How exactly does "assisted GPS" work?

I have seen two different types of behaviour with slow initial fix acquisition:

• the thing sees enough satellites and enough signal but can’t do anything with them – very common if powered up while moving
• the thing sees no satellites for a long fixed period (minutes) and then suddenly sees a lot of them and quickly gets a fix – the Nokia behaviour

Then of course there are devices on which one cannot see the constellation at all, which is the case for most IOS stuff… in fact all IOS stuff that uses the internal GPS, and then one can’t tell what’s happening.

Some tests I have done suggest that assisted GPS does not work with a dud SIM card – one which merely makes the phone display the name of the network but doesn’t deliver any connectivity.

The tests were difficult because this particular GPS evidently needs c. 5 minutes of a really good GPS signal, with no interruption, to get a fix if there is no GSM connectivity. In scenarios where it gets a rapid fix (e.g. near a window) it virtually never gets one if it has no working SIM card. It also throws away the satellite data when it is turned off.

There is no indication of a GPRS or 3G data connection so it looks like the GSM tower location is done over the GSM protocol and some level of “paid up” authentication is required.

It’s difficult to find anybody who actually knows how this works. Yet the expertise must exist in the telecomms business.

That’s a really interesting post Byteworks – thanks!

I have been doing more tests and when I start up a little GPS app, I can see the GPRS/3G or WIFI icons appear briefly. First the GPRS/3G one, then that one disappears and the WIFI one pops up. They vanish as soon as the GPS has a fix. The WIFI data transfer was about 17k bytes. I never noticed this before!

There is no evident config for this in my phone. It just brings up anything it can bring up, for a “covert” data transfer of some sort.

However – I have two identically configured phones, and the other one, no SIM card, doesn’t bring up the WIFI when the GPS app is started, which suggests that the phone uses the GSM connection to do something and then uses WIFI to do something that depends on whatever it got over GSM.

It turns out that Nokia use supl.nokia.com and have been for years.

There is a concern that in their quest to kill the Symbian scene and to move everybody over to WP phones they will kill this server, which will make the phone’s GPS not quite useless but much less good in marginal situations.

But one can configure other servers there, and I have just found, testing a SIM-less phone but with WIFI accessible, that it changed supl.nokia.com to h-slp.mnc015.mcc234.pub.3gppnetwork.org and it almost certainly got it over WIFI, even though having WIFI didn’t improve the time to fix at all.

Again, everything suggests that the phone needs a working data SIM and only then WIFI can be useful to it for subsequent transfers. So if somebody puts in one of the £5/month no-data Giffgaff SIMs, the GPS isn’t going to work too well.

The time to fix in a totally open site, no SIM, is 45-65 secs.

Just an update on an old topic…

I have just come across this PDF which on page 15 describes two methods of doing this.

So the previously mentioned idea that you merely need a functioning SIM card (including one that doesn’t need a valid account behind it, so e.g. a PAYG SIM with no money on it) doesn’t appear to be in use.

Whether any current model phone uses either or both 3GPP or SUPL, I have no idea. I did ask one guy inside Ericcson and he wouldn’t talk about it…

One aspect is that SUPL should not work if you are roaming and you are on one of the networks which delivers no data until you buy a roaming data pack (some of the EE bunch do this). SUPL data is not that small, especially as some devices send the SIM card details covertly to google, encrypted into the SUPL request ;)

Another thing I came across is contrary to the widespread belief that a mobile phone’s position is continuously available, by triangulation between nearby towers. Apparently, this would require a disconnection from the current tower and an attempted reconnection to other nearby ones. So all that is available (and is logged with every call, every sms, and every 10 minutes or so, for ever) is the location of the single currently connected tower. Whether the Govt (etc) has a way to get the phone’s position by forcing the disconnection process, I don’t know.

For sure a SIM card is not required for the GPS in an Ipad to “work”, but the Q is whether “GSM-assisted” GPS works without a SIM card. I don’t think you get any GSM tower comms without a SIM card, but could well be wrong.

The GPS should still work (there would be an uproar if you needed a SIM card for the GPS to work at all) but might take longer (possibly much longer) to get a fix. Same with WIFI-assisted GPS.

This gets us back to the other thread of how “assisted GPS” works. There is a lot of stuff online, most of it copied from another website which copied it from another website, as usual… Fairly obviously the real detail is not in the open and will be manufacturer dependent according to their view of “optimised user experience”. Then you get into the real fun where IMHO Samsung are forcing users to enable it in order to collect location data (and I would expect Apple to do exactly the same, but perhaps less crudely)