Using mobile phones as location beacons

[mujambee]

As there has been some discussion on how locating a phone works on Nasty's thread Follow up with the Ambulance Comms Centre, I've decided to split it, just to avoid cluttering that...


A cell phone is usually locked to one cell, but it has some soft link with all other cells around it, to check the signal strength it gets from each of them. That strengt is registered by the network, and it is used to decide when a phone is transfered from one cell to the next (notice that it is not your phone's decision but the network's). You phone is "talking" even with other network's cells, not ony with your own provider's, but connections to those cells are usually rejected (see below).

Cells have a catchment area (what is properly "the cell") that is wedge shaped, with the vertex of the wedge in the antenna; and overlap various other cells, so a mobile phone is usually inside several cells, and talking to all of them.

Now we know that our cell phone is talking to several cells, and that those cells can be drawn in a map. You just need to draw all those cells your phone is connected to, and see what area do all of them intersect on. You can tell for sure your mobile phone is inside that area.

The technology exists to locate any cell phone; in Spain it is freely and instantly available to Movistar users, so it is obvious it's not very expensive or overly complicated to do. If a tech retarded country as Spain has it, surely it is not rocket science.


There is also technology to locate cell phones "on ground" (don't know much about it). Any ambulance could be equipped with a phone finder, so if you crash and go off-road they sould be able to home on you even if you are not on plain sight.


The fact that your phone is talking to all cells in range means that not only your provider can track your phone, but their competitors can also do it. In an ideal world, a central emergency agency would be able to access that info from all networks, and use it to locate your phone much more acurately than using only your provider's.

Here in Spain cells reject connection attempts from phones that are not on thein network; but will service an emergency call from every phone, even if that phone is stolen, has been locked or is on another provider. That means that it is feasible to use competitor's networks.

[awayatc]

Don't crash in the south island then......
your phone will be talking to itself only most of the time...

Signal strength.....
What signal?...

[Radar]

Thanks for starting this thread Mujambee. On the 'ambulance follow up' thread you state: "my wife can track my phone by just sending an SMS and it takes seconds... " - ASFAIK this is not available here in NZ. Anyone know?

See also my thread about Emergency Locator Beacons.

[Jantar]

Yes, the technology does exist to triangulate cell phone signals and hence derive a location. The accuracy is down to a single city block where there are 4 or more cell phone towers within the signal reach of the phone. Its around 1km radius where there are 3 towers, and triangulation is not possible where there are less than 3 towers.

Thats great if you happen to be in a large city, but in most smaller cities in NZ that brings you down to only 2 or 3 towers within signal reach of a cell phone. In rural towns you will be down to 1 or 2, and that isn't sufficient to give even a hint of a location. Get out into the back country areas where location is most needed and usually there is no coverage at all.

The thing when comparing Spain and New Zealand is that Spain has a land area of double New Zealand's, but has 10 times the population. However the border length of both countries is almost the same, with New Zealand having a longer border length than Spain. New Zealand simply doesn't have the population density to justify full cell phone coverage, and even if we did we would need far more cell phone towers than Spain in order to cover the country. It isn't just a matter of technology and software, its also one of geography.

[Okey Dokey]

Yup, a cell phone is NOT an emergency beacon and people who rely on them as such are going to come seriously unstuck. Each year I read of both boaties and trampers who try to use cellphones when they get into difficulties. Not a good idea.

[CookMySock]

Reminder that many modern cell phones have true GPS built in.

Steve

[Radar]

Reminder that many modern cell phones have true GPS built in.

Steve

Does the GPS only work with a cellphone signal? In other words, do cellphones use satellites or just the cellphone network?

[Jantar]

The few that I have seen are true GPS. ie They calculate the position from satelites, but still require a regular cell site to communicate. And they are not cheap. an ELB would still be more practical as an emergency device.

[CookMySock]

Does the GPS only work with a cellphone signal? In other words, do cellphones use satellites or just the cellphone network?

I have not been in a position to try this. (always in coverage.) If the phone has a GPS reciever chip, then it will do GPS, otherwise not.

The unit does spend some minutes acquiring satellites one by one, and it has a satellite status menu. It is also daaaamn accurate in all respects, including altitude, speed, the lot. Also cellphone coverage here is not flash, so it would be physically impossible for it to be this accurate using a triangulation method. (not that many cell sites here - semi rural.)

It's my sons phone. It was a cheapie. I do not know if the cellular operator (telecon) can remotely activate its GPS, but I would suggest that they could. There are also Java applications you can download to the phone that will connect back to the internet for mapping etc, but these chew batteries real fast.

Steve

[MarkH]

Does the GPS only work with a cellphone signal? In other words, do cellphones use satellites or just the cellphone network?

I have a Nokia N95 (almost 2 years old) which has true GPS through its built in GPS antenna. I would still need coverage or access to a working phone to let an ambulance know help was needed and where to go.

