How Does Eclipse Season Affect Geostationary Satellites?
The Earth’s rotational axis is tilted 23.5 degrees from the perpendicular of its orbit. The equator is tilted at the same 23.5 degrees from the perpendicular of the Earth’s orbit as well. As the Earth travels around the Sun, the rotational axis actually stays pointed in the same celestial location—the North Star. And because of the rotational axis tilt angle, and as the Earth travels around the Sun, the Sun appears to travel up and down the Earth creating the seasons with the longer daylight in the summer and less daylight hours in the winter. The high and low points of this sun ‘travel’ are the Tropic of Cancer and Tropic of Capricorn.
Satellites rely on two power sources—solar panels, which convert the Sun’s solar energy into electrical power to run the satellite, and batteries, for when the solar panels cannot power the satellite. The solar panels also charge the batteries whenever they need it.
Satellites travel above Earth’s equator at approximately 22,300 miles (36,000 kilometers). They therefore also travel at the same 23.5 degree tilt. As the Sun reaches the two equinox seasons, autumnal and spring, the satellites and their solar panels are blocked from the Sun by the Earth. During these events, the satellite must rely on its batteries to function until the solar panels are again exposed to the Sun. The time when the satellite is blocked from the Sun is called the eclipse period.
The eclipse starts slowly. As the Sun travels from one of the Tropics to the equator, the satellite is blocked for a minute or two, at first. Gradually the eclipse increases until the Sun reaches fall or spring equinox and the satellite, and solar panels, are blocked for 72 minutes. As the Sun continues to travel to the other Tropic, the eclipse time becomes smaller and smaller until the solar panels are again exposed to the Sun 24×7.
Eclipse season occurs twice a year. For station-kept satellites, the spring eclipse season runs from approximately 26 February until 12 or 13 April. The fall eclipse season runs from approximately 30 or 31 August until 15 October. For inclined orbit satellites, the eclipse season starts and ends a little earlier, depending on the satellite’s inclination.
Satellites are designed and built with an extra percentage of battery capacity than will be needed when the satellite is at full load. This is to ensure that the satellite can function and continue to provide service to our customers, even when the battery power degrades over time.
Intelsat’s Satellite Engineering Group carefully monitors all Intelsat satellites, and their batteries’ performance parameters, well before an eclipse event. This is to ensure the batteries can handle the full satellite electrical load needed during the full eclipse event to satisfy our customers’ needs on that satellite.
Using our 50 years of experience, and depending on the satellite and the condition of its batteries, we may turn off non-service and/or flight affecting hardware on the satellite while in the eclipse event to provide an added margin of reserve power. After the satellite comes back around the Earth and the solar panels are exposed to the Sun again, we turn the hardware back on, as well as start the battery charging process. This is to ensure our full-time customers’ services are never compromised during an eclipse event.