Why is GPS almost unavailable without data connection?

Question

When travelling to foreign countries, ie. when a data connection is unavailable because I don't want to spend big bucks using a roaming data connection, I noticed that GPS is virtually never available.

It can take hours for GPS to show my location, and even then, only when I'm located in open areas for a long time, with no buildings or trees. The only time GPS is available is when the phone is connected to the Net through wifi, which is pretty much useless.

I am using a Galaxy Nexus running CyanogenMod, and the phone has the FasterGPS utility installed.

Can GPS be available in cities without a data connection?

Answer 1

In theory, yes.

However, you must be aware of a few things:

1) GPS does not work well on high latitude (GLONASS is better in this case)

2) When you change location, the GPS needs to get information about the position of the satellites (so called almanach) for efficient use.

3) in cities, especially with tall buildings, the satellites are "hidden" by the buildings. Therefore you may not have enough "visible" satellites (4 are necessary) to compute a position.

4) The GPS satellites are moving and you don't have the same number all the time. Usually in open areas you always have enough satellites, but as I said you need more chance in "closed" areas. There are softwares to know the best moments (based on the almanach, the location and the field of view)

5) Eventually, I don't think that Galaxy Nexus has a good GPS sensor. The price of a GPS may range from 100 to 20000 $ and this obviously leads to a large difference in terms of correction for multiple travels (signal bouncing on a building), precision (15 m to a few cm...). You use a phone for locating yourself, with a GPS add on. The phone based location (using GSM towers) is therefore far better than what you get with GPS signal.

Answer 2

The Faster GPS tool depends on network connectivity.

So you will have no luck in using GPS without a connection. If possible, disable it to get offline GPS reception. That is, connect to the satellites (for free), and not to the mobile cell transmitters (for payment).

Or use a GPS receiver without mobile connections.

To get a first fix, select a place open to the sky, and wait there without moving, until you have a fix.

Answer 3

GPS is obviously available (almost) anywhere there is clear view of the sky.

If your GPS receiver doesn't have a good position though, or doesn't have valid almanac, it is going to take longer to get a fix. If the initial position estimate isn't valid, it can take a long time to find some valid satellites (because your phone is assuming that the satellites in view will be based on almanac and the last position fix).

There could be bad effects from multiple things trying to update your GPS (e.g. try without the NTP based time sync, which could well be a problem if you don't have NTP server access).

It is also possible that there are particular software / firmware bugs on your particular device (google search shows multiple issues), but that is not in GIS.SE scope.

Otherwise, be patient - 15 minutes isn't unreasonable for a first fix after a cold start.

Answer 4

While @BradHard's Answer raises several important points, I'll like to mention something which has been missed. In cities with tall buildings, and very little view of the sky, it is quite possible that your device will not see enough of GPS satellites with an enough spread to make a valid calculation.

You should try to take points where you have a clear view of the large parts of the sky, maybe from an open park, or the terrace of a building.