First some good SPOT links:
Sequoia/Kings NP Advisory: SPOT emergency callsHow NOT to use SPOT: A newspaper
Article: SPOT Satellite Tracker. This includes the infamous Grand Canyon debacle.
I personally like my SPOT. I don't often go out on an adventure withOUT carrying it. It is not 100% fail safe all the time, but it DOES work. Pretty amazing for a little five-ounce thing, communicating with satellites hundreds of miles away.
SPOTS work best where the sky is completely open, with no obstructions, such as trees or canyon walls, or mountains. They work when there is a line-of-sight to the satellites they are sending to. So... understand that if you are in covered forest, your SPOT may not be able to determine its location, and it may not be able to get a signal out.
On the other hand, signals get out 1 to 2 out of three times when you are hiking. If you are in tracking mode, that means you will get a signal out every 20 or 30 minutes. And that is pretty good, actually.
If you want to be sure a signal gets out, such as at the end of a hiking day, you set it down, punch the OK/Check-in button, and leave it alone for at least half an hour. It sends a signal every 5 minutes, with the idea that at least one signal will reach a satellite. (There are times when there is no reachable satellite in view of the unit.)
The problems occur when people turn them on, punch the OK button, and then shut them off after it blinks that it has sent a signal. If no satellite was in view, no signal will be "seen". Or when people carry them on their belt -- signals will NOT get out. The SPOT II comes with a little case that I attach to my backpack shoulder strap, so it rides like a chip on my shoulder. I also use a second lanyard to secure it in case it breaks free -- that has happened! I am told it works inside a backpack as long as it is facing the sky and at the very top.
One last point: When you use your SPOT, try to designate a qualified person to try to keep an eye on your locations, and also inform them ahead of time, what any special signals might mean. The "Send Help" or Custom Message features have caused confusion in several tales I've seen.
I use mine for two primary reasons: one so that my family at home can see my progress, and be assured that all is well. Second, in the event of an emergency, I can use the 911 feature, and maybe be able to get out more quickly or safely.
I am confident the 911 calls would get out, but maybe just not in the first 5 minutes.
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Two SPOT stories from last June:
My son started the PCT from Canada last June, and he carried my SPOT. It came in handy several times, one to alert rangers in Washington, when his group bailed in white-out snow conditions, hiking down a side trail to a campground that had been closed for several years (unbeknown to the hikers). My call to the area ranger station got a ranger/sheriff up the closed road, and he hauled them out the last ten miles over the closed road. (Read more
here).
His friend and two others who started with him had a very difficult and slow time coming south, and bailed after 5 days. (Read the post
here) That fellow's SPOT was able to alert his wife where to go to pick him up. The biggest problem is that when he flagged down a jeep on the 4wd trail they were using for the exit, he turned his SPOT off! The lady tried driving a van up that 4wd road, while the hikers were safe in the nearest town. He had switched from SPOT to cell phone messages, but his wife was out of any cellular service area, stopping and calling only from pay phones along the road. By the way, this poor fellow ended up with a month-long recovery from painful trench foot.