PLBs and SPOT Beacons

I’ve found it interesting to read about the search in the Waitewaewae catchment area of the Tararuas over the weekend. Summarised, a police officer was overdue when setting up a SaR exercise which had been intended for next weekend. He and his dog were reported overdue at 9pm, about 10 search teams went in the following day, a location signal was received at 2.40pm, and an Air Force Iroquois winched him and his dog out of dense bush shortly afterwards. It turned out that his intended route had been taking longer than estimated, being very wet and with lots of tree-fall. Despite being relieved to be lifted out, he’d be prepared to be stuck for longer.

What I find most interesting is the media’s reporting of his use of a “Spot-Me” device, although I think it’s far more likely that they meant to say he was using a SPOT Beacon. (Google tells me that a “Spot-Me” device would be a kind’a dumb thing to take into the Tararuas.) The technology in use is interesting because whilst PLBs and SPOT Beacons both report positions and can be used for locating a person, they work differently and are generally intended for different purposes.

Unfortunately the reporting’s been unclear and inconsistent about exactly what technology was in use here, but it seems clear that a SPOT Beacon was in use, although a Herald article from this morning specifically states that he also had a “Personal Locator Beacon”, which probably means a 406 MHz Personal Locater Beacon (PLB), the current standard for requesting a rescue. Some sources (such as that article) state that both beacons were activated on Friday night despite no signal being received until the following afternoon, whereas other sources (such as this article from the DomPost) state that the “Personal Locator Beacon” wasn’t activated until Saturday afternoon. [Edit: Thanks to Heather, there’s now a more complete and detailed description of the sequence of events in the comment thread below.]

Whatever actually happened, it’s a nifty excuse to discuss the differences between a SPOT Beacon, and a 406 MHz PLB. SPOT Beacons are sometimes touted as drop-in replacements for PLBs, but they’re really not. Despite having learned informally, I’m not an expert on the differences between these two devices, so if anyone notices any errors or omissions in what’s below, please feel welcome to leave a comment and I’ll do my best to correct things. (In this respect, thanks Craig.)

Until very recently it was normal to head into the bush without any emergency communications technology at all, except perhaps a mountain radio and for various reasons they were few and far between. It’s no less safe to do so than previously simply because new technology is now available. Certainly in New Zealand’s back-country, safety is the responsibility of those taking the risks. In this case it’s hard to know from limited reports for how long the officer would have survived on his own, but at the very least it sounds as if he was prepared to survive for longer had he not been rescued so quickly, even if not in comfort.

The revolution with improving and more portable emergency communications technology is firstly that it allows for emergency assistance to be requested immediately when there’s an unexpected situation that’s either urgent (such as a head injury or possible secondary drowning) or otherwise means a party can’t get out by themselves. It also reduces the amount of time, resources and expense required for search and response services to locate someone prior to a rescue. For better or worse, this technology also makes it faster and easier to locate and rescue people who are otherwise less prepared than they should be.

Despite having some similarities, SPOT Beacons and 406 MHz PLBs are different devices intended for different purposes. While a PLB is a device meant never to be used until an emergency arises, a SPOT Beacon is designed with an intent of leaving it switched on for lengthy periods of time to continually indicate one’s position to outside observers. To achieve this, a SPOT Beacon will regularly transmit position information (obtained via GPS) using the Globalstar Satphone network. The associated service then plots these positions, as received, and makes it available through a website for people to follow. SPOT Devices provide a function to send pre-programmed “I’m OK” messages via SMS, and a popular use of a SPOT Beacon is to reassure people at home of where you are at any particular time.

A SPOT Beacon is not primarily designed as an emergency device, however. The emergency function for a SPOT Beacon is more of an appendage rather than part of the fundamental design. An emergency SOS button can be used to request an emergency response, but this signal is not monitored directly or officially by New Zealand emergency services. It gets sent to a company in Texas which then takes responsibility for notifying emergency services in the appropriate country of your position, and that you’re in distress. Batteries are subject to running out as a consequence of needing to leave the device switched on for the more primary uses, even though this might depend on how a person chooses to use the device.

Probably the biggest difference of SPOT Beacons, in an emergency situation, is their method of transmitting via the Globalstar Satphone network. The Globalstar Satellites are low orbit satellites, and consequently they cross any given localised part of the sky relatively quickly and therefore work most reliably when a large part of the sky is visible. Furthermore, a SPOT Beacon requires an adequate GPS signal to know its position if that position is to be transmitted. If the sky is obscured by terrain or otherwise (like if you’re in a deep hole or valley), satellites might not be immediately visible either for GPS, or for receiving transmitted messages.

