What is a Sun-synchronous orbit?
In a Sun-synchronous orbit, the satellite appears over an area at the same Sun angle in successive orbits, which is ideal for remote sensing. The orbital plane is always at the same angle relative to the Sun-Earth line during all seasons. It also implies that the satellite will cross given latitude at the same local time. The solar illumination angle desired for remote sensing or weather study can be fixed for the whole year in such orbits. Any numbers of sun-synchronous orbits are possible if some conditions are met. One vital condition is that the satellite has to go in an east-to-west direction i.e., in a retrograde orbit. The angle of inclination of the orbital plane to the Equator will vary, depending on the altitude of the orbit. Earth’s equatorial bulge will cause an eastward shift of the plane of the orbit. This eastward regression will nullify the orbital plane of a satellite caused by the Earth’s movement relative to the Sun during the annual revolution.
Thus, the Sun-synchronous orbit becomes possible by a combination of natural forces and choice of orbits. Satellites at altitudes of 565 km, 893 km and 1261 km, having inclinations of 97.60, 990 and 100.70 would complete 15, 14 and 13 revolutions per day respectively. It is obvious that but for the equatorial bulge of the Earth, there can be no Sun-synchronous orbits.
What is a geo-synchronous orbit (also referred to simply as synchronous orbit)?
Geo-synchronous orbit is important for communications and weather study. This is a circular equatorial orbit at about 36,000 km above the Equator. A satellite in this orbit moving from west to east takes about the same time as the Earth takes to complete one rotation i.e. 24 hours. The satellite therefore appears as a fixed ‘star’ if its plane coincides with that of the Equator. This idea was first suggested by Arthur C. Clarke, a science writer who propounded in 1945 the idea of covering the world with their satellite 120 degrees apart in a synchronous orbit, so that each can ‘see’ one-third of the globe. At this height, a satellite with nominal velocity of 3075 m/sec will synchronize with the speed at which the Earth rotates about its axis viz 450 m/sec, and will appear stationary in the sky.