Earth Eclipse of Satellite

If the earth’s equatorial plane coincided with the plane of the earth’s orbit around the sun (the ecliptic plane), geostationary satellites would be eclipsed by the earth once each day. As it is, the equatorial plane is tilted at an angle of 23.4° to the ecliptic plane, and this keeps the satellite in full view of the sun for most days of the year, as illustrated by position A in Fig. 3.8. Around the spring and autumnal equinoxes, when the sun is crossing the equator, the satellite does pass into the earth’s shadow at certain periods, these being periods of eclipse as illustrated in Fig. 3.8. The spring equinox is the first day of spring, and the autumnal equinox is the first day of autumn.

Eclipses begin 23 days before equinox and end 23 days after equinox. The eclipse lasts about 10 min at the beginning and end of the eclipse period and increases to a maximum duration of about 72 min at full eclipse (Spilker, 1977). During an eclipse, the solar cells do not function, and operating power must be supplied from batteries. This is discussed further in Sec. 7.2, and Fig. 7.3 shows eclipse time as a function of days of the year.

Where the satellite longitude is east of the earth station, the satellite enters eclipse during daylight (and early evening) hours for the earth station, as illustrated in Fig. 3.9. This can be undesirable if the satellite

has to operate on reduced battery power. Where the satellite longitude is west of the earth station, eclipse does not occur until the earth station is in darkness, (or early morning) when usage is likely to be low. Thus satellite longitudes which are west, rather than east, of the earth station are more desirable.