Why does GPS work when underground and under big buildings but radio signals, Wi-Fi, and cell phone signals struggle?
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I want to start off with everything people are saying about dead reckoning is true, modern phones especially use all sorts of signals to get more accurate positions in disadvantaged situations.
To explain how gps can work in a building or a short distance underground, you need to know of a property of the GPS signals. The term is autocorrelation, but what you need to know is if you do a correlation (think multiplication but for signals) with itself it will be boosted, but if you correlate with something else, like itself but shifted in time, it will just turn out to be noise. What this lets us do is take a signal that we shouldn’t be able to receive, like one that has less power than the noise around us, and boost it if we know the shape it should be.
With every generation of gps we have added faster (more accurate) and longer signals (more boosted), which require better clocks than what is on most consumer devices. Luckily you can use the older generation signals to make your local clock work better and lock on to the longer signals, which can be the difference between receiving the GPS and not.
Most things equipped with GPS also come equipped with a ‘dead reckoning’ system of some kind.
It uses your last known location and accelerometers and compass and such to make a best guess at where you are when you are out of contact.
Turn on airplane mode. GPS will still work with it on. Then, you can see how fragile gps signal actually is. Go to a parking garage or something and it will either take extremely long to locate you or fail all together
GPS doesn’t. I have an old mid 90’s GPS and it absolutely will not work if you aren’t standing in a field. It’ll continue to work if you go into heavy woods but it won’t like it and likely will lose connection to several satellites. Modern GPS works differently. Sure, the satellites are the very same ones (usually, there are newer ones up there now but the old ones work just fine). Your phone uses the term location services because it doesn’t rely on solely on GPS. It can base your location on where cell towers are, sometimes use data from other nearby phones (I think Apple does this) or it can make an educated guess. Using the accelerometers in your phone and the compass it can tell where you are going and how fast and keep your location accurate enough until you reconnect to the satellites.
Let’s start with your premise:
It doesn’t. GPS needs clear line of sight to at least 3 satellites.
All digital cellular devices have GNSS capabilities because the precise time signals broadcast by the navigation satellite systems (there are about 7 different systems in operation now, GPS (US) , GLONASS (Russia), and Galileo (EU) are the biggest ones. But they all operate on similar principles and frequencies, and a receiver can actually use signals from all of them.
But it requires clear line of sight to the sky.
Modern smartphones have a combination of systems that provide a location feed to the apps. This comes from the GNSS receiver in the cellular modem, WiFi signal triangulation based on signal strength of WiFi beacons that your phone can see, any external GPS receivers, etc, and combines those all into a single location stream that is then available to the applications.
Sorry but what I know is GPS don’t work underground, in my experience tunnels, roofs, and under the bridge will easily interfere my GPS any device.
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