51

This Newsweek article states that "GPS...will likely be nonexistent near the eclipse zone" because of all the people going to that area. While it makes sense that cell towers might be overloaded and it might be impossible to download maps or conduct Internet searches that find coordinates for an input address, and one shouldn't plan to be streaming music/videos over mobile connections during that time, why would GPS be affected?

I thought GPS didn't require communication from devices to satellites, it only required the devices to listen and compute, and thus the system was scalable to a very large number of devices, even when those devices are relatively close together.

PearsonArtPhoto
  • 121,132
  • 22
  • 347
  • 614
WBT
  • 1,144
  • 1
  • 11
  • 19
  • 21
    Poor reporting. The source is either misinformed or was talking about E911. – pericynthion Aug 06 '17 at 20:59
  • 8
    It is true that a GPS device is a pure receiver. The GPS system was designed this way, it should be impossible to determine the position of GPS device by listening to the communication between the device and the satellites, thus there is no such communication. As a consequence, the number of GPS devices per square mile of earth surface is not limited. – Uwe Aug 06 '17 at 21:30
  • 1
    There might be one issue though: I guess Ionosphere properties are changing rapidly due to missing sun light. Hence, location accuracy might be worse than usual. WAAS might not be able to cover it because it is a local and fast phenomenon. – asdfex Aug 07 '17 at 14:16
  • 33
    If some people call their browsers "the internet", calling their map apps "GPS" is not in the least bit surprising. – wedstrom Aug 07 '17 at 15:23
  • 2
    @wedstrom Not all map apps need internet access at use time (e.g. MapFactor Navigator does not). – WBT Aug 07 '17 at 17:00
  • Y2K problem all over again :-P – szulat Aug 07 '17 at 22:28
  • 1
    @pericynthion Or a phone GPS - see point 1 over on electronics.SE – Izkata Aug 08 '17 at 18:09

3 Answers3

62

GPS isn't affected by demand, as it is transmitting only from satellites, and the receivers only receive, they do no transmit to the satellite at all. At best, there is a slight degradation by having antennas in really close proximity. A million man march, each with a GPS device, might cause some degradation as each device will absorb a bit of the energy that others might. If it were to happen, it would only absorb the signals at low angles, which aren't really required, although they do help to get a better solution. There could also be some noise generated, as receivers can transmit a very small amount of signal in the range of the intended signal as generated by their oscillators. But in large part, it won't be degraded, and at worst case, would only be reducing the number of satellites slightly. And even that effect is probably more due to the large number of people around you, and not the electronics which they possess.

Differential GPS, on the other hand, might have some degradation. This requires that GPS corrections be sent out, and the network traffic that these are sent out on might not work as well. Still, that would only lead to uncertainty in the highest order, it would still be able to figure out which street you are on.

From the article comes the following quote:

print out directions since GPS (especially Google Maps) likely won’t be an option

Many people associate GPS with turn by turn directions from something like Google Maps. And that has the potential to be really affected. I suspect the author made the same mistake.

PearsonArtPhoto
  • 121,132
  • 22
  • 347
  • 614
  • 4
    However, if people download an offline map application like MapFactor Navigator, and download maps for the appropriate states in advance, they should NOT be thinking that the GPS signal will be crowded out. (Paper maps are still a good idea in any case, but not that much more in this case than others.) – WBT Aug 06 '17 at 22:05
  • 10
    The wavelengths of GPS L1 and L2 are 19cm and 23cm respectively, that's 7.5 and 9.2 inches, which is a heck of a lot closer than people are going to be holding their GPS units even in a crowd. The speculation about mutual interaction and signal loss due to receiver proximity is not based in science. The rest of the answer is great! – uhoh Aug 07 '17 at 02:47
  • You can download offline areas for google maps (on Android) when you're on wifi. – Chris H Aug 07 '17 at 08:18
  • If it caused any degradation, it would be from the low elevation satellites, where the LOS is affected. Still, I bet that the people in the area would cause as much degradation as the antennas. – PearsonArtPhoto Aug 07 '17 at 09:11
  • What about frequency saturation? GPS used BPSK, and with an increasing number of devices in the same area, the chances for collisions in the signal skyrockets. Might be worth adressing in your post how GPS avoids collisions in the signal if too many devices are nearby. – Polygnome Aug 07 '17 at 11:17
  • 3
    It doesn't apply, as the GPS devices are only receiving, not transmitting. Does quality degrade based on how many people listen to a radio station? – PearsonArtPhoto Aug 07 '17 at 11:19
  • @PearsonArtPhoto It might be worthwhile to include this piece of information in your answer. Because its one of the key differences between phone communication and GPS ;) – Polygnome Aug 07 '17 at 11:34
  • 4
    Cold fix probably will be affected. You can't download the ephemeris at the last moment over GSM or WiFi if there are ten thousand people around you trying to do the same. – John Dvorak Aug 07 '17 at 16:47
  • 1
    @JohnDvorak for large events that aren't deserving of permanent equipment (i.e. Lollapalooza is 4 days/year, not the Cubs' Wrigley Field which is 80-90 days/year), Cells On Wheels (COWs) are set up in advance to handle the overwhelming network traffic. – Nick T Aug 07 '17 at 23:10
  • @WBT , you can also <a href="https://support.google.com/maps/answer/6291838?co=GENIE.Platform%3DiOS&hl=en" Download Offline Areas using the Google Maps app. – Map Man Aug 08 '17 at 19:05
39

As PearsonArtPhoto says, it's not the GPS protocol itself that causes the problem.

