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In a separate question on Travel.SE I asked why my phone's GPS didn't seem to work in Beijing, China. Turns out that it was simply an "urban canyon" problem, and that the GPS did in fact work when I moved to higher and/or more open areas. What I'm wondering, is why is my phone capable of receiving GPS satellite signal in New York City, a much more dense city than Beijing especially in terms of skyscraper density downtown, but not so in Beijing? It seems if anything that it should be the other way around, or that I shouldn't be able to receive signal in either city.

Please note: I am not asking about location services on the phone, which can be influenced by things such as nearby WiFi networks and certain service carriers. I am specifically asking about GPS satellite reception coverage.

Clarification:

Outside of downtown Beijing my phone is able to receive GPS signal just fine. This question is not an issue of the GPS receiver on my phone altering the signal or any type of Chinese law. I know that the GPS receiver works perfectly. I'm simply confused as to why downtown Beijing has particular problems compared to a denser city such as downtown NYC.

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  • @MichaelMiles-Stimson thanks for that map! Is each number how many orbits a satellite makes in that spot? Also, considering it is military purposed, I would have thought there would be a high need to have satellites over potential adversaries
    – Matthew Herbst
    Commented Mar 30, 2015 at 1:34
  • The numbers in the map Michael links to are individual sat IDs. If you go to the host page of the map there's a user guide link at the top right. That page also refreshes every two minutes to show sat movement. Note that 'directly over' doesn't really matter, just needs line of sight over the horizon, though granted the higher the better to a point.
    – Chris W
    Commented Mar 31, 2015 at 0:47
  • The global positioning system was developed by the U.S. military and then opened to civilian use. It is used today to track planes, ships, trains, cars or literally anything that moves. Anyone can buy a receiver and track their exact location by using a GPS receiver. reformation.org/geostationary-satellites.html The WAAS satellites are geostationary en.wikipedia.org/wiki/Wide_Area_Augmentation_System. They are not 'parked' over the U.S. but the orbits originally chosen favored the U.S. mainland.
    – Michael Stimson
    Commented Mar 31, 2015 at 0:47
  • You get better positioning if the satellites are just above the horizon than all overhead. Overhead is great for 'urban canyoning' but in a plane or walking the plains you want one overhead and the rest fairly low on the horizon.
    – Michael Stimson
    Commented Mar 31, 2015 at 0:49
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    My understanding of the GPS receiver in my phone is that is can use both the US made GPS system and the Russian made GLONASS system. Further, I have read that some receivers are able to use both system in tandem to provide more accurate results than just using a single system.
    – Matthew Herbst
    Commented Mar 31, 2015 at 2:01

2 Answers 2

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There are a variety of reasons why your phone is giving imprecise coordinates when in china, and we can't straight away blame multi-path errors in the Urban canopy for those errors.

I feel that you should also know the following before jumping to conclusions.

1) Firstly, As a foreigner, it is illegal to use a GPS device in China

2) Many manufacturers introduce a deliberate error in the GPS location when in china.

3) It is known that Companies like Apple have a predictable error in the GPS location recorded in China.

I feel that it is reasonable to assume that the error that you see, has a far greater chance of being a result of China's Laws, rather than any technical issue.

Further reading:

