The Iridium and the Inmarsat satellite system both boast global coverage however they both have completely different structures. The Iridium system works by operating 66 active satellites (with spares ready to replace any that fail) that are located in a Low Earth Orbit (LEO). The Inmarsat system is a series of only 3 satellites in a Geostationary orbit. At first glance you would think that the Iridium system would offer superior service however its not quiet that simple.
With the low earth orbit of the Iridium system the satellites are constantly moving and this can be an advantage and a disadvantage. It does help in getting a signal since you can be on a couple of different satellites at a time but it also means you dont know exactly where the satellites are, so in the real world when you find a clearing and try to use the phone you get a signal and start to make a call but 2 minutes into the call the satellite moves over the horizon and the other satellite is not visible so the call gets dropped. One of the major advantages though is that regardless of where you are in the world (including the north/south pole) you will still get great coverage.
With the Geosynchronous orbit that the Inmarsat system uses the satellites are considerably more powerful and because the satellite is higher you can still give global coverage. The advantage of this setup is that you know exactly where the satellite is so once you have a connection you could stay on the network all day. The disadvantage is that the further you get from the equator the lower the satellite is on the horizon so you need a clearer sky to get a full reception.
The best way to think of the different systems is to think of the worst case scenario. If you fall off a cliff and are sitting at the base with your satellite phone and need to call for help, the iridium will have the better chance of getting the call out but you may need to make several calls as the satellites move past, where as with the inmarsat phone you will need to move clear of the cliff to get the signal but once there you can keep talking until help arrives.
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