http://www.ledauphine.com/haute-savoie/2012/02/16/avalanches-votre-iphone-pourrait-bientot-vous-sauver-la-vie reports an IPhone App that is currently being field tested in collaboration with the PGHM.

My rough translation of the important bit is

Operation is simple: once the application is downloaded, the iPhone works independently. Each time the skier stops, it analyzes the course before the stop. If it corresponds to an avalanche or a fall (whose characteristics will be pre-registered), it automatically sends an SOS message. Ideal if the victim is unconscious, when we know that in case of avalanche, the first fifteen minutes are vital. If conscious, it can also manually trigger the alert. This product is intended primarily for off-piste skiers.
“With my application, even if the skier does not know where he is, his phone does. And finally the language barrier: the laptop directly transmits the GPS coordinates to the rescue services. “
If the phone has no signal from a base station, The iPhone switches to Bluetooth and sends an SOS to all iPhones equipped with the application within a radius of 100 meters.

There are quotes from the PGHM underlining the obvious importance of achieving an extremely low false alarm rate (they ask for zero) and some other tricky areas. The Bluetooth mode sounds easier to implement robustly. The article suggests the app could be available in a month, but I suspect that more time will be neeeded for field testing and algorithm refinement. 

I don’t know how accurate GPS in an iPhone is: the Wikipedia article http://en.wikipedia.org/wiki/Error_analysis_for_the_Global_Positioning_System
says 5m. If this is right, a GPS location would not replace a transceiver, but might speed up the initial search.

If the iPhone uses satellite-based GPS, how does it get line-of-sight to any satellites if it’s in your coat pocket under a few layers of warm clothing to begin with; under 2 metres of snow to end up with?

If the iPhone uses masts (the maths is similar, but slightly more complex than triangulation) to calculate a location, is 5 metres (as suggested by the link) still a valid estimate?  And are there going to be enough masts with a reachable signal to to the maths (three I’m guessing)?

We are thinking of trialling a similar system on housing estates (north faces especially avalanche-prone*).  Will let you all know how it goes…

* in fact, to protect staff who make home visits.

The Apple website says that the iPhone has both GPS and the Russian equivalent (GLONASS) as well as location by “3G”, which presumably means by reference to base stations.
GPS signals are around 1.5GHz. Snow is transparent at these frequencies (The harmonic received by Recco is at 1.83GHz, not very far away). Even the thickest clothing is also transparent at these frequencies. A quick look suggests that 3G frequencies are similar. 

If the iPhone is as clever as I think it probably is, it may use all these methods simultaneously to provide a location, which would have a significant impact on reducing the error. In addition, I realised after my last post that many of the errors in GPS would be common with other nearby receivers (i.e. those being used by searchers) so the relative accuracy should be a good deal better than I suggested.

It would be easy to find out experimentally what the accuracy is if someone can round up a couple of iPhones.

I actually patented pretty much the same idea about 12 years ago in the days before Bluetooth and long before iPhones. The patent is now lapsed, but the idea was the same; to re-transmit GPS co-ordinates of a buried skier in such a way that they could be received by rescuers who could then locate them pretty much immediately.

I did a load of experiments in Val d’Isere digging some very deep holes! Yes, GPS does still work under at least 9 feet of snow, and although the *absolute* accuracy of GPS is not that good (back then in the days of Selective Availability it was even worse) the *relative* accuracy is very good. Provided the victim and rescuers can see the same set of satellites, yes, it is possible to get accuracies of around 30cm. In my scheme I re-transmitted the GPS info by modulating the standard 457kHz avalanche tranceiver frequency so that a “GPS enable” tranceiver could still be located by and standard tranceiver.

Beginning to wish I hadn’t let the patent lapse, but at the time commercial version wasn’t practical (GPS power consumption, battery life etc). So good luck to the App. I might even be tempted to buy an iPhone!

