> They are in 12 hour orbits because that is the result of optimizing the number of satellites needed to cover the Earth, plus having at least 4 satellites visible at the same time from a spot on the earth, with the satellites being spread out across the sky. It also helps that they move fairly slowly, so the Doppler is changing slowly during the PN code acquisition phase. Remember that this system was basically designed in the late 1970s, and we didn’t have 2 GHz processors in wristwatches - a GPS receiver had to acquire and process the signals to get a fix, and it had to be something that a single person could carry.
> If you go lower, you need a lot more satellites (see, e.g., Iridium or Teledesic with 77 and hundreds, respectively), and they move faster across the sky. Each GPS satellite carries an atomic clock (Cesium or Rubidium, depending on the batch) and they’re not cheap to build. The fast motion also makes acquisition harder, and low orbits are more affected by the imperfect Earth’s gravity - in order to calculate your GPS position, your receiver needs to know EXACTLY where the satellite is - that’s harder in a low orbit.
> If you go higher, 3 satellites in GEO will cover the equatorial earth, but you also need satellites in inclined orbits - you want your satellites to be spread across the sky, so your position accuracy isn’t degraded because they’re in a line or a clump. I think it actually takes more satellites to get the required coverage from GEO height (think about needing to cover the polar regions) - you could probably do some Molniya orbits, but that makes things more complex for other reasons.
https://www.quora.com/Why-are-GPS-satellites-placed-below-ge...
(albeit these are slightly elliptical)