Number of GPS satellites required to give 100% coverage I am a Belgian school student in sixth grade of secondary school (the Belgian equivalent of High school). Together with two class mates we were assigned to write some kind of essay about the Global Position System.
During the research about this subject we did not manage to find a (mathematically sufficient) reason why there are about 21 satellites required to provide a good coverage on earth.
We know that there are four satellites required to pinpoint one location, how could that information be used to calculate the minimum number of satellites required? Is there a general calculation for this magic 21 number?
Apologies if we overlooked the answer for this problem.
 A: Most of the following info can be found in the GPS Standard Positioning Service Performance Standard
First, I fear I'll have to correct your number, because you need 24 satellites for 100% coverage. 
100% coverage means that there are at least 4 GPS satellites in sight at every point on Earth at all times. 
Your position is calculated in the following way: Every satellite sends out a signal containing the time and the position of the satellite relative to the center of the Earth. You need three satellites for the three unknowns in the coordinate, and one more satellite to account for the unknown amount of time the signal needs to travel to your personal position.
The current configuration of the satellites is as follows: The satellites cover six orbital planes with 4 satellites each. The orbital planes are inclined by 55° against the equator. While the orbital planes are distributed evenly, the satellites within them are not! At this point, I'd like to quote Wikipedia on GPS:

The orbital period is one-half a sidereal day, i.e., 11 hours and 58 minutes so that the satellites pass over the same locations or almost the same locations every day. The orbits are arranged so that at least six satellites are always within line of sight from everywhere on the Earth's surface. The result of this objective is that the four satellites are not evenly spaced (90°) apart within each orbit. In general terms, the angular difference between satellites in each orbit is 30°, 105°, 120°, and 105° apart, which sum to 360°. Orbiting at an altitude of approximately 20,200 km (12,600 mi); orbital radius of approximately 26,600 km (16,500 mi), each SV makes two complete orbits each sidereal day, repeating the same ground track each day. This was very helpful during development because even with only four satellites, correct alignment means all four are visible from one spot for a few hours each day. For military operations, the ground track repeat can be used to ensure good coverage in combat zones. 

Here is an image to visualize the configuration (also taken from Wiki):

So the answer to your question is: The number 24 is valid for the current configuration and takes into account the position of each satellite in the respective orbital plane. I do not think it is the mathematical optimum. Note that the system is also setup in a redundant way. There are at least 6 more active satellites to guarantee coverage when satellites need to be repaired.
A: The number of satellites required for 100% coverage of any network is a function of 
a number of variables such as orbit radius, capacity of each satellite, footprint of their beams, etc. The number required for GPS, for example, is rather different to the number required for the Iridium constellation or Starlink.
However, GPS, GLONASS and Galileo all use approximately 24 satellites so you could make the case that for a positioning network based around TDOA this is the de-facto norm. You may wish to look into the Walker Constellation topology if you want more detail on this popular design.