Peter

Don’t forget Pieps are giving GPS a go as well :

http://www.pieps.com/en/avalanche-transceivers/vector

& a (slightly confused) review here ;

http://www.earnyourturns.com/959/pieps-vector-includes-gps/

This sounds like the “Buddy Beacon” option on ViewRanger a mapping application available for smart phones. I hadn’t realised that gps worked under 9 feet of snow. ViewRanger would also require a phone data connection as well. The main difference is that the Buddy Beacon option needs to be switched on manually.

RichardH - 17 February 2012 04:13 PM

GPS signals are around 1.5GHz. Snow is transparent at these frequencies (The harmonic received by Recco is at 1.83GHz, not very far away). Even the thickest clothing is also transparent at these frequencies. A quick look suggests that 3G frequencies are similar. 

Interesting. GPS seems to have difficulty under trees so I’d like to see some tests. I’ve read a couple of papers tonight I found through google which suggests GPS signals can penetrate snow. This paper was the most readable on the subject

http://www.insidegnss.com/auto/SepOct07-GNSS-Solutions.pdf

is the iPhone GPS as accurate / sensitive as a recreational unit? How do iPhones cope with the cold?

ise - 17 February 2012 06:42 PM

Don’t forget Pieps are giving GPS a go as well :

http://www.pieps.com/en/avalanche-transceivers/vector

& a (slightly confused) review here ;

http://www.earnyourturns.com/959/pieps-vector-includes-gps/

I must admit I didn’t understand a word of what Craig Dostie was saying but he doesn’t seem to understand how GPS works - that is a receiver using timing differences to calculate a location, it doesn’t transmit the location to a satellite 50 miles above the earth. Erm earth to Dosie, the GPS satellites are at around 20,000 km altitude. In the case of SPOT type emergency beacons, they retransmit decoded GPS locations via another signal, either to satellites or other radio receivers.

You could envisage an avalanche beacon that does the same thing over a LINK type signal (used by the Pulse and Arva beacons) to beacons in proximity. Thus GPS could be used to give you a rough position and the 457khz signal for a fine search. Taking bobskis figures, a 5 meter accuracy is going to leave you with a big hole to dig! Or a lot of probing.

I see the Vector has 4 antennas, we seem to be going down the Gillette route at the moment with avalanche beacons. Reading the link Ise posted it says GPS is used for the initial search phase to keep rescuers on track in their search strips and also to give a topomap view of the search area. If someone can explain “vector triangulation” more clearly than Dostie I would be interested.

davidof - 20 February 2012 12:13 AM

ise - 17 February 2012 06:42 PM

Don’t forget Pieps are giving GPS a go as well :

http://www.pieps.com/en/avalanche-transceivers/vector

& a (slightly confused) review here ;

http://www.earnyourturns.com/959/pieps-vector-includes-gps/

I must admit I didn’t understand a word of what Craig Dostie was saying but he doesn’t seem to understand how GPS works - that is a receiver using timing differences to calculate a location, it doesn’t transmit the location to a satellite 50 miles above the earth. Erm earth to Dosie, the GPS satellites are at around 20,000 km altitude. In the case of SPOT type emergency beacons, they retransmit decoded GPS locations via another signal, either to satellites or other radio receivers.

I was more thinking of his expectations of transceivers and the range. There’s some physics about transceivers being a near field application which define the range of devices, you’re not going to get a transceiver at 457 that works at 250m or from a helicopter. So there’s a theoretical maximum range and there’s a practical limit on antenna size, optimum size is half a wavelength (λ/2) which is 327 meters at 457 kHz. So the devices we have work really well.

davidof - 20 February 2012 12:13 AM

You could envisage an avalanche beacon that does the same thing over a LINK type signal (used by the Pulse and Arva beacons) to beacons in proximity. Thus GPS could be used to give you a rough position and the 457khz signal for a fine search. Taking bobskis figures, a 5 meter accuracy is going to leave you with a big hole to dig! Or a lot of probing.

I see the Vector has 4 antennas, we seem to be going down the Gillette route at the moment with avalanche beacons. Reading the link Ise posted it says GPS is used for the initial search phase to keep rescuers on track in their search strips and also to give a topomap view of the search area. If someone can explain “vector triangulation” more clearly than Dostie I would be interested.

