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, single hop (up and down) 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 geostationary 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. 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.
In recent years there has been increasing interest in low earth orbit (LEO) and medium earth orbit (MEO) satellite networks. These satellites move across the sky and need tracking (or low gain omni-directional) antennas on the ground. Coverage visibility of the earth is less than for geostationary satellites and to provide continuous coverage multiple satellites are used for each network, configured in constellations with complex interlocking orbits designed to give users visibility of one or two satellites at all times. As many as 1000 satellites may be proposed in a such constellation.
O3b is an operating constellation of 12 satellites, spaced along in a single medium height orbit directly above the equator. The height is 8062km and this results in lower latency compared with geostationary. This is a key selling point. If a typical single hop path involves sloping path lengths of 11000km the single hop distance is 22000km with a latency of 73 mS. O3b claim a round trip latency of better then 150mS. 150 mS round trip refers to a double hop distance of 11250 + 11250 + 11250 + 11250 km.
This is a proposed LEO satellite constellation with some 648 satellites at a height of 1200km. Typical single hop latency, corresponding to 1500km + 1500km would be 10 mS. If the terrestrial end points are not within the coverage of a single satellite (approx 1000km x 1000km) then the distance will be greater, with inter-satellite links via other satellites.
The use of the TCP/IP protocol over satellite may be improved and a number of companies have developed ways of changing the protocol over the satellite link to achieve IP acceleration.
Page created 4 Apr 2004 Amended 16th July 2016
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