Satellite Internet Versus Ground-Based Wireless

The range of ground-based transmitters is restricted by the curvature of the Earth and other obstacles that get in the way such as hills or buildings that absorb the signal. Unless a mobile phone company has an enormous amount of money to provide a lot of transmitters then there will always be areas with patchy or no coverage.

The idea of a satellite is to put a transmitter in the sky where there are no obstacles between the transmitter and the receiver. If the transmitter is placed high enough it is directly visible to an extremely wide area, such as an entire country or even an entire continent. This means there is signal coverage everywhere with no gaps.

If the signal from a satellite is not focused then it is dispersed everywhere that the satellite is visible. The signal from a satellite can be thought of as a single broadband connection. If the satellite is visible to an entire continent then that means everyone on that continent has to share one broadband connection. Needless to say, there is not very much broadband to go around.

Older satellites could not focus the signal very well hence this made satellite internet somewhat impractical. Only a very slow connection would be available for a very large amount of money. Modern satellites though have a much better capability to be able to focus the signal on a smaller area. This means fewer people have to be served by one signal and there is more broadband available for each customer. A single satellite can potentially transmit many focused signals simultaneously to different areas.

It is very difficult to keep a satellite in one place, it has a tendency to orbit or rotate around the earth. Thrusters can be used to keep a satellite in position but this uses fuel and eventually the fuel will be exhausted. The fuel in a satellite cannot easily be replaced.

When satellites are far away the signal is weak and an antenna (dish) that is intending to receive the signal needs to be very precisely pointed directly at the satellite. If the satellite is constantly moving around, this is a problem, as the antenna would need to be constantly adjusted.

There is a particularly good position in which to place a satellite which is about 35,000 kilometres up. At this particular height a satellite will naturally orbit the Earth at the same speed at which the Earth rotates. This causes the satellite to appear completely stationary in the sky and is known as a geostationary orbit. The satellite is still moving very fast but because of the exact synchronisation with the rotation speed of the Earth it appears to be motionless. This means it’s effective position in the sky can be maintained with little fuel. This particular height also has a good view of the earth with wide visibility.

A problem with the geostationary orbit is the length of time that the signal takes to get to the satellite and back to Earth again. Geostationary satellites are so far away that this takes about a quarter of a second even at the speed at which radio waves travel which is close to the speed of light. This is too long for many internet applications and in practice can be even longer. Video conferencing and voice calls are very difficult with a delay and it is also unsuitable for online games. Consequently there has been interest in placing satellites at much lower heights where there is little delay getting the signal to the satellite and back again.

The problem with low orbits is that the satellite is visible to much less of the Earth. This means many more satellites are required to provide reasonable coverage. If you imagine pointing a spot lamp at a table, when the lamp is close to the surface the light only covers a small area, however when it is moved further away the beam of light expands to cover a much larger proportion of the desk. Hundreds of satellites in low orbits, possibly even thousands, are required to replace just one geostationary satellite.

Satellites have been historically extremely expensive to both build and launch. New technology though has made satellites cheaper to produce and reduced the launch costs. This has made it more financially feasible to position large constellations of satellites in low orbits.

As mentioned previously, if a satellite is not in geostationary orbit then it moves around in the sky. A satellite in a low orbit may be visible to a fixed position on earth for as little as 10 minutes before it has overflown the position. An antenna pointing in a fixed direction is therefore of no use and will never receive a signal because the satellite moves too fast. Mechanically moving the antenna with precision is possible but requires a sophisticated and expensive mechanism that would make the antenna unaffordable to many. The problem though has been solved by using a large collection of small fixed position antennas working together that provide the effect of an antenna that can be electronically pointed in a wide range of directions. This allows a fast moving satellite to be followed very precisely at low cost.

The improvements in antenna technology and the lowering of the launch cost of satellites has enabled low orbit satellites to present more financially viable competition to ground based transmitters. However due to the considerably lower cost of installation, a ground-based transmitter will probably always be able to deliver lower costs in urban environments where there are many customers in a small area. A ground-based transmitter can also produce a stronger signal that can penetrate the walls of a building, which is something that a distant and weak satellite signal struggles with. Satellite technololgy is an excellent solution for rural areas though where there are few customers dispersed over a wide area and it becomes more expensive to install a large collection of ground transmitters than launch a constellation of satellites.