There would be an interesting dividend with data collected form Vectors in terms of understanding how searches are performed.

BCA, amongst many great papers, published something about this :

457 KHz ELECTROMAGNETISM AND THE FUTURE OF AVALANCHE TRANSCEIVERS, Avalanche Gazette, Vol I, Issue 3

https://s3.amazonaws.com/BackcountryAccess/content/papers/457andFuture_000.pdf

davidof - 20 February 2012 12:00 AM

is the iPhone GPS as accurate / sensitive as a recreational unit? How do iPhones cope with the cold?

Not far off nowadays, the small GPS chipsets were pretty awful, you’ll recall all the skier reckoning they were doing 120 kph on the green run back to La Plagne. But they’re pretty reasonable now, they probably perform as well as the etrex vista or forerunner I have but not quite as good as the Oregon 550t.

Phones just eat batteries though, I’d not rely on one being turned on.

The SPOT is a little different from most 406 MHz beacons, the latter generically called EPIRB’s in the trade, in that the SPOT uses the satellite phone network where the 406 MHz beacons use GEOSAR optionally encoding GPS or GLONAS coordinates with a unique device id. The practical difference is that a lot of 406 MHz beacons don’t use GPS data and the initial GEOSAR location is fairly coarse, location being done by receivers with local rescue resources. Far field applications not requiring sub 100m accuracy basically.

The SPOT is a very affordable device though, under 200€ with a strong footprint in the areas we’re all skiing in. I’ve not seen any case of use after an avalanche yet though.

The Dauphiné Libérée has recently reported the next installment in this story. http://www.ledauphine.com/haute-savoie/2012/04/05/application-iphone-pour-avalanches-le-site-en-ligne . The app is available to download as a beta test version at http://www.nextinov.com/isis/ .

Ise gave us a reference to the BCA article

BCA, amongst many great papers, published something about this :
457 KHz ELECTROMAGNETISM AND THE FUTURE OF AVALANCHE TRANSCEIVERS, Avalanche Gazette, Vol I, Issue 3
https://s3.amazonaws.com/BackcountryAccess/content/papers/457andFuture_000.pdf

David gave us a reference to a scholarly article

http://www.insidegnss.com/auto/SepOct07-GNSS-Solutions.pdf

which quantifies and supports my rather wooly statement that snow is transparent to GPS signals. This, and Peter S’s experiments, refute the statement in the BCA article that “[GPS signals] are severely attenuated or limited by snow”.

The appendix of the BCA article is standard undergraduate physics, but is unnecessarily complicated to describe how a transceiver works for a wider audience. To my mind, the references to radiated power and the Poynting vector (although technically correct) are positively misleading: an avalanche transceiver detects the quasistatic magnetic field. The radiated field is irrelevant, except that any power that is radiated by the transmitting antenna is an undesirable drain on the battery.

So what are the relative merits of GPS compared to avalanche transceivers?

Peter S says that the relative accuracy of GPS is around 30cm. I guess this is comparable to the accuracy of a transceiver for a burial depth of about 1 meter, so at this superficial level GPS might replace a transceiver for positional accuracy. However, one would need to be a lot more precise than this (a small factor of higher accuracy for one method over the other would be a decisive advantage), which would need careful experiments and analysis.

There are two clear advantages of GPS that I can see:

(1) It tells you where the burial is, and a straight line to get there (no curved flux lines, and no ambiguity about the best direction)
(2) It can give you a true map of all the burials in a multiple burial scenario, without the need to mark. This offers the potential for optimum deployment of manpower in a really serious situation.

Against these, Ise pointed out that GPS is power hungry, so you probably need to recharge your battery daily, with the obvious risk that you’ll forget. Also David has pointed out that trees and canyons can be a problem for GPS, so I will be hanging on to my transceiver for a while yet (anyway, my phone is old fashioned, like me: all it does is make phone calls )

I learnt a little bit about GPS today. Great info guys