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One of the arguments made against PLBs is that people will use them in non-emergencies and put rescuers in unnecessary peril. This definitely does happen from time to time as in this case in the Grand Canyon,

Once again, nightfall prevented a response by park helicopter, so an Arizona DPS helicopter whose crew utilized night vision goggles was brought in. They found that the members of the group were concerned about possible dehydration because the water they’d found tasted salty, but no actual emergency existed. The helicopter crew declined their request for a night evacuation but provided them with water before departing. On the following morning, another SPOT “help” activation came in from the group. This time they were flown out by park helicopter. All four refused medical assessment or treatment. The group’s leader had reportedly hiked once at the Grand Canyon; the other adult had no Grand Canyon and very little backpacking experience.

Source

Are there any statistics as to how often a PLB alarm is used frivolously as opposed to in a real emergency?

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    As against Spot units, real personal PLBs were deliberately manufactured as sealed units. Once you use them, you have to send them away for re-setting and it's not cheap - around USD 100 from memory. I would imagine that this was done deliberately to discourage frivolous calls, as part of the negotiation to make them legal for personal use. – Tullochgorum Aug 28 '17 at 21:45
  • @Tullochgorum, another reason is that the batteries must be replaced after an activation, so as to be sure they will transmit for the advertised 24 hours if used again. – Toby Speight Aug 31 '17 at 9:28
  • @Toby Speight - True - but they've made the unit sealed so you can't replace it yourself. – Tullochgorum Aug 31 '17 at 18:57
  • @Tullochgorum: We're agreeing, I think. The fact it's sealed means there's some QA on the replacement battery, and that the manufacturer can stand by the refurbishment. I don't know very much about the proprietary Spot system - is it intended to pass non-distress messages, too? (e.g. "Safely back - stand down"). If so, there would be a need for user maintenance of battery. I'm partly playing devil's advocate, as my own PLB will eventually need sending away when it reaches a certain age, and I'm sure I'll be gouged for that. – Toby Speight Sep 1 '17 at 7:39
  • @TobySpeight - yes, the Spot is primarily a tracker and check-in device, with SOS as a secondary function. The ones I've seen run off AAA batteries which are user replaceable. Not as powerful or reliable as a true PLB though it works well enough in some areas. – Tullochgorum Sep 2 '17 at 19:18
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In 2003, the International Maritime Organisation stated in 2003 that fewer than 1 in 20 alerts related to persons genuinely in distress:

1.4 False alerts in the IAMSAR Manual are defined as: Any alert received by the SAR system indicating an actual or potential distress situation, when no such situation actually exists.
1.5 Due to an increasing problem it was decided to start collection of data on the causes for false alerts.
1.6 Statistics from (M)RCCs show that the percentage of false alerts are approximately 95-100% of the total alerts received, mainly caused by lack of knowledge of the relevant conventions, codes and regulations.

The European Radiocommunications Committee gives a breakdown of root causes:

Mishandling: 40%
Beacon Malfunction: 10%
Mounting Failure: 6%
Environmental Conditions: 4%
Unknown: 40%

It doesn't include figures for intentional, genuine alerts, but does list some cause descriptions that mostly indicate accidental, rather than frivolous, activation.

A report by the US FCC in 1995 states that around 10% of activations were genuine - this may indicate an increase in false alerts over that period (this one is interesting reading, with some notable examples in the text - start at page 10, labelled "18").

An undated presentation from US Coast Guard and NOAA says

  • 96% of 406MHz EPIRB Alerts are false
  • 85% Resolved by RCCs with registration and good detective work

Note that many of the above incidents measured above relate to automatically-activated beacons carried on boats, rather than PLBs, so it may be invalid to suggest that the PLB false-activation rate is the same as for EPIRBs in general. For example, the last document above says that

69% Of False Alerts [are due to] Bracket Interface Failure

This clearly doesn't apply to PLBs.

A report that distinguishes PLBs from other EPIRBs is a January 2014 report by ICOA which says

Based on the data provided by Participants, Cospas-Sarsat calculates two false alert rates, identified for convenience as the “SAR false alert rate” and the “beacon false alert rate”. The SAR false alert rate, which characterises the impact of false alerts on SAR services, is the percentage of false alerts plus undetermined alerts (no person in distress fo und; no beacon found) over the total number of alerts transmitted to SAR authorities. [...] In 2012, the false alert rate was 96%, i.e. about one real alert in 25 alerts received.

In that year, the "beacon false alert rate" is listed at 0.4% for PLBs, compared to 0.9% for other EPIRBs and 4.9% for the old 121.5 MHz ELTs (i.e. one in 250 PLBs in the world transmitted a false alert; we can estimate that one in 5000 beacons was used in a real emergency in that year).

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