Wednesday, June 20, 2012

The Clarke Orbit

Geosynchronous Orbit n. 
An orbital position with an orbital period that matches the rotation rate of the Earth measured relatively to  fixed stars. 
That orbital period isn't a 24 hour day actually. it's what they call a mean sidereal day, which is 23 hours 56 minutes and 4.091 seconds in length. It's not an exactly fixed interval because the Earth wobbles on it's axis, which is measured as two variables called Precession and nutation.  Btu that's getting picky. This is just the time it takes for the Earth to rotate once around its polar axis relative to a distant fixed point. Our sun is not a distant fixed point, so they use the vernal equinox. So far.. not about radio. Don't worry I'm getting there.

The first man-made anything in space was a V4 rocket in 1942. the first satellite was Sputnik in 1957. But the first geosynchronous orbit wasn't achieved until 1964 with the launch of Syncom 3. It was an American communications satellite.

The very idea of a geosynchronous communications satellite is quite old, almost as old as broadcasting. It was first considered and published by Herman Potocnikin 1928. Being an ethnic Slovak in an area now called Croatia... his national identity is up for some debate. But he sometimes used the name Hermann Noordung. He was a real-deal rocket scientist. He attended the University of Technology in Vienna and became a mechanical engineer. He was inspired by the research of Hermann Oberth and went into rocketry.  But Arthur C. Clarke stole his thunder. In a 1945 paper published in Wireless World magazine, titled "Extra-Terrestrial Relays — Can Rocket Stations Give Worldwide Radio Coverage?" More here. He even predicted the frequency band we'd be using. He was only off a few hundred GHz on the future location of the C and Ku bands. Not bad for a guy who writes fiction. the first US TV network to move all it's affiliate feeds to the Ku band was NBC in 1983. Read on:
"It may be argued that we have as yet no direct evidence of radio waves passing between the surface of the earth and outer space; all we can say with certainty is that the shorter wavelengths are not reflected back to the earth... Medium high frequencies go through the E layer twice to be reflected from the F layer and echoes have been received from meteors in or above the F layer. It seems fairly certain that frequencies from, say, 50 MHz to 100,000 MHz could be used without undue absorption in the atmosphere or the ionosphere."
In the piece Clarke first described a geosynchronous orbit as useful for broadcast and relay communications. Because of this much more widely read publication a geosynchronous orbit  is sometimes called a Clarke Orbit. Likewise the region of Earth orbit where near-geostationary orbits are possible is called the Clarke Belt. it is about 22,000 miles above sea level along the equator. To his credit, there are presently over 700 radio communications satellites in a Clarke Orbit. More here.