- A satellite is said to be in eclipse when the earth or moon prevents sunlight from reaching it.
- If the earth’s equatorial plane coincides with the plane of earth’s orbit around sun, the geostationary orbit will be eclipsed by the earth. This is called the earth eclipse of satellite.
- For a geostationary satellite, the solar eclipse due to earth occurs during two periods that begin 23 days before equinox and ends 23 days after equinox. Because during equinox (autumn and spring) the sun, earth and the satellite are in the same plane.
- Solar eclipses are important as they affect the working of the satellite because during eclipse satellite receives no power from its solar panels and it has to operate on its onboard standby batteries which reduce satellite life.
- Satellite failure is more at such times when satellite enters into eclipse (sudden switch to no solar power region) and when it moves out of eclipse (suddenly large amount of solar power is bombarded on satellite) as this creates thermal stress on satellite.
- Eclipse caused by moon occurs when moon passes in front of sun but that is less important as it takes place for short duration (twice in every 24 hours for an average of few minutes).
Way to avoid eclipse during satellite lifetime:
Satellite longitudes which are west rather than east of the earth station are most desirable.
- When satellite longitude is east of the earth station, the satellite enters eclipse during daylight and early morning hours of the earth station. This can be undesirable if the satellite has to operate on reduced battery power
- When satellite longitude is west of the earth station, eclipse does not occur until the earth station is in darkness when usage is likely to be low.