Why does GPS work when underground and under big buildings but radio signals, Wi-Fi, and cell phone signals struggle?
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It doesn’t always.
GPS runs off of a satellite system, which is also used for things like satellite TV.
If you’ve ever used satellite TV or tried to use a vehicle’s GPS in a storm (even heavy cloud cover with a lot of rain or precipitation in it), they are notorious for losing connection.
Many “GPS” applications still store location data in phones and other devices so if they lose connection it will still show your last location. Most modern location services combine GPS, cell signal triangulation, data from Wi-Fi or other networks, as well as other devices nearby to confirm your location.
Otherwise, the lower frequency signals are likely to travel further (like an x-ray going through almost everything but lead) than higher frequency ones. High frequency signals oscillate (go up & down making a full wave) much more quickly than low frequency ones, and because of that they’re likeliest to be deflected by thin layers of metal or other things
GPS doesn’t. It’s really flaky if you don’t have a clear view of the sky.
However, most GPS systems are augmented. For instance, they can use accelerometers to know the speed and direction of travel and thus extrapolate from the last GPS position.
So when you’re in a tunnel your GPS can’t locate you, but the positioning system takes the last GPS read and adds your movement since then, plus the assumption you’re still on the same road, and plots your position in the tunnel.
These “inertial” navigation systems actually predate GPS, but they loose accuracy over time from when they were last calibrated at a fixed position.
Edit: as mentioned by many, smartphones have very basic estimation this way – they can’t estimate position accurately long after loosing connection. And phones also use a few other techniques to augment GPS like WiFi (there’s essentially a shared list of WiFi base stations and their locations), Bluetooth and cell phone towers.
Edit2: I’d like to emphasize this bit
> assumption you’re still on the same road, and plots your position in the tunnel.
Inertial navigation, even with expensive big systems, is not very accurate. Assuming you’re sticking to the road network is pretty key to making it work well (plus you can probably assume they’re travelling at roughly the same speed). It often freaks out if you’re in a complicated tunnel network with forks and turns offs
Edit3: leaving lose as loose for all the redditors who need to point it out 🙂
GPS itself doesn’t work well or at all underground or in buildings. However most devices these days don’t rely on the GPS signal alone. Smartphones also triangulate using cell towers and known wifi hotspots. Additionally there are signal repeaters that can be installed that will boost the signal in areas where there are known blockages. The same can be done for cell phone, WiFi, and radio.
GPS is notorious for needing a very clear view of the sky in order to work. I don’t know where you’re getting the idea that it works underground.
Another aspect is that GPS is a one way radio communication. You only need to be able to receive the signal from the satellite, and don’t need to transmit back to it. For the other systems you talked about your device needs to be able to receive and have a powerful enough transmitter to send signals back.
It definitely doesn’t always work better, but one reason it can work better is because modern GPS, at least for Google, uses multiple sources of location. If you have the settings turned on, they will use GPS, WI-FI signals, which have known locations, Bluetooth signals, and other mobile signals, which also may have known fixed locations.
So because of this, Google maps can continue to estimate your locations if GPS goes out, if it picks up a nearby Wi-fi or something that it already knows the location of.
Many tunnels in California have Bluetooth beacons at regular intervals, according to Waze at least. This combined with speed information is probably giving you a good estimate of your position during the loss of GPS fix.
GPS bands are
L1 1575.42 MHz
L2 1227.60 MHz
Both of these are much higher than FM radio and struggle to penetrate far through buildings, so they are almost always blocked by concrete, brick and tinted windows.
GPS doesn’t work under ground or in large building especially well. If you are referring to your phones’s location services they use a whole suite of features to provide the appearance of precisely determined location that include GPS/cell tower identification/Wi-Fi Mac identification from data bases such a skyhook /and inertial navigation and even as a last ditch geo ip.
All of these add up to the devices ability to locate itself fairly well, model the world and continue to update location even when one or more of them fail, and fading in or out or report spurious data.
We use the term “GPS” as a shorthand for “Location Services.” It’s actually one of several means used to determine your location.
Seeing WiFi that is at a known location, cellular triangulation, etc. all augment that ability to show your location. (I don’t know all the ways). In some cars, it will use the speed of the wheels and the turning of the steering wheel to improve the estimate of your location. The system can also fix up inaccuracies by assuming you are still on the freeway when you go into a tunnel.
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