GPS when roaming with no data

Question

What kind of GPS functionality should I expect on a roaming Android device that has mobile data disabled when I am abroad?

I have just found myself a bit stuck with Google Offline Maps because my rooted Galaxy Note 3 running Resurrection Remix would only get a correct GPS fix once mobile data roaming was turned on - before turning on mobile data, the phone thought it was at its previous good location. Once mobile data was turned on, the phone got a new GPS fix very quickly despite being in a heavily built-up area. Turning off mobile data would cause the GPS fix to be stuck at this new location.

As an example:

1. come out of a Metro Station A, fix from previous good location
2. turn on data, fix very quickly
3. turn off data, fix sticks at location of Metro Station A
4. go into Metro Station A, and travel to Metro Station B
5. come out of Metro Station B, fix still shows location of Metro Station A

I have tried changing the settings on the GPS to tell it to only use the GPS satellites (and not wifi/cellular) but this didn't help.

Is this expected behaviour, a quirk of the Note 3 or Resurrection Remix, or something else?

Answer 1

All current GPS chips can work in one of two modes: GPS Standalone or AGPS (either MS-Based or MS-Assisted), where AGPS is the abbreviation of Assisted-GPS. The latter requires internet connection to contact an AGPS server which helps the chip to get its first fix.

When on gps standalone mode (in your case when you deactivate mobile data), the chip is not assisted in any way, and it may take about 12.5 minutes to resolve its location.

Read more on wikipedia.

Answer 2

Yes, this behavior is expected.

As EHH has pointed out, the time to get a location fix using unassisted GPS can be long. This is because the receiver does not know where it is and needs to do a search for which satellites are in view.

When you have mobile data turned on, there are actually multiple sources of location information that your phone can use.

1. GPS (assisted or unassisted)
2. WiFi - Location fixes from WiFI are much faster than GPS. The accuracy is coarser, however, at ~50m.
3. Cellular - Not as frequently used because it requires action by the network, and is less accurate than either GPS or WiFi.

Applications frequently use multiple sources. They will take the first location fix, usually from WiFi, then they will refine the location estimate if other sources become available. Once GPS has a fix, location can be updated very frequently and accurately.

I speculate that in your situation you are not waiting long enough to get a GPS fix, or that your GPS may be unable to fix and you are simply using WiFi location. Android will store the last available location, so this will be used until your device can get a new location fix.

Answer 3

Using Samsung Galaxy Note 3 ( unrooted, original ROM, SM-N900)

I have downloaded offline maps using google maps & It takes around 15-30 seconds to fix without any internet connection. In my experience yet with all others phones, most of phones require internet connection for first fix as pointed above. But high end samsung phones do not require internet... I have tried in my local as well as on my 7-8 days long tours outside, Offline maps worked very good for me...

I am not sure why are you facing this issue... Please check if any library problems with custom ROM you are using... Notify the ROM developer on XDA.. He might fix that problem for you...