The Eclipse Season and what it means for satellites.

As we welcome the Spring Equinox and longer and brighter days in the northern hemisphere, explore how our satellite operators meticulously navigate the THOR fleet through the eclipse season to ensure that we keep our connectivity services unaffected.

Spring Eclipse Season

The spring equinox is the time when those of us in the northern hemisphere start to look forward to the longer and brighter days of summer. In fact, like its autumn counterpart, the equinox (named from the latin words equus (equal) and nox (night) is the rare occasion when day and night are equal. Here Telenor Satellite’s Richard James Buckley, Director Satellite Operations, explains how the equinox and its related eclipse season affects both satellites and the people operating them.

While everyone else is preparing for either summer or winter, for the engineers looking after our fleet it is the period during which all GEO stationary satellites will enter the shadow of the earth (an eclipse) for between one and 72 minutes per day for a duration of 44 days. The precise timing of this will depend on their exact location but, as our THOR fleet are all based at 1⁰ West, the satellites will all be in shadow at the same time. This is a challenge for the operators as the satellite cannot use its array of solar panels when the sun is unavailable.

Effects on the satellite.

Entering an eclipse (or period of darkness) results in a rapid decrease in temperature onboard the satellite. Fortunately, this is mitigated by the thermal blankets within which the satellite’s essential equipment is wrapped to moderate extremes of temperature.

When the satellite is in the sun, reflectors are used to repel the heat while elsewhere heat pipes absorb heat ready to distribute warmth to areas of the satellite which are cold. Engineers make the decision whether to switch heaters on or off while the satellite is operating and so maintain active thermal control over the equipment and ensure the satellite remains at an optimum temperature at all times.

The intricated dance of light and shadow.

The ideal time for the satellite is when the sun is directly behind it as there is an equal amount of illumination on all the solar panels which means that electrical power generated by the panels can be optimised. However, it therefore stands to reason that the worst time for the satellite is when it is in darkness such as it experiences during an eclipse and this lack of solar energy means that the satellite must be powered with batteries.

However, it’s not only the satellite itself that can be affected by spring equinox season. Signals between the satellite and the ground antennas can be disrupted due to the angle of the sun.  If the sun shines directly onto the ground antenna, it can be momentarily blinded by the light.

On the plus side however, inclination manoeuvres are slightly smaller during this season and, additionally, there may be a reduction in the roll and yaw torques (pivots the satellite makes on its axis) – both of which may ease the demands on fuel.

How does the eclipse season affect us as satellite operators?

There are a number of steps that satellite operators have to take during equinox season to ensure that the satellite continues to operate effectively and smoothly:

  • Our satellite operators are responsible for the thermal balance onboard the satellite and so have to adjust heating of the various working parts.
  • They ensure that there is sufficient electrical power to perform the manoeuvre
  • We monitor the ground antennas, so that they can be taken offline if they are subjected to sun blinding.
  • Operators monitor the health of the satellite, especially the battery, carefully.
  • We plan to avoid having manoeuvres or other major procedures/activities during the daily Eclipse.
  • After each eclipse we carefully charge the batteries again, ensuring they are fully charged before the satellite enters the earth’s shadow during the following day’s eclipse.
  • After the season is complete and the satellite is back in 24-hour sunlight, satellite operators put the batteries into storage ready for the next time they are needed. (This may mean cooling the battery and reducing the charge slightly so that wear and tear on the battery is reduced to a minimum).

So, as you look forward to brighter mornings and sunny evenings, spare a thought for our satellite operators. We might simply be entering spring , but they are entering their busiest time of the year.


In Oslo, Norway, the Spring Equinox will begin on March 20, 2024 at 04:06 CET.