I can give you the information I've gathered so far, but I cannot verify it. Hopefully this will move the discussion along and others can confirm or correct what I'm about to write...
The SPOT device uses the Globalstar satellite constellation. Globalstar has had a lot of problems over the past several years and their voice coverage kept dropping to a point where if you wanted to make a call, you usually had to wait up to an hour for a satellite to come into view, and then you'd only get a few minutes of call time before the satellite passed out of view. They recently successfully launched 6 new satellites and those should be operational by summer 2011 and more launches are planned.
As of March 2011, for most of North America it appears that you will typically have to wait about 20 to 30 minutes for a satellite to come into view, and then you'll have about 13 minutes or less to make your voice call. You can see how this works by going to
http://calltimes.globalstar.com/ Globalstar has had so many voice customers quit the service that they are currently offering a $20 a month unlimited voice plan with one year commitment that is good through the end of March 2011, in order to keep customers. Not a bad deal at all compared to Iridium and especially if you don't need to make instant calls and can wait 20 to 30 minutes for a signal. As a comparison, if you turn on your Iridium phone, which uses Iridium's constellation, you will almost instantly acquire a signal and be able to make long calls immediately. This helps explain why Iridium is twice the cost of Globalstar for the phone, and 100 times or more expensive for the usage minutes. So Iridium is expensive, but for a good reason.
Here's where it gets tricky - Globalstar has both duplex and simplex transmission. Globalstar sat phones are duplex (two way) and that is what I described in the paragraph above. The SPOT device uses simplex transmission (one way) so theoretically it should have better coverage than voice. It's the same satellite, but the SPOT only uses the simplex side of the satellite.
Simple deduction, and using Globalstar's own calltimes tool, would imply that the worst case scenario for a SPOT transmission would be that it would take a maximum of 20 to 30 minutes to acquire a signal and then transmit a message. That's as of March 2011 and should, according to Globalstar, improve by the summer of 2011 and get even better with the next satellite launch.
Globalstar publishes maps of both simplex and duplex coverage and the simplex coverage is much more dense, especially in outlying areas like Alaska and Canada. This would imply that it is easier to acquire a satellite for simplex than it is for duplex.
http://www.globalstar.com/en/index.php?cid=101&sidenav=85
What I described above - the 20 to 30 minute wait times to acquire a signal, matches some of the implied performance that is written into the SPOT user manual, however I haven't found any hard data from SPOT. The most I've found is a press release saying "There is a proven 99.4% reliability with the SPOT system. They process over 6 million messages a month or approximately 2.3 messages per second." This same press release says the AA lithium batteries have a 12 month standby capability. The press release is a couple years old so the 99.4% number may be inaccurate and is most likely low since the successful launch of six satellites in October of 2010.
Again, most of what I've written above is anecdotal and hopefully we'll get some experts on here to confirm or refute all of this.
