Mumbai University > EXTC > Sem 8 > Satellite Communication and Networks

Marks: 5M

A satellite is said to be in eclipse when the earth or moon prevent sunlight from reaching it.

Solar eclipses are important as they affect the working of satellites and in particular the energy generated by solar cells is greatly affected. It should be noted that solar eclipse due to earth is of more importance than due to moon, as the former lasts for several days.

For geostationary satellites, the solar eclipse due to earth occurs during two periods that begin 23 days before equinox (about March 21 and about September 21) and ends 23 days after equinox. The geometric positions of the earth, sun and satellite during eclipse are illustrated in the fig.

The satellite eclipse occurs during equinox because these are the times when the sun, earth and satellite are nearly in the same plane. During full eclipse the satellite receives no power from its solar panels and hence it must be operate entirely from its onboard standby batteries. This reduces the available prime power as the satellite nears the end of its life. This may force the shutting down of some transponders during the eclipse period.

Spacecraft designers must guard against harmful transients as solar power fluctuates sharply at the beginning and end of an eclipse. In addition, passage into out of the earth’s shadow can place severe thermal stress on a satellite. For these reasons satellite failure is more likely to occur during eclipse than at any other time following deployment.

The solar eclipse caused by the moon occurs when the moon passes in front of the sun. The eclipse occurs irregularly in time of duration and depth front of the sun. Often eclipses may occur twice within a 24 hours period. Eclipse duration may range from a few minutes to over two hours with an average duration of about 40 minutes. Compared to earth solar eclipse, the number of moon solar eclipses range from 0 to 4 with an average of 2 per year.

Satellite longitudes, which are west, rather than east, of the earth station, are most desirable.

Justification:

When the satellite longitude is east of the earth station, the satellite enters eclipse during daylight (and early evening) hours of the earth station. This can be undesirable if the satellite has to operate on reduced battery power. When the 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. Thus, Satellite longitudes, which are west, rather than east, of the earth station, are more desirable.

Figure: A satellite east of the earth station enters eclipse during daylight (busy) hours at the earth station. A satellite west of the earth station enters eclipse during night and early morning (non busy) hours.