This question appears in Mumbai University > Satellite Communication and Network subject

Marks: 10 M

Year: Dec 2013, May 2013

• A satellite moving in the equatorial orbit at a height of 35786 km above the earth in the same direction as the earth’s surface with a forward velocity of 3.07km/sec will complete one orbit in about 24 hours (precisely 23 hours, 56 min and 4 seconds). The satellite will therefore appear stationary with respect to earth and hence called a geostationary satellite and the orbit is referred to as geostationary orbit or Clarke orbit.
• There exists only one geo stationary orbit.
• Three satellites in a geostationary orbit can cover the entire globe except the Polar Regions.
• A geosynchronous satellite is one which has a period of rotation equal to the sidereal period of rotation $(T_p)$ of its primary body which is Earth. For Earth the sidereal period is 23 hours 56 minutes and 4 seconds.
• The following point differ geostationary and geosynchronous orbit
• The geostationary orbit is a special case of geosynchronous orbit.
• The geosynchronous orbit may be inclined, polar or may be elliptical but geostationary orbit is always circular.
• Geostationary orbit is always stationary but geosynchronous need not be always stationary.

Maximum daily eclipse duration for GEO orbit:

• The sun is considered to be at infinity with respect to earth. So the earth’s shadow becomes a cylinder of constant diameter. The maximum shadow angles at equinoxes is given by:

$∅_max=180°-2 cos^{(-1)} \frac{r_e}{r_s}$

$∅_max= 180°-2 cos^{(-1)} (\frac{6378.15}{42164.2})=17.4°$

• Geostationary satellite period is 24 hours, the maximum shadow angle is equivalent to maximum daily eclipse duration $(τ_{max})$.

$τ_{max}= \frac{17.4°}{360°} ×24=1.16 h$