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    Is that a form of censorship? Aren't you allowed to know where you are in China? What possible justification could the Chinese government have for banning GPS units when they have launched GPS satellites... perhaps it's deemed 'military technology'. Perhaps it's just more in the petty argument that the Chinese government is having with Google.
    – Michael Stimson
    Commented Mar 31, 2015 at 4:48
  • It's not completely illegal, just for foreigners. I disagree it's just laws rather than technical issues. I would agree that some of the technical issues are caused by laws (re the deliberate errors and such), and some of it depends on where the device was built for/sold. However if you have a capable unit (not firmware blocked) there's nothing short of a local jammer that can stop it from working. Getting caught doing so is another story. @Michael yes it is censorship (selective - Chinese National civilians can use it) and their GNSS system is not GPS (just like GLONASS isn't either).
    – Chris W
    Commented Mar 31, 2015 at 4:59
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    On reading the article, it's not illegal to know where you are but it is illegal to keep a record (tracklog) without a license. The equipment mentioned scramble or deactivate when in or near PRC, like the Nissan GTR which will not run at 100% unless the inbuilt GPS detects that it's at a race track. gizmodo.com/337096/….
    – Michael Stimson
    Commented Mar 31, 2015 at 5:06
  • Some corrections: 1. as a foreigner, it's illegal to make maps in China. I can't imagine it's illegal as a foreign individual to use AutoNavi to find your way around Beijing. 2. I haven't yet found a reference showing that GPS manufacturers introduce deliberate errors in the GPS location if it falls within China's boundaries (aside from the Oogle Earth article showing the 5 of our 10 camera manufacturers that block GPS completely in China; but those aren't GPS chips per se). Any reference would be great for the Wikipedia article.
    – Dan Dascalescu
    Commented Apr 8, 2015 at 1:06
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There are several factors that could be at play here, and I strongly doubt it's strictly an 'urban canyon' issue. For one thing, there may be more sources of interference (beyond things like buildings blocking LoS) in Beijing than in New York. For example the FCC here controls who can use what frequencies and at what strengths and such. Their rules and regulations have no applicability in China.

You don't mention what phone you have or are referring to, but they don't all have 'true' GPS chips. Some of them rely on those networks (see assisted GPS) to get location information (related: Tablet or smart phone GPS vs "real" GPS). Even if you do have an actual GPS chip, the antenna for it isn't going to be that great in a phone. They often rely on the network to get GPS sat location data (ephemeris) to more quickly determine their location. See the above question or these other related ones: https://gis.stackexchange.com/search?q=gps+Ephemeris

I don't believe you can 'turn off' the assist part of things on most phones - if it's available, it's used. So it might be available in New York, but not when you're in Beijing. And if it's not available, it might take much longer to get a lock than if it is (see If the GPS navigation message takes 12 1/2 minutes to cycle, how can receivers update your position every second?). Or that particular receiver may simply not be able to get a lock at all without that supplemental information.

A dedicated GPS unit (or a phone you know you can set to operate solely on GPS receiver chip reception) is really the only way you could compare signal in different locations.

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  • That's a sound and rational argument. Smartphone GPS units tend to be very weak, location is a secondary intended purpose for these devices. Are you saying Chris that if you had a 'true' GPS unit (survey quality, I'm not talking about Sat-Nav, they're nearly as bad as phones) you wouldn't get 'urban canyoning' to the same extent irrespective of where you are in the world? I suppose that's why surveyors walk around with heavy and expensive GPS units instead of their iPhone X...
    – Michael Stimson
    Commented Mar 31, 2015 at 1:52
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    @MichaelMiles-Stimson Absent any assist from something other than the GPS chip in the phone itself, even a navigation/consumer level GPS might be more accurate than a smartphone. Biggest factor aside from chipset is antenna design - where, how big, what components are around it, what blocks it when you're holding, etc. There's a reason survey has a huge antenna on a pole up away from everything else. Survey grade also gets you more access to augmentation systems like DGPS (though that relies on ground stations, broadcast or your own). Urban canyon is a local problem and can be anywhere.
    – Chris W
    Commented Mar 31, 2015 at 2:02
  • My phone: android.stackexchange.com/questions/103714/…
    – Matthew Herbst
    Commented Mar 31, 2015 at 2:03
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    @MatthewHerbst That phone does have a GPS that can operate without assist (note what I said about turning assist off and whether or not it's present still stands), and according to specs can use GLONASS as well as GPS (the whole tandem thing is an entirely different discussion). The actual details on chipset and such are extremely difficult to find. While this isn't your specific model, this thread I found might be worth a read: forums.androidcentral.com/htc-one-x/…
    – Chris W
    Commented Mar 31, 2015 at 2:15
  • Hey @ChrisW, I just found colorado.edu/geography/gcraft/notes/gps/gif/svs27.gif that shows that GPS satellites are not U.S. centric. The whole earth is covered (fairly) evenly. Looks like I was completely wrong, the tracks aren't designed to pass directly over U.S. all the time like they used to 15 years ago. Sorry guys, I haven't been this wrong in a long time.
    – Michael Stimson
    Commented Mar 31, 2015 at 3:51

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