“A satellite, orbiting around the Earth, would continue to orbit forever if gravity were the only force acting on it. However, satellites below 2000 kilometers, are actually travelling through the Earth's atmosphere. Collisions with air particles, even at these high altitudes, slowly act to circularize the orbit and slow down the spacecraft causing it to drop to lower altitudes.”
http://www.windows.ucar.edu/spaceweather/sat_drag.htmlAltitude Classifications
Low Earth Orbit (LEO) - Geocentric orbits ranging in altitude from 0 - 2,000 km (0 - 1,240 miles)
Medium Earth Orbit (MEO) - Geocentric orbits ranging in altitude from 2,000 km (1,240 miles) - to just below geosynchronous orbit at 35,786 km (22,240 miles). Also known as an intermediate circular orbit.
High Earth Orbit (HEO) - Geocentric orbits above the altitude of geosynchronous orbit 35,786 km (22,240 miles).
http://en.wikipedia.org/wiki/SatelliteSynchronous Orbit is about 36000km high
So atmosphere drags satellites on Low Earth Orbit mainly, almost does not touch synchronous satellites.