Low Earth Orbit

Geosynchronous (or parking) orbits are at about +/- 22,400 miles up.

While this is quite handy for things like TV broadcasting, where a signal doesn't need to make a round trip, it kind of sucks for high-speed interactive communications.

On the good side, once the object is in place, little additional attention is required to keep it on station or, more importantly, to track it in the sky. A significant fraction of the mass of most geosynchronous satellites is dedicated to station-keeping fuel. Before the XIPS IonThruster, this fraction was roughly 1/4 to 1/2, for satellites with a 20 year useful life. The XIPS IonThruster reduced this fraction to 2% - 5%.

On the bad side, a round trip signal, and its response (for interactive) is 4 trips over this 22,400 miles, meaning that (given speed of light) the total travel time (not including any ground switching) is at least two-thirds of a second. This latency is why it sucks for interactive comms. Telnet at that speed is truly painful.

To overcome this latency, it is necessary to station participating satellites at a much lower altitude -- a "low Earth orbit" -- so that the latency from radio wave travel time is minimized.

There is no one set altitude. Anything above the majority of the Earth's atmosphere, say 200 km, works.

On the good side, the latency can be practically eliminated.

On the bad side, you don't get a useful communications constellation until you have *all* the satellites in place and, because they *can't* be stationary at this lower altitude, you have the "cell phone handoff" problem in reverse as satellites pass into and out of the aperture of the ground station(s).

Other low orbit applications would include such things as satellites tasked with photographic missions, weather tracking, and so on.

A LowEarthOrbit is thus a suitable orbital altitude, below the "parking orbit" altitude of "ClarkesConstellation?" for a given application of satellite resources where the higher orbit is inconvenient or useless.