Satellite Formation Flying

Applications
Title	Satellite Formation Flying
Author(s)	GMV.
Level	Medium
Year of Publication	2011

All space missions are difficult. Docking a pair of spacecraft is tough but flying multiple satellites together in formation is the real cutting edge. In formation flying separate expensive pieces of hardware, each one zipping through space at several kilometres per second, may have to manoeuvre to within metres of each other to achieve their goals.

The relative positions of the satellites must be maintained precisely as they close in: lose control of one part of the formation, even momentarily, and the satellites risk destruction. And orbital dynamics dictate the satellites’ orbits will tend to cross as they circle Earth, another worrying factor for their controllers.

Application Architecture

In GNSS formation flying 2 different approaches can be possible depending on the formation characteristics:

• Ground-based control - In ground based control, the GNSS measurements are sent by each satellite on the formation to the ground control center that will commands the satellites to maneuver into the adequate position in the formation. This approach is adequate for formation with separation between spacecrafts on the order of kilometers that don't require dynamic adjust of the orbits. The interval between adjustment maneuvers might be of months.
• Autonomous - In autonomous flying formations the measurements are broadcasted between spacecrafts allowing the spacecrafts to calculate their relative positions in the formation and use the Attitude and Orbit Control Systems to maneuver into the adequate position. This approach is adequate for tighter formations that require continuous adjustment of their relative positions. This approach is more difficult and riskier since the tight link between GNSS receiver and the guidance systems can lead to unforseen interactions causing the loss of control or destruction of the spacecrafts.

GNSS formation flying benefits from the fact that relative positioning is usual more accurate that absolute positioning since being the spacecrafts in proximity it is likely that the measurements from each spacecraft share correlated errors that would be canceled in the relative positioning solution.

Application Characterization

Different types of formation can be envisioned for formation flying. The type of formation, the separation between vehicles and the intended application will have an impact on the formation flying approach. The three most common type of formations are:

• Trailing - Trailling formation are formations were the same orbit is shared and the spacecrafts follow each other path at a specified distance.
• Clusters - In which a group of satellites are located close to each other following orbits that permit them to remain as a cluster.
• Constellation -

Application Examples

Landsat 7 Cluster mission Prima Proba-3