McMurdo Personal Locator Beacons are used for emitting a distress call with your location to emergency personnel via satellites. They are frequently used in small airplanes, sailboats, or by backwoods outdoor enthusiasts. There are newer similar devices that have advantages but these units have some distinct advantages. Namely, they do not require a monthly subscription. That coupled with their cheap resale price on secondary markets means cost shouldn’t be a factor in having this extra level of safety. However, there is a gotcha. The batteries for these devices are rated for 5 or 10 years and are proprietary. Instead, the company would prefer to have you return the product to them for battery replacement and testing. Even without the service, the battery module alone, if you can find one, may cost more than $200. There are many cases, maybe most, where this is the prudent course of action. But as a backup, maybe a cheaper alternative is acceptable.