A comprehensive survey of the GEO-belt using simultaneous four-colour observations with STING

Colour light curves of resident space objects (RSOs) encapsulate distinctive features that can offer insights into an object’s structure and design, making them an invaluable tool for classification and characterisation. We present the results of the first large systematic colour survey of the GEO (Geostationary Earth Orbit) belt in which we obtain full-night multi-colour light curves for 112 active geostationary objects between April and May 2023. This survey was undertaken during the ‘glint’ season using the STING telescope located at the Roque de Los Muchachos observatory in the Canary Islands.

Colour light curve maps were created to compare and contrast the colours between different satellites and bus configurations. We find that satellites with BSS-702 and STAR-2 buses can be effectively distinguished from the colour measurements on these maps, but comparing the average colour of individual satellites within given solar equatorial phase angle ranges shows that it is difficult to distinguish between bus configurations based on colour alone. We also find tentative evidence to suggest that there is a relationship between colour and time spent on orbit for the Eurostar-3000 class satellites, which is unseen behaviour within other bus configuration classes. The satellites in our sample exhibit ‘redder’ colours than the Sun, which is in agreement with previous findings.

We found common light curve features such as symmetrical colour changes as well as unique regions of short timescale glinting which are ‘bluer’ than other regimes within the colour light curves. If these features are indeed seasonal, this would be a powerful characterisation tool. We are able to detect and resolve features in the light curve of the LDPE-3A satellite related to manoeuvres being performed.

Finally, we measured the solar panel offsets of 54 satellites in our sample and found variation in the type of colour response. The majority of which did not exhibit any colour change across the solar panel glints compared to them shifting towards ’redder’ or ’bluer’ colours.