Something I noticed when driving around.
My phone’s GPS usually does a great job at telling what lane I’m on and how far I am to an intersection, but it does a terrible job at telling if I’m on or under an elevated expressway.
Why can it tell my horizontal position so much better than my vertical position?
The phenomenon is independent to the model and make of the phone as well. I’ve switched from Android to Apple, and both phones seem to be terrible at telling if I’m on an elevated expressway or on the road under it.
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GPS works by listening to the signal of many satellites that broadcast their own very exact time and position over radio frequency. Since the the exact position and time of each satellite is known, trigonometry can be used to calculate the GPS receiver’s exact position. The delay of the signals as they arrive through the air will tell how far away each satellite is, as the farther the distance from a satellite, the longer the delay will be.
This works great as long as the radio signal travels in a straight line, but as soon as the receiver is close to large structures such as buildings, bridges og overpasses, where some or all of the radio waves will bounce, the wave will travel further than the actual distance to reach the receiver, disturbing the calculation.
GPS software compensates for these inaccuracies by using additional information to determine where you are, such as the direction you are travelling and if there are any roads in that direction nearby it can “snap” to the nearest road.
This breaks down when multiple roads are crossing, or you enter a tunnel where the signal is weaker/very bouncy or maybe you don’t move fast enough for the software to confidently know your direction.
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