Traditional communications satellites orbit at what is known as a geosynchronous (GEO) orbit at a height above the earth of 22,300 miles (36,000 km). The advantage to this very specific location is that it takes 24 hours for the satellite to orbit the earth, which means that the satellite remains at the same location above the earth at all times and appears to remain stationary to an observer on the ground.
This orbit is very convenient in allowing the user on the ground to fix an antenna to a particular location in the sky. This orbit also provides continuous coverage for any location that can see the satellite and allows the operator to focus on coverage for particular countries or population centers. The disadvantage is the distance itself, which is about three times the diameter of the earth or about 10% of the distance to the moon. In contrast, the International Space Station orbits at an altitude of approximately 250 miles (400 km), and the earth’s atmosphere extends out to only about 600 miles (approximately 1000 km).
GEO orbit is extremely high, which makes GEO satellites expensive to launch and impossible to repair in orbit. But most importantly of all, GEO orbit is so far away that it takes light about 1/4 of a second to travel from earth to the satellite and back down to the receiver, adding a noticeable delay to voice communications and interfering with TCP’s round-trip time based algorithms.
If the distance to GEO satellites causes problems, the obvious solution is to move the satellites closer to the ground in LEO (low earth orbit) or MEO (medium earth orbit) orbit. There is no single definition of LEO and MEO orbits, but in general LEO extends from the ground up to about a thousand miles and MEO extends from there up to GEO orbit.
In addition to lower delay, the cost of launching LEO and MEO satellites is generally much less than for GEO satellites. A LEO satellite can potentially be repaired in orbit from the Space Shuttle.
Iridium, Inmarsat and Globalstar satellite phone systems were designed on the premise that a LEO satellite constellation was necessary to meet latency requirements, as was the proposed Teledesic Internet system. The Iridium satellite phone system is in a 450 mile (780 km) low earth orbit. But Iridium also clearly illustrates the disadvantage to this approach.
Because LEO and MEO satellites move in relationship to the ground, multiple satellites are
required to provide continuous coverage so that at least one satellite is in view at all times.
The lower the satellite is to the ground, the more satellites are necessary to cover the earth. The Iridium system is a constellation of 66 satellites. Store-and-forward tracking systems can work with only a few satellites, but for voice or Internet service, the full constellation must be in orbit before the system can be operational since service which is available for a few minutes out of each hour as the satellite goes overhead will not find many customers. In contrast, a GEO satellite can provide coverage to users on about 1/3 of the earth with only a single satellite. So while it may cost less to launch a single LEO satellite, the whole fleet can cost billions of dollars before the system can be switched on and begin generating revenue.
Also, due to the movement of the satellites relative to the users, a sophisticated hand-off system is necessary to periodically move the user from one satellite that is disappearing over the horizon to another satellite that is still visible. On the ground, a sophisticated antenna which can track moving satellites and switch between satellites on-the-fly may be required, which would likely make the customer premise equipment prohibitively expensive for consumers. Satellite telephone systems solved this problem by using a unidirectional antenna which is sufficient for low power phone service (although the subsequent inability of the phone to work indoors or even in the shadow of tall buildings may have been a large contributor to the failure of the businesses) but this type of unidirectional antenna would be unlikely to work for Internet systems operating at high data rates.
Lastly, while LEO satellites do reduce the round-trip time to just a few tens of milliseconds, the round-trip time will be highly variable depending on whether the satellite is directly overhead or on the horizon. Since TCP’s retransmission mechanisms are tied to the round-trip time, TCP can be highly sensitive to variability in the round-trip time.
Overall, by bringing the satellite much closer to the ground, LEO and MEO satellites are able to resolve most TCP performance limitations by reducing the satellite latency to a value typical of terrestrial networks. However, LEO and MEO satellite networks introduce other technical challenges regarding antenna design, connection hand-over, and satellite-to-satellite communications. Most importantly, the cost of a constellation of LEO satellites is nearly impossible to justify with any rational business plan, especially when GEO satellites can be made to work just as well by using some basic protocol enhancements.