Lets make this simple: (its all in the details of can you (antenna) see the satellite(s))
1. Inmarsat (ISatPhone antenna) must see the geostationary satellite
over the EQUATOR. Hence, your position and view to the "south" must be a clear line of sight - not blocked, else the connection is dropped or impossible.
2. Iridium uses Low Earth Orbiting (LEO) satellites which appear over the North horizon and travel southward disappearing over the south horizon - elapse time is about 3-5 minutes - the connection is then handed-off to any available Iridium satellite
in an attempt to keep it up and running. There in lies the challenge - with only 66 world satellites it is possible that the next satellite in view (to the antenna) will not arrive in time to make a transparent 'hand-off' and the connection will drop. The good news is the protocol supports break-restart so you can continue with SLOW speed data exchanges, i.e. text email
or GRIB weather
files etc. For voice comms you will need to re-dial.
So your choice is simple - can you ALWAYS see towards the 'south' (SE/SW) (check coverage map for Longitude for the Atlantic or Pacific or Indian Ocean
satellites, i.e. 15W, 31W, 65W and 182E) at a given elevation (Google Inmarsat elevation map). The further North you are the smaller the elevation angle above the horizon and the greater chance it will be blocked by objects such as buildings and mountains.
So the simple answer is Iridium offers the greatest and most flexible satcom connection solution be it on land or sea unless you are at a fixed location with an unrestricted view to the Inmarsat satellite over the equator. Outside mounted antennas offer the greatest and best connection chance.
Questions? Use Google
- its your friend.
BTW - I use Iridium on my boat
because of the many north/south worldwide cruising locations and in particular anchorages
which are typically protected and only overhead Iridium satellites are visible for satcom connections. If you require high-speed Internet
then Inmarsat Fleetbroadband is a great system - for a price