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#263200 - 09/03/13 09:04 PM 911 Calls
hikermor Offline
Geezer in Chief
Geezer

Registered: 08/26/06
Posts: 7144
Loc: southern Cal
I recently took a first aid course in order to have some sort of current certification, and our instructor mentioned that, if making a 911 call, it was preferable to use a land line, rather than a cell phone, since the operator would have your address automatically displayed on the screen.

We didn't discuss this in detail, so I am curious...Is this true throughout the
USA, common in the developed world, or just about everywhere? Cell phones do have a sort of GPS capability (pinging from towers in the system)...How easily accessible is this information to first responders? or do they have to contact NSA first (just kidding, mods!)?
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#263204 - 09/03/13 10:18 PM Re: 911 Calls [Re: hikermor]
chaosmagnet Offline
Sheriff
Carpal Tunnel

Registered: 12/03/09
Posts: 3261
Loc: USA
In the USA, if you dial 911 from a landline phone, the Public Safety Answering Point (PSAP) will immediately have your address. For multi-tenant residential buildings they'll get the unit number. With few exceptions, in large commercial buildings they'll get a location (like "3rd floor, SE quadrant") or even a room number.

If your phone has GPS capabilities, they're turned on, and the phone has a good-enough view of the sky to get a fix, the PSAP will get the GPS location of your phone. If you haven't been using any features that use GPS, it's possible that later in the call the PSAP will get a better fix. Phones that use aGPS (like the iPhone) will provide the more precise location fix that aGPS provides if it's available. My understanding is that on the iPhone, if Location Services is turned completely off that navigation is off and this feature will not work for 911 calls. If Location Services is turned on at all (even if it's turned off for all apps that can use it) the location is provided to the network for 911 calls.

Without regard to whether Location Services is turned on the PSAP will obtain cell tower location data. If you're in range of only a single tower, they'll get the tower's location. If you're in range of three or more towers, they'll get an approximate phone location based on triangulation.

Two of my daughters are old enough to carry cellphones. I have trained them that when calling 911 the first words out of their mouths should be the location they are calling from.

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#263206 - 09/03/13 11:14 PM Re: 911 Calls [Re: hikermor]
WB2QGZ Offline
Newbie

Registered: 03/01/03
Posts: 25
Loc: FN30gq Long Island, NY
Cellular E911 uses handset GPS on most CDMA networks (which can not be disabled by the user) and on some GSM networks location is determined pretty quickly by differential time of arrival at a couple of nearby sites. If no GPS or TDOA info is available the PSAP gets cell tower location and bearing derived from the site and sector serving you.

Your location info is supposed to appear immediately at the turret position that gets your call, in CDMA the 911 operator can "re-bid" the phone manually to force it to resend location info while your connected to them.

I have heard instances where the 911 operator was relaying GPS coordinates to a SAR helo in decimal degrees (41.002343/-73.213456 for example) and the crew was inputting the location as Degrees-minutes-seconds (41 deg 32 min 45 sec/-73 deg 31 min 22 sec). Unless its up on a map display and you have everyone on the same page for coordinates this is another potential point of failure.

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#263214 - 09/04/13 01:54 AM Re: 911 Calls [Re: WB2QGZ]
AKSAR Offline
Old Hand

Registered: 08/31/11
Posts: 1156
Loc: Alaska
Originally Posted By: WB2QGZ
I have heard instances where the 911 operator was relaying GPS coordinates to a SAR helo in decimal degrees (41.002343/-73.213456 for example) and the crew was inputting the location as Degrees-minutes-seconds (41 deg 32 min 45 sec/-73 deg 31 min 22 sec). Unless its up on a map display and you have everyone on the same page for coordinates this is another potential point of failure.
GPS coordinates are always a potential source of confusion. Some possible errors include:

A digit gets garbled and misunderstood. (Depending on which digit, and which coordinate system you are using, this can throw your location off a little bit....or a long way.)
or
The coordinate system gets confused, or not is understood at all. (Some dispatchers may not understand UTM for example.)
or
The recieving end must convert coordinates and makes a simple arithmetic error.
or
Using different (wrong) datums. (For example you are using NAD27 and dispatch assumes you mean WGS84.)


