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sra · Sep 10, 2009, 08:00 AM

The GPS in my 3Gs iPhone works fine in the US: I use GPS Track and Navigon, plus Google Maps and so on and it never takes as much as a minute to get a good fix. But I'm currently traveling in Cyprus, and it can't get a fix at all. Every app that tries can only find the last fix made in the US (my home in Connecticut), and when it tries to do anything else says "your current location cannot be determined" or "No satellites" or the like. I really can't believe there are no satellites visible from Nicosia.... I have an app with Cyprus maps that claims to be able to locate you on the map with GPS, but when I try it reports, reasonably enough, that what it gets from the GPS (coordinates for my home in Connecticut) isn't within the area mapped.

I have Location Services on, but Data Roaming off. When I try to get the position, I'm outside with a clear view of the sky, with no WiFi or other network (though I do have a Phone Carrier, and there are cells galore around here). I've tried resetting the iPhone, resetting Location Services Warnings, and so on, but nothing works.

I can't imagine that GPS services are somehow limited to the US. What's up? And is there a way to fix it?

ghporter · Sep 10, 2009, 09:05 AM

The GPS satellite segment is designed to provide 100% coverage over at least 90% of the globe, with the remainder getting something like 80-90% coverage (at least lat/long but not necessarily elevation) all of the time. There ARE satellites over Cyprus pretty much all the time, so that's not the problem. I'd reboot the phone and see if that helps.

sra · Sep 10, 2009, 09:19 AM

Thanks, but I tried that (twice), with no luck. It still tells me I'm in Connecticut (41.26 degrees north, 72.69 degrees west), MotionX-GPS says it can't find any satellites, etc. I know there's GPS service available here, because there are GPS units sold in stores, GPS maps of Cyprus are available for a variety of systems, etc.

Exactly what services have to be available in order for the GPS to work? I'm really reluctant to enable anything except Location Services if I don't have to, to avoid AT&T's ridiculous foreign roaming charges.

turtle777 · Sep 10, 2009, 11:04 AM

How long dod you wait to acquire the fix ?

Give it some time, with such a big distance from the last location, it might take a while for the iPhone to get a fix on the satellites.

-t

cgc · Sep 10, 2009, 12:56 PM

This isn't a problem with the GPS constellation or geometry of the satellites in reference to Cyprus... Are you in a city or could there be some sort of interferer nearby (though I doubt you'd get interference on the GPS frequencies). If rebooting your iPhone doesn't solve the problem, and it's not a software issue (I don't have an iPhone so I wouldn't know), I'd recommend finding a GPS unit and seeing if it works, then trying your iPhone in the same location.

cgc · Sep 10, 2009, 01:00 PM

Originally Posted by turtle777

How long dod you wait to acquire the fix ?

Give it some time, with such a big distance from the last location, it might take a while for the iPhone to get a fix on the satellites.

Distance from last locations shouldn't matter, it's just gotta get a fix from 3 satellites (4 if you want altitude).

Simon · Sep 10, 2009, 01:41 PM

Originally Posted by cgc

Distance from last locations shouldn't matter, it's just gotta get a fix from 3 satellites (4 if you want altitude).

Yeah sure, but first it needs to know where in the sky those satellites are. And that changes with time and your location. Have you ever noticed how long an old-fashioned GPS receiver needs till it starts locating satellite signals and gives you a first fix? That's why.

Now the iPhone has A-GPS and in principle that should help with this problem. My guess is though that the assistance the iPhone's A-GPS needs from the cellular network might be insufficient where the OP is. He should be able to find out though by simply giving the GPS a lot of time to acquire that first fix.

mduell · Sep 10, 2009, 01:43 PM

Without the cell network for assistance (preloading the almanac, etc), it's going to take a while to cold start.

sra · Sep 10, 2009, 02:34 PM

Suddenly the problem resolved itself. First I should say that I had given it plenty of time to find the satellites, I thought, and also that I have been trying in an area where the sky view is relatively unobstructed - not too many tall building in the Nicosia Old Town. But no luck.

Then tonight I was on a university campus, where for some reason there was an open wifi network to which I could connect. Soon after I did so, I found that I had a quite good GPS signal, even inside the building where we were having a meeting! And the signal has persisted even after I'm not connected to wifi or anything else.

It seems that there was some information the system needed before it could find the satellites in a part of the world quite far from the last fix, and it got that over wifi, and now that it has it things are OK.

Tomchu · Sep 10, 2009, 03:20 PM

It's lame that "assisted" GPS is really "won't work without a data connection GPS". It kind of defeats the purpose of things like the TomTom app if you go to another part of the world where you don't want to pay data roaming charges.

Simon · Sep 10, 2009, 03:22 PM

It's actually the other way around. A-GPS needs data to be more than just GPS, that's the whole point. Regular GPS works w/o data (GPS fix, not maps), but you need to be more patient.

cgc · Sep 10, 2009, 03:40 PM

Regular GPS works fine without the "assistance" and can work with maps. I think you're saying the iPhone doesn't store maps internally and therefore needs to DL maps from Google Earth (for example) in order to graphically show location.

So even without the "assistance" portion the iPhone should show lat/long/alt (unless Apple decided not to allow that).

If you've moved a long distance from the last use you shouldn't have to wait a long time to get an initial location fix (for a normal GPS, not sure about the iPhone GPS) as all your receiver has to do is locate 3-4 satellites and do some "simple" math...it can do that irregardless of previous location because all that matters is the current location.

turtle777 · Sep 10, 2009, 06:30 PM

So, the lesson learnt here is that iPhone GPS solutions are not as mature as standalone GPS units.

