How GNSS works

Your receiver does not talk to anything. It listens, and solves for time.

GPS is one system among several. The general term is GNSS, Global Navigation Satellite System, and the constellations in use are GPS (United States), GLONASS (Russia), Galileo (European Union) and BeiDou (China), with regional systems in Japan and India. Modern receivers use several at once, which is why fixes are faster and more reliable than they were a decade ago.

It is a clock problem

Each satellite broadcasts its position and the exact time the signal left. Your receiver measures how long the signal took, multiplies by the speed of light, and gets a distance. Three distances would fix you in space, except your receiver's clock is nothing like accurate enough. So it solves for time as a fourth unknown, which is why you need at least four satellites for a position, and why a GNSS receiver is really a very good clock that outputs coordinates as a side effect.

Receive only

Nothing is transmitted from your device. It follows that positioning works with no mobile network, no SIM, and in flight mode, which is the single most useful practical fact in this entire section. It also follows that a GNSS receiver cannot be tracked by the satellites.

Why accuracy varies

Sky view. Satellites low on the horizon or blocked by a ridge reduce the geometry you have to work with. A narrow Norwegian valley is close to the worst case for this.
Multipath. Signals bouncing off rock walls arrive late, so the receiver computes a distance that is too long. This is why a track recorded along a cliff base wanders.
Ionosphere. The upper atmosphere delays signals by a variable amount. Dual-frequency receivers, which pick up two bands from the same satellite, can largely cancel this and reject multipath much better. This is the single biggest real improvement in consumer positioning in years.

Cold, warm and hot starts

A receiver needs to know where the satellites are. With current almanac data it fixes in seconds. Without it, after a long period off or a big move, it has to download that data from the satellites themselves at a very low bit rate, which takes minutes. Phones normally cheat by fetching it over the mobile network, called assisted GNSS, which is exactly why your phone fixes instantly in town and appears to struggle at a trailhead with no coverage. Turn the device on before you leave the road.

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