In the comments to my last post, a question was asked about my experience with the SPOT Satellite Messenger, and whether or not it was hampered by a canopy of trees overhead.
That is a great question. Every type of satellite communication device, whether it be a GPS a Personal Locator Beacon, or the SPOT Satellite Messenger depends to a certain degree on having a clear shot at the open sky. These are not high-powered devices, so the signal can be interrupted by overhead obstructions.
That said, I have successfully used my SPOT in deep forest as well as on open terrain. With a normal PLB, the only way to verify that the unit worked is to listen for the sound of helicopters coming to rescue you. But the SPOT works differently, having a couple modes of operation other than just calling for the rescue team. You can send a "check in" message that allows your team of people back home to follow your progress on a Google Map. Or you can send a "I need help, but don't call the rescue team" message in the event you have a non-life threatening emergency such as a flat tire, or other minor mishap. That message gives your GPS coordinates to your team who can then come and give you a hand. Of course, the final recourse, when you life is at stake, is to press the button that calls in the helicopter rescue teams.
So far (knock on wood), I haven't needed to call for rescue. But I have used the other functions.
Keep in mind that my unit is a first edition — an old model that has since been updated by the company. The newer units are more powerful, smaller and lighter. I paid $149 for mine and maintain a $99 per year subscription to the service. The alternative is to buy a PLB that costs several times that much but requires no subscription fee ever after.
One other difference between my SPOT and a PLB is that I can change batteries myself, while PLB owners need to send their units in for service.
I hope that helps answer the question. I encourage you to do more research. The websites to check out are: www.findmespot.com for the SPOT Satellite Messenger, www.acrelectronics.com and www.mcmurdo.co.uk for PLB information.
No matter which unit you buy, it is money well spent. Hope you never need to use it, but the peace of mind alone for you and your loved ones will be a huge benefit.