SPOT’s technique of only working in one direction (transmitting to the satellites without receiving) also makes it impossible for the beacon to know if a particular transmission has been received. Except for an emergency signal which is transmitted continually, a SPOT Beacon will transmit a position 3 times, then it stops trying. In certain parts of the world this works better than others, and (sadly for us) New Zealand tends to be near the edge of the reach of Globalstar satellites meaning that satellites are more likely to be nearer to the northern horizon and obscured by hills or mountains. One response to this post [from Craig] reckons that about 70% of SPOT Beacon positions tend to get through with typical use within New Zealand, compared with near 100% in Texas or California.

This is fine for the SPOT Beacon’s primary purpose of non-emergency situations, for which it’s reasonable to presume it unlikely that a person will remain in an obscured location for long, and it doesn’t matter too much if some transmissions aren’t received. If it’s been a while since a transmission managed to get through, however, and an emergency occurs, then it might not be practical to reach a location where the SPOT Beacon can either receive a suitable GPS fix, or adequately transmit a message.

PLBs, on the other hand, are designed to almost never be used, until there’s an emergency. As long as the device has been serviced properly, the battery shouldn’t be a problem when activated. A 406 MHz PLB will transmit to the Cospas-Sarsat satellite system, which includes both low-earth orbit and high-orbit geostationary satellites. The satellites are more visible and so it’s more likely that a signal will be received soon after activation. Unlike SPOT, if a PLB cannot obtain its position via GPS, the source of a 406MHz signal can still be derived over a period of hours by analysing the signal using a Doppler Shift technique.

The down-side of a 406 MHz PLB, compared with a SPOT Beacon, is that PLBs really are meant for emergency distress signals, and nothing else. They’re designed so a charged battery can be left inside for years at a time, knowing that it’ll work well if it’s ever needed. Then if and when something happens, you’ll have the best chance of getting a useful signal out, directly to the most relevant people and and irrespective of the availability of GPS. The device can’t be switched on for convenience beforehand to do other things, however, as is the case with a SPOT Beacon.

Generally if you activate a PLB (as with a SPOT SOS signal), you should expect to inconvenience many people and cost someone money. In New Zealand, it’ll nearly always be a combination of taxpayers’ money and often the voluntary time and resources of sponsor organisations and regular (but trained) people taking time off work. The good news is that you’re likely to be found and assisted from whatever bad predicament you’re in, within a matter of hours if weather allows. If it wasn’t activated it for a needless reason, the people involved will usually be happier to have helped out than not. Generally though, people are expected to prepare with the intent of not needing it for anything besides real emergencies, and that doesn’t include problems such as being delayed or inconvenienced behind swollen rivers, for instance.

In this particular instance the reports suggest that “It is not known why the technology signal failed”. Behind the scenes, if both devices really were activated on Friday night, it seems most likely that he was probably in a location without a clear enough view of the sky. Or perhaps the SPOT Beacon was activated initially with an I’m Okay signal and the PLB was only activated following day when it was clear that many people were out searching regardless. Maybe someone in the know who reads this can clear it up. Irrespective of the details, either of these devices has potential to succeed or fail, but a PLB is most likely to succeed in getting out an emergency signal quickly, or at all. It doesn’t make it a critical device to be carrying on any and every trip, but it’s always a good idea to consider what’s appropriate in the context of where one’s going, how many people are involved, conditions, and endless other circumstances that can affect things. Maybe you’d even be better off with a Mountain Radio.

Update, 1.44pm 9-May-2011: Shortly after posting this I noticed that this explanation has popped up on Stuff as of a couple of hours ago, courtesy of the Manawatu Standard. The Police Press Release about the incident also clarifies some things. It confirms that the officer was happily continuing (though possibly ‘happily’ is an overstatement), and presuming that the SPOT Beacon was transmitting the “I’m OK” signals based on the device’s instructions which indicated that only a clear sky was needed. It turns out they weren’t getting through, and he activated his PLB at 2.40pm on Saturday once he correctly inferred that overhead helicopters were searching for him, helping them to quickly find him and avoid a drawn out search effort. Given the search was already underway, it was a good idea to activate the PLB.

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