Some things you can do to minimize these kinds of problems:

1. Make sure you know how to use your GPS! (Do you understand datums, coordinate systems etc? Do you know how to convert using your GPS?)

2. Find out (if possible) if there is a prefered coordinate system in your area. (Pilots often prefer Degrees, Minutes and Decimal Minutes. Since in Alaska SAR frequently involves aviation, this is our locally recommended default.)

3. Speak slowly and clearly, and always give complete information, including datum.

4. Always ask dispatch to read back your coordinates. (Make sure they have it right before you end the conversation!)

5. If possible give a geographic reference besides the GPS coordinates. This gives a way to cross check your GPS location. ("We are at Latitude blah blah, Longitude yada yada, WGS84, on the ridge of Sheep Mountain approximately two miles north of the peak...")
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"Toto, I've a feeling we're not in Kansas any more."
-Dorothy, in The Wizard of Oz

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#263216 - 09/04/13 02:38 AM Re: 911 Calls [Re: WB2QGZ]
chaosmagnet Offline
Sheriff
Carpal Tunnel

Registered: 12/03/09
Posts: 3261
Loc: USA
Originally Posted By: WB2QGZ
Cellular E911 uses handset GPS on most CDMA networks (which can not be disabled by the user)


Are you sure that's right? I believe that whether GPS can be disabled is determined by the handset, not the network. Some handsets do not permit disabling GPS, while others do.

73 de chaosmagnet

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#263218 - 09/04/13 02:50 AM Re: 911 Calls [Re: AKSAR]
hikermor Offline
Geezer in Chief
Geezer

Registered: 08/26/06
Posts: 7144
Loc: southern Cal
Back in the 90's, a hiker on Santa Cruz Island was caught by darkness and rain, bivvied up in a convenient rock shelter, and then dialed 911 for assistance. There must have been zero GPS protocols working then, because the operator relayed the call to the city of Santa Cruz, in northern California (only a few hundred miles distant).

Once we got the location straightened out, the Park sent a boat out and a dauntless party braved a dark and stormy night to reach the beleaguered hiker. They were glad to warm themselves at the fire he had built in the meantime.
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#263219 - 09/04/13 03:26 AM Re: 911 Calls [Re: hikermor]
Pete Offline
Veteran

Registered: 02/20/09
Posts: 1372
darn good topic

AKSAR - let's talk.
My GPS puts out degrees, minutes, decimal minutes.
I've got no problem passing that on to a dispatcher. but what words exactly do I say to them ...

"here are my coordinates.
I'm giving you degrees, minutes, and decimal minutes.
do you understand over?"

WILL this conversation make sense to them?
Or do I describe the system some other way??

Pete2

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#263223 - 09/04/13 03:59 AM Re: 911 Calls [Re: Pete]
hikermor Offline
Geezer in Chief
Geezer

Registered: 08/26/06
Posts: 7144
Loc: southern Cal
I believe that the US Coast Guard prefers to operate in degrees, minutes, and decimal minutes. Can anyone confirm?
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Geezer in Chief

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#263227 - 09/04/13 04:45 AM Re: 911 Calls [Re: hikermor]
Russ Offline
Geezer

Registered: 06/02/06
Posts: 5264
Loc: SOCAL
Navy also.
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Better is the Enemy of Good Enough.
Okay, what’s your point??

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#263228 - 09/04/13 04:50 AM Re: 911 Calls [Re: hikermor]
Roarmeister Offline
Old Hand

Registered: 09/12/01
Posts: 931
Loc: Saskatchewan, Canada
Originally Posted By: hikermor
I believe that the US Coast Guard prefers to operate in degrees, minutes, and decimal minutes. Can anyone confirm?


You can also set the GPSr to output MGRS coordinates, would that be better or worse than D-M-S coordinates? I agree with the above that the caller should also provide the datum system as well, that way the SAR team can set their equipment to the same.

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