Sending messages without a GPS location.

Can I send a message even if the GPS location can't be acquired?

My new inReach mini seems to only send a message once it's got a GPS fix....as a default behaviour I think that's good.

However if I had an accident where a GPS fix couldn't be acquired, I would still want to be able to send messages so I could describe where I was. Is that possible?

0 twolpert over 6 years ago

I don't know. However, as a practical matter, if you are in a situation where you cannot acquire a GPS fix, your chances of successfully sending a message via the Iridium satellite network are very poor. In general, messages require a better view of the sky than a GPS fix.

0 geoffmozz over 6 years ago

Thanks.

I'm not surprised that's the case but I like to understand my kit and what it can and can't do.

Can you easily explain why communicating with the iridium satellite constellation is trickier than receiving GPS signals?....or point me at some techie info I can read?

I'm guessing the transmitter power on the inReach mini will be the weakest link. It is genuinely astonishing that an inReach mini can successfully send a signal to a speeding satellite nearly 800km above the earth. Clever.

0 twolpert over 6 years ago

Don't remember the technical details and don't have a link readily available. IIRC, the major difference is that the GPS constellation is designed so that multiple satellites (many more than the minimum needed to a acquire a fix) are visible at any given location/time. The receiver can use all visible satellites to acquire. The Iridium constellation is such that only 1 (I believe - this is the part I can't confirm without research) is visible at any given time. It transits horizon to horizon in about 15 minutes. This means that visibility in challenging conditions (for example, a slot canyon or bluffs blocking a significant portion of the sky) is subject to very short time frames.

0 CowboySlim over 6 years ago

I agree with the above. IIRC, 3 satellites are required to determine horizontal location and a 4th to determine elevation. After that, the more available will help define the location's position, both horizontally and vertically. OTOH, only one Iridium should be required to convey the message.

0 geoffmozz over 6 years ago

Thanks both for your input.

I've had a quick look through the inReach mini options and can't find any settings that force messages to be sent regardless of whether a GPS fix has been acquired or not.

My experience with the inReach mini so far has been good (a couple of days out in the hills, one good weather, one bad). Reliable GPS position acquisition with good trackpoint/message sending and internal logging. I hope that continues.

Always good to know what it can..... and can't do :-) Cheers.