Cell phones use Assisted GPS, where cellular data is used to speed up obtaining a GPS fix. This should be just a few kb per session though.

Many mapping applications also download map data as you go along, again causing lots of network traffic.

This may be an aberration, but I'm leaving it here for future reference: in my own tests an iPhone went through 100 Mb of data just for AGPS in 1 hour.

Hobbes
  • 127,529
  • 4
  • 396
  • 565
  • 4
    This is the correct answer. The article specifically puts it in that context: "Cellular service towers aren’t meant to handle the capacity of an additional half-million to a million people per state. Cellphone, GPS and smartphone internet services will likely be nonexistent near the eclipse zone..." +1 – rasher Aug 06 '17 at 23:51
  • 5
    Thats a lot. Are you sure it is just the agps and not maps (the foot version as even vector maps are not that big usually)? If it were so much, many Pokemon go players would be broke long time ago. – jkavalik Aug 07 '17 at 06:09
  • @rasher even agps should only "need" the first fix imho. Then it should work quite normally (possibly with a bit lower precision than when data are available). Source: I used GPS without data connection on my current phone before. – jkavalik Aug 07 '17 at 06:12
  • the iPhone lists data use per application. If the data were due to map downloads, I'd expect the data to show up as downloaded by Apple Maps, and not as 'Time and Location services'. – Hobbes Aug 07 '17 at 07:44
  • Cell phones can use AGPS. At least in most models AGPS isn't necessary. If I go to another country and turn off mobile data (no wifi either) the only noticable effect is an increased time to the first fix. About the same time as my old Garmin in fact, because it works the same way as the ephemeris (and possibly almanac) are downloaded the slow way. The precision should be unaffected however in some areas signal propagation effects mean GPS isn't as good as it should be. These areas are often cities, where the backup methods (cell triangulation) are most accurate – Chris H Aug 07 '17 at 08:17
  • 21
    100 Mb per hour can't be right. The whole almanac has just 15 kBit, plus a few more kB for WAAS/EGNOS data. The alamanc doesn't need regular updates, WAAS should be updated every few minutes. – asdfex Aug 07 '17 at 08:26
  • My sentiments exactly. Still, 100 Mb is the figure reported by the phone, I'm trying to troubleshoot this, but not making much headway. – Hobbes Aug 07 '17 at 13:12
  • 1
    That's crazy. Like asdfex says, a SUPL response isn't more than a few kilobytes, and once the GPS lock is acquired there's no use in making any more requests for the acquisition parameters (so no continuous use there), and the ionospheric stuff doesn't change rapidly enough to need frequent updates. Either whatever you're measuring is the data usage of some other feature, or Apple is being terribly wasteful of their customers' money. – hobbs Aug 07 '17 at 16:32
  • So, can the assisted part of Assisted GPS get switched off? – NoDataDumpNoContribution Aug 08 '17 at 07:15
  • No, not specifically. You can disable mobile data and the phone will function in normal GPS mode. You can also get an external GPS module that doesn't use AGPS. – Hobbes Aug 08 '17 at 09:02
  • 3
    100 Mb per hour is indeed insane. I implemented ephemeris distribution for a well-known Portable Navigation Device vendor, and that was measured in kilobytes per week. (ephemeris prediction works quite well). You have a clock error when it's 100 Mb per hour - the device thinks it always has stale data, even after receiving current data. – MSalters Aug 08 '17 at 11:12
  • An iPhone shouldn't go through significant AGPS, but a cheap older "AGPS" phone might. Some older phones don't have a GPS chipset, but instead record a short burst of RF from the GPS radio, then send it to the cellular tower so a separate computer can decode the location. This is usually only used for emergency calls, and only for older cheaper phones which don't have a real GPS chipset, and the servers usually limit requests because this decoding is expensive, so it's not used for navigation, only emergency response. So even then it shouldn't consume this much data. – Adam Davis Aug 08 '17 at 20:58
2

It isn't the GPS, but the applications using this data. On your phone, you generally wouldn't interact with GPS, but with a Map, in some form. This map data is not stored on your phone. The applications using the GPS are what puts the load on the network.

James Heacock
  • 41
  • 1