and C

Swarm A and C are slowly drifting apart from 0° in October 2021. This drift will be stopped but it has not yet been decided at which DRAAN this shall be.

Stopping DRAAN drift of Swarm A and C