A variety of emergency locating and messaging devices are available for backcountry travelers as means of signaling for help. These devices include Mountain Locator Units (MLUs), Personal Locator Beacons (PLBs), GPS locating devices (ie. the SPOT Satellite Messenger), and cell phones and Smart Phones. There is, however, no one perfect device. Each has its own set of advantages and disadvantages, not to mention the fact that all electronic devices may fail and aren't a guarantee that rescuers will be able to reach a victim's location in time. These devices are no substitute for adequate preparation and at least one non-electronic backup plan should things go wrong.
The Mountain Locator Unit:
Oregon's Mount Hood is the only mountain in the world on which the MLU is currently used. The unit, which has a long range and long battery life, can be rented for about $5 at most Portland outdoor stores and from the Mount Hood Inn at Government Camp.
The Mountain Locator Unit was designed after a school group of two adults and seven children died on Mount Hood in 1986. The bodies of some of the group were found in a snow cave a day after searchers had passed within fifteen feet of their shelter without noticing them.
The MLU is easy to use but must be manually activated, and a call has to be placed to 911 in order for a rescue to be initiated, because no one is listening for the signal. There are only a few receivers in the state of Oregon, which are stored away. When someone calls 911, those who have the receivers will then activate them. But cell phone service is limited on Mt. Hood and varies widely among providers, not to mention that cell phone batteries don't last long in the cold. And an MLU won't be of any help if the victim is unconscious or unable to pull the cord and there is no one to activate it for them.
MLUs work with line of sight and provide a direction but not coordinates. MLUs do penetrate snow, however, so they will work in a snow cave, but the signal can bounce off of or be blocked by terrain.
Despite those limitations, the Mountain Locator Unit has saved lives on Mt. Hood. Here's a news video, including film taken by the rescuers when they were called out to search for missing climbers one February night during a blizzard with 60mph winds and white-out conditions.
The Personal Locator Beacon:
When activated, a Personal Locator Beacon sends a high-strength satellite signal, which gives law enforcement a broad location of the device. The PLB emits a homing signal, which rescuers follow with the use of a receiver to locate the victim. Newer models provide a complete set of GPS coordinates, and, unlike the MLU, simply activating a PLB initiates a rescue response without a call to 911. PLBs have a shorter battery life than MLUs, lasting 1 or 2 days at most once activated and must be sent back to the manufacturer for battery replacement. The units require an unobstructed view of the sky and registration with the appropriate national authority.
PLBs, such as the Revere McMurdo FastFindcan be purchased for retail prices of about $200 and up.
Here's an informative video about MLUs, PLBs and other signaling and GPS tracking devices, presented by the Clackamas County Sheriff's Office and Portland Mountain Rescue.
What is a PLB?
Mountain Locator Unit Can Help, But Doesn't Guarantee a Safe Return
The Use and Misuse of the SPOT Satellite Receiver