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Matching a satellite network to your needs
Lets start on your journey to find the ideal satellite network. Start by reviewing the "Mobile Satellite World" article Here.
Each network has it's unique personality so lets review them.
Inmarsat is a geostationary network and based on the distance to the satellite, requires a higher gain antenna as well as a larger antenna. Also, there has been limited outside M2M development and integration of Inmarsat technology because of the lack of a core module or integratable modem. The largest M2M integrator today is Skywave/Orbcomm who designed their own modems and technology to operate on the Inmarsat network. There are a few others as well such as Honeywell but this does limit available hardware and options.
Iridium is a low earth orbiting network that is quite close to earth and allows the use of small omnidirectional antennas and low latency. The satellites tend to be at an angle to the user, and rarely overhead as they are quite low. The satellites are always moving in relation to the user as well so blockage is less of a concern. They also offer a variety of embedded modems for integrators to design into hardware which means there is the largest amount of M2M hardware designed for Iridium.
Globalstar is also a low earth orbiting network, and operates in the same band and antenna characteristics as Iridium but with limited coverage. The key difference is that the satellites cannot cross link so each satellite must be within range of a groundstation. Also there are technical issues with transponders on the current network that does not allow the groundstation to transmit to units in the field, but the terminals can transmit to the groundstation to send positions.
Thuraya is a geostationary network that has doesn't quite cover the globe, but does cover Australia, Asia, Middle East, Europe, and Africa. They do offer modules for integration but there has not been a large development of hardware by the satellite community. Their module does operate on a smaller omnidirectional antenna and provides approx 60 kb/sec service so depending on your needs, may be a good alternative.
Now that you have a network or two in mind, lets narrow down those options more by your requirements.
Is a medium size tracking device (6" diameter) acceptable?
You can use any satellite network and the device costs are lower for the medium sized trackers than the small trackers.
Do you need a small (< 4" diameter) device or a covert/flat antenna?
Iridium is your only option for two way communication, or GlobalStar for simplex communication.
Does your application need high throughput or large packets?
Inmarsat or Thuraya are your options. Throughput is not often a requirement for M2M or tracking devices, but may be a limitation for SCADA applications.
Are you operating above or below 82 degrees latitude?
Iridium has good coverage, and GlobalStar might if you are within coverage of a groundstation. Inmarsat and Thuraya are not options as they are geostationary satellites that are shadowed by earth at that elevation.
Are you operating on land, within a few hundred miles of land, and only need one way transmission?
Have a look at the Globalstar coverage maps as you likely have coverage. There is little coverage in large ocean regions and polar regions though.
Is the functionality of the hardware the most important factor of your application?
Look at Iridium options, then Inmarsat, then GlobalStar.
Is pricing of the hardware or the airtime the most important factor?
Look at Globalstar, then Inmarsat, then Iridium options.
Enjoy your next satellite project!