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Since I checked this option in Settings > Personal > Location, GPS finally works when no data connection is available, eg. when travelling in foreign countries. It's still slower than when a data connection is available, but at least, it does locate me when I'm standing in open skies.

However, I was wondering what this option really means: Does it mean that it will never use A-GPS to download data from the Net even when a data connection is available through GSM *?

IOW, if I'm travelling in my country where a GSM data connection is available, will Android download A-GPS data to speed things up?

Thank you.

  • GSM, as opposed to wifi; I don't know a better term
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The "Wifi" option means that A-GPS gets used only when on Wifi, such as at a cafe. Meaning, you're out of luck when using any form of cellular data including GSM data.

The GPS satellites send all required data, but they do so slowly. So you can get much faster "first time to fix" by getting the same information for your area over a data connection (either WiFi or cellular data).

See also https://en.wikipedia.org/wiki/Assisted_GPS The specific data is the "ephemerides" and the "almanac", both of which are required for a GPS position fix.

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AGPS uses not only GPS satellites, but also nearby cell towers and Wi-Fi networks to lock onto your phones location, so your location should be more accurate.

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GSM, CDMA, WCDMA and LTE are all mobile network connections used for acquiring aGPS data.

So if your device has a standalone or hybrid GPS as well, you can use GPS without any problems (meaning as long as there is satellite reception). With aGPS your time-to-first-fix (TTFF) is much quicker, especially if its been awhile since last using GPS.

So to answer your "IOW", yes you will lock onto signal very fast if mobile data is enabled, as opposed to disabled.

As to: " Does it mean that it will never use A-GPS to download data from the Net even when a data connection is available through GSM ? ". No, it will use available mobile network connection for acquisition of needed data.

Take a look at this, since you said travelling around your country (could help if accuracy becomes an issue) https://stackoverflow.com/questions/14548707/android-how-to-reset-and-download-a-gps-data

Standalone GPS units depend only on radio from satellites. A-GPS does that as well as using cell tower data to enhance quality and precision when there is poor satellite signal conditions. A GPS unit may require as long as 12.5 minutes (the time needed to download the data - GPS almanac and ephimerides) to resolve the problem and be able to provide a correct location. To be more precise, "A-GPS" features are mostly dependent on an internet network and/or connection to an ISP. A mobile device featured with "A-GPS" (no additional GPS feature to be selected as alternative) can work only when there is an internet link/connection to an ISP.

For a little more info look at aGPS Wiki

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