Geosynchronous Orbits - Communications Satellites:
This article follows on from my previous article about Geosynchrous and ‘Clarke’ (Geostationary) orbits and is a digression based on the distances involved in these highly specialised orbits and how they effect satellite communications. Hopefully, some of the information here will be useful to people who want to provide a more realistic communications environment for their science fiction stories.
As you may remember a Geostationary orbit means that the satellite appears to sit suspended over a specific point on the Earth’s equator at an altitude of about 36,000 kms. This is quite a distance. Even light travelling at approximately 300,000 kms per second will take over a tenth of a second to cover this distance and you will have all seen this demonstrated in television satellite links where a remote interviewee’s response will noticeably lag a question. In fact, with processing overheads and the travel time both ways this delay can be as much as a quarter of a second; a situation certainly not conducive to casual dialogue.
Geostationary satellites are good for certain types of communication - the mainly one way type. This explains why a satellite television dish, once set up, doesn’t move. It’s pointing at a geostationary satellite orbiting in a particular spot above the equator. However, Geostationary satellites are not so good for telephony and locations at high latitude where the satellite is too close to the horizon to be practicable. Here’s where the other type of geosynchronous orbit comes into play.
The definition of geosynchronous orbit tells us that a satellite in such an orbit will always be over a particular point on the surface at a particular time of the day; its orbit is synchronised to the rotation of the Earth. Networks (also commonly called ‘constellations’) of such geosynchronous satellites in lower orbits (so that the travel time for signals is much, much less) can work together to provide a better coverage of the Earth’s surface with less communication’s lag. Notable examples of these types of systems are the satellite telephone services: Iridium and Inmarsat and the Global Positioning System (GPS).
The ‘visible sat’ number is the number of GPS satellites
visible from the indicated point on the surface.
Image Courtesy of El Pak from the Wikimedia Commons.
However, having said all this, most telephony and other two-way communications systems (e.g. the Internet) use the vast network of cabling that has been laid both under the ground and sea to handle the world’s long distance communications. These cables provide both a more secure (access to major cables is restricted) and more reliable (unaffected by sunspot activity, which plays havoc with radio) service.
Well, enough about satellites. A new topic for discussion next week. Promise.
For a more complete discussion on Geosynchronous Orbits:
http://en.wikipedia.org/wiki/Geosynchronous_orbit
And for information about communications satellites:
http://en.wikipedia.org/wiki/Communications_satellite
N.B. Please note that I although I use the Wikipedia (and WikiMedia Commons) a lot for references, this is for expediency and the familiarity of my readers. Anyone interested in further studies should make use of the references where available and understand the Wikipedia is a co-operative project contributable to by anyone and must always be looked at in that light.
Phill Berrie, July, 2008.