It does occur to me that even if you are in range of only one cell tower it would be good if 111 comms could see which tower you were calling through as this would eliminate some silly mistakes (e.g. Palmerston vs Palmerston Nth). Even if you were hard to hear or incoherent they would know the overall area you were talking about and if you could give them just the name of the road there may be only one or two roads with that name that were in range of the cell tower you were connected to. If you were in range of 2 towers that would narrow down the area you could be in even more. 3 or more towers and they could actually triangulate your position.

[mujambee]

Yes, the technology does exist to triangulate cell phone signals and hence derive a location. The accuracy is down to a single city block where there are 4 or more cell phone towers within the signal reach of the phone. Its around 1km radius where there are 3 towers, and triangulation is not possible where there are less than 3 towers.

Seems like I didn't explain well. I'm not talking about triangulation here, but about what cells you are in.

I've attached a crude sketch.

Cell A is the area inside the red lines, B the blue ones and C the greens.

If your cell phone is talking to all three cells, it must be in the cyan area.

If it is only talking to cells A and B, it must be somewhere within the yellow lines. Note that the fact that you are not talking to cell C can not discard the cyan area, as you could be hidden behind a rock that prevents your cell from contacting that antenna.

If you overlay that section with a road map, it can be relatively easy to locate the phone.

Thats great if you happen to be in a large city, but in most smaller cities in NZ that brings you down to only 2 or 3 towers within signal reach of a cell phone. In rural towns you will be down to 1 or 2, and that isn't sufficient to give even a hint of a location. Get out into the back country areas where location is most needed and usually there is no coverage at all.

The thing when comparing Spain and New Zealand is that Spain has a land area of double New Zealand's, but has 10 times the population. However the border length of both countries is almost the same, with New Zealand having a longer border length than Spain. New Zealand simply doesn't have the population density to justify full cell phone coverage, and even if we did we would need far more cell phone towers than Spain in order to cover the country. It isn't just a matter of technology and software, its also one of geography.

Mmmmm. Central Spain is just a big mesa with huge flatlands. There are a few cells that cover enormous areas, so cell tracking is not that useful there (last year a man died because of this. He dialed 112 but as he had a broken jaw he could not tell where he was; he was in the middle of the plains, so the area to sweep was so vast that they couldn't arrive in time).

Then again, there are huge montain ranges like those on the South Island, and there is little cell coverage there. (Google "Picos de europa" and you'll see where I grew up).

Yup, a cell phone is NOT an emergency beacon and people who rely on them as such are going to come seriously unstuck. Each year I read of both boaties and trampers who try to use cellphones when they get into difficulties. Not a good idea.

Anyhow, most people carry a mobile phone today, but few carry a beacon. I'm not proposing that people use their phones as beacons, but that emergency services should be prepared to use phones to aid on rescue operations.

[enigma51]

If any one wants a read of the nokia or siemens gsm spec/training material you can bring a couple of beers over to my place and have a read.

I realy cant be asked explaining on here how it all works. The bottom line is that what you see on tv you should not always believe. Using your phone as Beacon is as dumb as spear fishing with a crossbow. You might get lucky.

I have a spec for GPS. but will need to look for it first. I think i borrowed it to someone at work. Again having GPS in your phone only helps if you can phone someone and tell them you coordinates. The SMS thing works on the by asking your phone where it is by a "smart tag" sms. It then responds with the coordinates which (depending on your software) converts it into city street and number.

GPS dont have forwards and backwards talking (Well not completely true but for this exercise think of it as one way) You want a beacon go to a shop and buy a emergency beacon.

[mujambee]

Thanks for starting this thread Mujambee. On the 'ambulance follow up' thread you state: "my wife can track my phone by just sending an SMS and it takes seconds... " - ASFAIK this is not available here in NZ. Anyone know?

It has a trip mode in which you give a start and end time. During that period, it is constantly updating your position to a monitor phone. I usually set it up when I go on a lone ride (quite often). If my wife receives two messages with the same location she phones me, and if I didn't answer then she would start thinghs up (never happened).

See also my thread about Emergency Locator Beacons.

I voted yes on the 525$ (that's about 200€). I'll try to locate those things here; I know they are quite common in Switzerland, many people carry one to help save them after avalanches.

[Wingnut]

When in need of emergency services and reception is a little dodgey. Dial 112 and it will scan for any available network telecom - vodafone whatever is available. Just something I learned the other day that may interest some. Out.

[Radar]

When in need of emergency services and reception is a little dodgey. Dial 112 and it will scan for any available network telecom - vodafone whatever is available. Just something I learned the other day that may interest some. Out.

Really?

If true, this is vital information, something everyone should know.

I guess someone will have to ask the emergency call centre, or Vodafone/Telecom.