-t

Phileas · Sep 10, 2009, 08:35 PM

Originally Posted by turtle777

So, the lesson learnt here is that iPhone GPS solutions are not as mature as standalone GPS units.

-t

No, the lesson learned is that this particular iPhone had problems picking up a satellite signal. Personally I have never encountered a similar issue.

Simon · 03:40 AM

Originally Posted by cgc

Regular GPS works fine without the "assistance" and can work with maps. I think you're saying the iPhone doesn't store maps internally and therefore needs to DL maps from Google Earth (for example) in order to graphically show location.

First, you need to distinguish between a GPS fix (the blue dot) and the map. The latter requires data, but there is also some caching involved so you can preload an area of interest. The GPS fix however is independent of that. You need only satellites to get coordinates.

The important point is however, that what allows the iPhone's GPS receiver to get an initial fix so quickly is the assistance it gets from the data network (A-GPS rather than just regular GPS). Without the data network it's as slow as any old-fashioned GPS receiver. Getting a first fix could require minutes rather than seconds.

So even without the "assistance" portion the iPhone should show lat/long/alt (unless Apple decided not to allow that).

Yes, it does. But it could take time.

If you've moved a long distance from the last use you shouldn't have to wait a long time to get an initial location fix (for a normal GPS, not sure about the iPhone GPS) as all your receiver has to do is locate 3-4 satellites and do some "simple" math...it can do that irregardless of previous location because all that matters is the current location.

This is not really correct. A GPS receiver builds a cache of satellite locations and it has some basic knowledge of how they change position as a function of time. If you move it to an entirely different location on the planet that mechanism breaks and it has to first acquire the available satellites from scratch. This can take a very long time. An old-fashioned GPS receiver would easily take two minutes for this even if the circumstances were otherwise excellent (clear unobstructed view of the sky, no interference, good antenna).

Enter A-GPS which basically replaces the GPS' caching with a a set of data the GPS receiver downloads from a data network. Since the data network knows where it is, it knows roughly where the phone with the A-GPS receiver is. It also knows the exact time. It can therefore tell the A-GPS receiver where the satellites are right now as seen from the location of the A-GPS receiver. The A-GPS receiver can then immediately start collecting data and get an initial fix very quickly.

This explains what the OP observed. As soon as he had a data connection the iPhone got a rough idea of where it is and where the satellites are. Once it had that information it immediately acquired an initial fix.

Simon · Sep 11, 2009, 03:44 AM

Originally Posted by turtle777

So, the lesson learnt here is that iPhone GPS solutions are not as mature as standalone GPS units.

No, both stand-alone GPS receivers and the iPhone in principle work the same. The difference is the type of GPS being used.

The iPhone like many cell phones uses A-GPS which requires a data connection to speed up the process of getting an initial fix. Without it it will eventually work too, but it could take a long time and is prone to interference.

An old-fashioned regular GPS receiver OTOH doesn't need that data connection. The flip side is the long time required to get the initial fix and the fact that its caching is rendered useless when it is moved large distances (for example shipping it form the US to Cyprus).

Simon · Sep 11, 2009, 03:46 AM

Originally Posted by Phileas

No, the lesson learned is that this particular iPhone had problems picking up a satellite signal.

Yes, that's exactly what it sounds like.

Personally I have never encountered a similar issue.

Me neither. But it's not the first time I hear a story like it. It's rare, but it can happen. Bummer when it does.

sra · Sep 11, 2009, 06:41 AM

Well, the GPS in my iPhone never took more than 10-15 seconds to get a fix, while I was in the US, even the very first time. But I was (almost) always on some network. In comparison with other GPS units I have used, I have been amazed at how quickly it works.

Here in Cyprus, with no network, five minutes of waiting yielded nothing. Once it connected to a data network (on wifi), I got a fix almost immediately, and that was inside a building. And since then with no subsequent net access, I also get quite quick fixes, suggesting that whatever it found last night on the net was cached and still helps.

Simon · Sep 11, 2009, 08:11 AM

Keep in mind that only the single solid dark blue dot is the real GPS fix. The light blue circle (even if it has a dot at its center) is acquired through wifi or cellular triangulation.

cgc · Sep 11, 2009, 09:49 AM

I understand the location fix versus the map issue and I understand how GPS works and the constellation geometry (I worked space operations for several years for DoD). I don't think 2-3 minutes is a long time to wait to get an initial lat/long/alt fix...it sounded like the OP was never getting any GPS location information which is why I was asking the questions I was asking.

Good to hear the OP's problem seems to have been resolved.

ghporter · Sep 11, 2009, 10:10 AM

Originally Posted by sra

Well, the GPS in my iPhone never took more than 10-15 seconds to get a fix, while I was in the US, even the very first time. But I was (almost) always on some network. In comparison with other GPS units I have used, I have been amazed at how quickly it works.

Here in Cyprus, with no network, five minutes of waiting yielded nothing. Once it connected to a data network (on wifi), I got a fix almost immediately, and that was inside a building. And since then with no subsequent net access, I also get quite quick fixes, suggesting that whatever it found last night on the net was cached and still helps.

The satellite constellation over Cyprus will be VERY different from what was seen the last time you used GPS in Connecticut. It took a LONG time for the phone's GPS system to get almanac data, which it needed to know what satellites to tune into. Once it got that, it had to sync the signals from those visible satellites. Just as Simon described in reference to older, stand-alone GPS receivers, it took a long time to become useful because it had to find the satellites, sync with them, and then receive data. Your "10-15 seconds" fix time was due to the iPhone's system maintaining a long history of almanac data-but only for the small range of locales you'd already used it in. It's all about the GPS receiver knowing which of the 30+ satellites to look for, and then synchronizing its receivers to those signals-not a trivial process.