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Posted by Bill S on November 7, 2007 at 6:16:02 pm EST
In reply to TracMe locator beacon posted by MTB416 on October 29, 2007 at 10:39:28 pm EDT

The SPOT product is a new one that utilizes the Navstar Global Positioning System (a project I spent a number of years working on as a system analyst and designer) to determine your location and then transmits it via a satcom system (Globalstar according to the website) that feeds it via Internet to people you designate. I looked at it briefly at the Summer Outdoor Retailer Show. Some things are not clear about its limitations - for example, GPS receivers are subject to canopy and canyon problems. That is, heavy vegetation canopy can block the signal from a receiver, and canyons not only block the signals (the radio waves do not travel through the rock of canyon walls and mountains), but also reflect signals to cause what is called multipath, which results in an erroneous position determination (sometimes off by kilometers, not just a few meters). The newest receivers, using SiRF-III and other high sensitivity chips can largely overcome the canopy problems, but not the canyon problems.

Globalstar has been around for quite a while, since shortly after Iridium (another project I worked on when I was in aerospace). Because of the lack of customer base, both Iridium and Globalstar have been operating under bankruptcy for a number of years. Iridium keeps going because of government contracts, while Globalstar keeps hobbling along with reduced capability (SPOT uses the simplex side of Globalstar, which is more reliable than the duplex side used for voice). Iridium is much more reliable as a satphone, and is available over the entire planet, while Globalstar often has 20 minute outages, and at best covers from 75 deg north to 75 deg south latitude. Still, you might consider a satellite phone. We used Iridium when I was in Antarctica last December, and I had considered Iridium or Globalstar for my upcoming Africa trip. However, at $1.99 a minute, plus the rental fee of a couple hundred dollars a month, I can't really justify it. The SPOT unit sends data (which is cheaper than voice over the satphones) and the basic unit is much cheaper than renting or buying a satphone.

There are other personal locater emergency systems as well, though that isn't clear from the website. Note that SPOT is $100/year for service, plus the $150 initial cost of the unit, plus more for some other special services. The thing that SPOT offers over the other emergency locater systems is that it transmits your GPS-determined location, while the others for the most part are like ELTs for aircraft and avalanche beacons for skiers - they depend on someone having a receiver and being skilled in search techniques for this method. The marine-style EPIRBs and the PLB (TracMe) mentioned in the first post in this thread transmit a signal that can be received by a system of satellites that estimate position from the doppler shift and its variation during the satellites orbit. (Actually, the TracMe is not a true PLB, in that it uses FRS radio, which is not monitored by the satellites, and is short range enough that a ground searcher has to be quite close to detect it, then requires a detailed grid search to actually locate). This takes an hour or so to figure out and the position is only good to a few miles, compared to the SPOT position which is instantaneous (the GPS-determined position) and is good to a few feet.

So each approach has its advantages and disadvantages. The best approach is don't get lost, don't get injured, and mostly don't depend on electronic widgets to bail you out - take responsibility for yourself and your party.

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