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The Mobile Satellite World
There is little education outside of a small group of people in the satcoms industry about the various satellite networks. One of the first things is understanding what the major differences and benefits are of each network. Here we will run over the various technologies and come to a few conclusions based on your needs to select the best network. There are many networks available but the most popular in my industry are Inmarsat, Iridium, GlobalStar, and Thuraya so we will cover those.
Why are satellite orbits so important?
They define the behavior of the network including polar coverage, antenna angle, type of antenna, delay in the communications link, and satellite life.
To start with the most basic orbit, a geostationary orbit, a satellite is launched on some of the largest rockets available to reach a height of about 26,000 miles and a speed of about 2 miles per second. The benefit is that the satellite appears to be frozen in it’s location based on observation from the ground as the satellite falls and rotates at the exact same speed as earth so you can point an antenna at the satellite and it will never move. This allows easy use of high-gain antennas which can transmit at higher data rates. Also, the lifetime of these satellites are good as there is little fuel required to maintain the orbit/speed/direction the satellite is pointing. The downsides are the weight, size, fuel required to launch, and distance from earth add a significant delay to the transmission path as the signal travels up to the satellite and back down to a ground station. Also, the poles are not covered as the earth shields the signal at +-82 degrees latitude. Operation above/below these areas requires a MEO or LEO satellite covered next.
Now moving to a more complex orbit are lower earth orbits called MEO (medium earth orbit) or LEO (low earth orbit) where the orbits can be quite regular or irregular/complex. These satellites are orbiting much closer to earth and are travelling at a much higher speed, about 5 miles/sec. You can imagine that they are shot from a slingshot at such a speed that they never hit the ground and can have very complex orbits because of this. They can be shot in nearly any direction and can change orbital heights as they traverse the planet by adding “eccentricity” described best here: Milankovitch cycles. These satellites move quite rapidly in relation to the earth surface, so either an omnidirectional antenna must be used, or a tracking antenna that stays locked on the satellite. Because of the antenna complexity, there is less signal margin and lower bandwidth available, but much better latency and visibility to the user. As the satellite moves in relation to the user, blocked signal paths are not much of a problem as you will get coverage as the network moves around you. Also, the satellites can pass over the poles so you have coverage over the entire planet. The lifetime of these satellites is lower as there is more atmosphere but weigh significantly less as their orbital periods are short, which means the onboard solar+battery packs are small to power them through the period they cannot see the sun.
- Inmarsat uses geostationary orbits
- Iridium uses low earth orbits
- GlobalStar uses low earth orbits
- Thuraya uses geostationary orbits
Why are ground stations and the type of satellite so important?
The ground stations create the link between the satellite and ground communications for terminating calls and data in general. Based on what technology is on the satellite, it will determine the type of groundstation required to run that satellite. In reality, a basic satellite or intelligent satellite does not make the network better. It is dependant on the orbit AND the type of satellite. LEO/MEO orbits are best serviced by intelligent satellites, and geostationary orbits are best serviced by basic satellites.
A standard satellite is a reflector that does a “copy and paste” of the signal it is receiving back down to the ground for processing. Satellite’s typically have many smaller antenna’s called beams that get combined and sent to the ground. This allows for flexibility in how the available frequency is utilized by updating the waveform at the ground station, and the satellite’s have much less processing onboard, and weighs less, as this is done at the ground. Adding new technology or adapting is much easier as changing the ground station hardware is much easier than launching a new satellite. The downside is that since the satellite has no idea what the signal is, and therefore can’t process and forward the traffic across satellites to reach a ground station. For this reason, single hop communications where a mobile user can communicate with a mobile user directly without doing a double hop (user 1->satellite->ground station->satellite->user 2) is not possible. Also, the user must be covered by a satellite that a neighboring ground station is also connected to. In the case of a geostationary satellite network, the theoretical minimum is 3 ground stations for global coverage, and a LEO network would be 20+ ground stations and still some coverage missing in the oceans and polar regions.
An intelligent satellite that has signal processing onboard, can decode what is contained in the signal and perform some call routing and functionality onboard the satellite. This allows for single hop communications, cross linking of satellites to reach a ground station, and you can potentially operate an entire satellite network from a single ground station. The downside is that the signal technology is relatively fixed at the time of manufacturing as the call processors are launched onboard the satellite. Also, they weigh more by including more electronics and batteries onboard.
- Inmarsat uses basic satellites
- Iridium uses intelligent satellites
- GlobalStar uses basic satellites
- Thuraya uses basic satellites