911 Calls

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!)?

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.

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.

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...")

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

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.

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

I believe that the US Coast Guard prefers to operate in degrees, minutes, and decimal minutes. Can anyone confirm?

Navy also.

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.

I have never seen a CDMA handset not provide GPS info to the PSAP as long as it was not in a bad position for a fix, including iPhones on the metroPCS and Sprint networks. I have no experience testing on Verizon. I was always advised from the beginning of the phase 1 hi res location rollout that the 911 location function was not able to be defeated in the handset through normal means. Considering the gyrations the carriers have to go through to insure proper configuration and continual verification of the 911 location system it wouldn't make sense, IMO, to allow handsets on the network that allow you to disable location on the system. It's kind of a big deal to carriers when a PSAP can't obtain location info from the handset in a 911 call, and if they get a few of them the places near me are usually initiating troubleshooting with the carrier and location service vendor.

There may be some handsets somewhere I have not seen that let you turn it off.

E911cannot be turned off. It is required by the fcc:

http://www.fcc.gov/guides/wireless-911-services:

I searched to see if there was a hack to turn it off, very unwise if you have an emergency and can't get help, and you can't short of using a tin foil hat.

I use a find my phone app that if you text a code word will return your gps coordinates and the street address for the police. I use it to send my last known address when I'm playing in the woods. If you have a signal to make a call you can text yourself first or during the call to get more detailed info of where you are. It is also important to download the google map of where you are traveling so the street address can be found if you lose data.

This is my 100th post!!! After over 10 years of reading the forum I'm now officially a "member" :-)
Mods where should I send my address for my free toaster?

I don't think that the FCC regulations state that -- it appears to me that the provider must have the capability.

The only modern phone I know much about is the AT&T iPhone, and I'm pretty confident that when Location Services is turned completely off that the GPS chip is turned completely off. If I'm right about that, iPhone users who do that would be left with the network's location-finding ability rather than the handset's onboard reporting of location.



Let me be clear that I do not advocate turning this feature off when calling 911!

"Mods where should I send my address for my free toaster?"
Just stand outside in shorts on a sunny day. As soon as enough satellites are clustered overhead, the toasting will commence. =^P

I explored this kind of thing with some local agencies. My chief concern was which horizontal datum they used.

Funny conversation I had with a medical transport helicopter pilot:

G-A-R: "What datum does your onboard GPS nav system use?"

pilot: "Degrees, minutes, fractions of minutes."

G-A-R: "OK, thanks..." <shakes head>

While land-based people often switch their GPS unit's horizontal datum between NAD27 (or other local datum) and WGS84 to jive with the paper maps they have, aviation and naval assets almost always stick with WGS84. Of course I am quite open to correction on this, I know we have some solid SMEs on the forum.

Regardless, I would always include the horizontal datum when reporting coordinates. If the people on the other end of the line have the knowhow to use that info, great; if they don't then it won't hurt them to hear the info.

Unfortunately with the ETS forum budget cuts we can only send out a cookie. The good news is that we've already delivered it to you. If you are using Firefox on Windows, look in the %APPDATA%\Mozilla\Firefox\Profiles\ directory, open your Firefox profile, and look inside the cookies.sqlite file. If you're using some other OS/browser combo, let me know which one and I'll help you find your cookie.

Those cookies don't taste very good.

Not 100% sure about at&t, but the GSM network I currently service uses TDOA, or automated differential time of arrival at 4 or 5 sites to determine location. Accuracy to 3 m or better, similar to GPS. Not sure if there's a technical reason that you can't use GPS over the GSM like you can on CDMA though. Considering that at&t handsets can be configured for the network I'm at I'd guess they might also use TDOA in which case GPS doesn't matter.

Anyone else know for sure how at&t does 911 location?

I would do it like the following example:

"My location is six one degrees, one one point six nine three minutes North, and one four nine degrees, five four point three five six minutes West, WGS84. Please read that back to me to confirm."

Speak slowly and clearly, and say each digit separately (say "six one", not "sixty one").

(That location (61° 11.693'N and 149° 54.356'W) is in the parking lot in front of the Anchorage REI store. )

Modern nautical charts are now in WGS84, and scaled in degrees, minutes, and tenths of minutes. I believe aviation charts are likewise.

Note that one minute of latitude equals one nautical mile.

I use an iphone app called "Basic GPS". It will give you a readout of your location in Lat Lon (several options), UTM and MGRS. It has both a text and email option. Sometimes when I do a spur of the moment hike by myself I will text my location to my wife from the trailhead (if I have cell reception). That way if I don't show up at home that night she at least knows what trail I started up. If SAR doesn't find me, she can at least recover the car!

How did you know oatmeal is my favorite? Thanks



I took your advice and my cookie is now toasted. Thank you too

The PSAP for my area does not - last time I was there - have the direct ability to handle cell phone delivered GPS coordinates. They use Google Maps (really) if they have coordinates available, but the CAD system simply can't handle that data input so (last I heard) it was ignored.This is a known issue and it's been over a year since I last asked about it, but it was definitely a problem, especially along the river which is the border between two states and cell sites pick up across borders.

I've got Verizon and with my handset, your options are to allow location services or 911 only.