Wow, this is already part 12 in my blog series on the Garmin InReach Mini 2 and the underlying Iridium satellite constellation. Perhaps you remember, my main use case for the Mini 2 is to have a means to communicate with family members in other countries should there be a longer network outage, be it local or more widespread. While the service works great during normal times, I wonder of course if and how quickly Iridium would get overloaded, should there be a widespread terrestrial network outage. So I made a number of assumptions to chase the answer, or to get at least a basic feeling for this.
Let’s say the InReach service could use the full capacity of the satellite network. This is obviously not the case but let’s assume this for the moment and get back to this at the end of my thought experiment. On Wikipedia, it is stated that one Iridium satellite supports 1100 concurrent phone calls at a data rate of 2400 bps. At any given time, there are (only) one or two Iridium satellites over Europe, so let’s assume there are two satellites and a wide area network outage. That means that 2200 phone calls can be made simultaneously over all of Europe. Multiplying the 2400 bps per phone calls with the 2200 possible concurrent calls, that’s 5,25 Mbps or 5280 kbps.
Now let’s say an active InReach device sends or receives 1 message per hour during such an outage. Let’s further say the message has a length of 160 bytes + signaling overhead. Let’s say this results in 1 kb of data that is used per message. Note that already at this point, I took a lot of assumptions and made a lot of simplifications, as I have no idea what the exact values are. At 5280 kbps, 528 devices could then send a message simultaneously per second. Multiply this by 60 seconds and 60 minutes and you end up with 2 million InReach devices that could send one message per hour over Europe (if they had the complete Iridium capacity over Europe for themselves). Another question: Could that many devices per second actually start a communication with 2 satellites or would this overwhelm the random access part of the channel?
Obviously not all InReach devices are used all the time. Mine for example is just used for a couple of hours or days a month and I guess most others as well, as I assume that many are only used for special or emergency communication. So the number of sold devices could perhaps be an order of magnitude or two more than the 2 million devices above. But, of course, Iridium is used for many other things like, phone calls, on planes for data backhaul, IoT sensors, etc. etc. And given all those other devices and applications, it’s really hard to estimate from the outside how much capacity they use today.
Let’s try another angle to estimate some numbers by estimating how many InReach devices might be out there in Europe. Let’s say about 500 million people live in Europe. If 1 in 1000 people had an InReach device, that would be 500.000 InReach devices in Europe, and I guess a major part of those would be used if networks failed in a larger part of the continent. Personally, my feeling is that this number is much too high. But be that as it may, at least the number of concurrent devices and the number of devices sold could be well within range of each other, so there’s a good chance the system would not be overloaded.
Coming back to the ‘use of other services’ which limits capacity for InReach. In a terrestrial network outage event, there will obviously be a lot of phone calls over Iridium, which would take a significant amount of the overall capacity. Also, I don’t know if the resources that the satellite assigns for phone calls could also be used for the the data service that is used by the InReach service. So there are many many question marks, and I can’t really answer the question. But at least the numbers do not make it impossible that the satellites would not be overloaded in such an emergency scenario.