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Impact of collision avoidance manoeuvres on large satellite constellations.

Jonas Radtke · 3 min read · 11 September 2023

Impact of collision avoidance manoeuvres on large satellite constellations.
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In the last few years, the launch of several satellite mega-constellations has been announced, which will place thousands of satellites in the already crowded Low Earth Orbit. In this context, collision avoidance manoeuvres (CAM) are essential to protect both the space environment and the mission of the constellation. However, satellite constellations are usually bound to very tight geometrical constraints, which can be disturbed by these manoeuvres. The aim of this work is to analyse the impact of different manoeuvre strategies on constellation performance.

Study setup

A generic communications Walker Delta constellation was defined for the study: a 67°: 1080/24/12 constellation at 600 km altitude, 1080 satellites in 24 orbital planes with a phasing factor of 12, giving a 4° phase difference between satellites in adjacent planes. Every satellite communicates with 4 others via inter-satellite laser links (ISLLs). The study investigated the impact of CAMs on both coverage and ISLL performance.

Six conjunction cases

  • Case 1: One object crossing one orbital plane.
  • Case 2: Two objects crossing the same orbital plane.
  • Case 3: Two objects crossing two adjacent orbital planes.
  • Case 4: One object crossing several orbital planes.
  • Case 5: One object orbiting in one of the constellation planes.
  • Case 6: Collision cloud in one of the constellation planes (fragmentation event).

CDMs from ESA's Collision Avoidance Challenge were used and modified to fit the constellation geometry, providing realistic covariance evolution over time.

Radial vs. in-track strategies

Two manoeuvre types were compared: a short-term radial strategy aiming for 0.2 km radial separation, and a long-term in-track strategy aiming for 1 km in-track separation. The results showed that deviations due to the radial strategy have a much bigger magnitude than those due to the in-track strategy.

Distance, azimuth and elevation variation for in-track and radial manoeuvres
Distance, azimuth and elevation variation for the in-track (blue) and radial (red) manoeuvres with Δt = 2 orbits before TCA, return over 4 orbital revolutions.
Distance variation for in-track and radial manoeuvres at different times before TCA
Distance variation for in-track (left) and radial (right) manoeuvres for different times before TCA.
Distance variation for in-track and radial manoeuvres with different return orbit counts
Distance variation for in-track (left) and radial (right) manoeuvres for different numbers of phasing orbits.
Distance variation from radial manoeuvre with respect to 4 linked satellites
Distance variation for the radial manoeuvre with respect to the 4 satellites linked via inter-satellite laser links.
Visibility loss percentage for in-track and radial manoeuvres
Visibility loss (%) for the in-track and radial manoeuvres. The radial strategy causes significantly larger coverage loss.

Key findings

When two satellites must manoeuvre (Cases 2 and 3), the chosen direction matters critically: if both manoeuvre in the same direction, the effects compensate each other. If they manoeuvre in opposite directions, the effects add up, potentially problematic. For Case 6 (fragmentation cloud), reducing manoeuvre time reduced the number of problematic encounters by nearly 70%. This demonstrates the operational benefit of short-term manoeuvre strategies in specific scenarios.

Conclusion

The importance of proper manoeuvre design to avoid unnecessary harmful impacts has been demonstrated. The whole constellation needs to be taken into account when any manoeuvre is planned. Additionally, CAM strategies should be considered from the design stages to include both adequate fuel budget and flexibility on geometrical constraints.

Article by Lucía Ayala Fernández.
Lucía spent 6 months at OKAPI:Orbits at the end of her dual degree Spacemaster programme at Luleå University and University Paul Sabatier Toulouse, working on analyzing the impact of collision avoidance manoeuvres on the service delivery of large satellite constellations. This article summarises the resulting Master's thesis.
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