How GNSS Positioning Works

Global Navigation Satellite Systems (GNSS) work by measuring the time it takes for a signal to travel between a GNSS satellite and a receiver.

Determining an accurate position on earth requires two pieces of information from at least 4 satellites. The precise orbit of the GNSS satellite (called the ephemeris) and the time it has taken for the signal to reach the receiver.

The GNSS Receiver uses the time a signal is received minus the time the signal is sent to determine the elapsed time. This elapsed time is then multipied by the speed of the signal (speed of light) to estimate the distance.

One Satellite Receiving information from a single satellite defines a sphere around that particular satellite. Based on the information provided by this satellite, the GNSS receiver could be located anywhere within this sphere.
Two Satellites The Receiver uses information to define an individul sphere around each of the two satellites.
This is more useful than a single satellite. However, the receiver determines it’s position to be anywhere within the intersect point of both spheres.
Three Satellites The Receiver uses  information to define individual spheres around each of the three satellites. There are three possible intersect points between the three spheres. Working on the principle that only one of these intersect points is on the earth’s surface it is possible to use the information from three satellites to determine the receiver’s position.
Four Satellites
The Receiver uses the information to define individual spheres around each of the four satellites.
This provides a difinitive location as this sphere is only likely to intersect the other three spheres at one location.
In Conclusion
The more satellites the GNSS Receiver has access to, the greater the accuracy due to the greater number of intersect points between the idividual satellites.
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