Geostationary satellite latency and time delay ms
Most communications satellites are located in the Geostationary Orbit (GSO) at an altitude of approximately 35,786 km above the equator. At this height the satellites go around the earth in a west to east direction at the same angular speed at the earth's rotation, so they appear to be almost fixed in the sky to an observer on the ground.
The time for one satellite orbit and the time for the earth to rotate is 1 sidereal day or 23 h 56 m 4 seconds.
Radio waves go at the speed of light which is 300,000 km per second.
If you are located on the equator and are communicating with a satellite directly overhead then the total distance (up and down again) is nearly 72,000 km so the time delay is 240 ms. ms means millisecond or 1 thousandth of a second so 240 ms is just under a quarter of a second.
A satellite is visible from a little less than one third of the earth's surface and if you are located at the edge of this area the satellite appears to be just above the horizon. (see my satellite dish pointing calculator for slant range and azimuth and elevation angle calculations ). The distance to the satellite is greater and for earth stations at the extreme edge of the coverage area, the distance to the satellite is approx 41756 km. If you were to communicate with another similarly located site, the total distance is nearly 84,000 km so the end to end delay is almost 280 ms, which is a little over quarter of a second.
Extra delays occur due to the length of cable extensions at either end, and very much so if a signals is routed by more than one satellite hop.
Significant delay can also be added in routers, switches and signal processing points along the route..
The use of the TCP/IP protocol over satellite may be improved and a number of companies have developed ways of temporarily changing the protocol to XTP over the satellite link to achieve IP acceleration. See http://www.packeteer.com/resources/?attrvalue=75 for an example.
Satellite broadband internet access - how it works
Page created 4 Apr 2004, Amended 1 April 2008
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