What is Geostationary orbit?

Example 27.10 (The geostationary altitude)

Kepler’s third law, read backward, dictates the radius: $r = \left(GMT^2/4\pi^2\right)^{1/3} = \left(3.98 \times 10^{14} \times (86\,164)^2/4\pi^2\right)^{1/3} \approx 4.22 \times 10^{7}\,\mathrm{m}$ — an altitude $h = r - R \approx 3.58 \times 10^{7}\,\mathrm{m} \approx 35\,800\,\mathrm{km}$, traveled at $v = 2\pi r/T \approx 3.1\,\mathrm{km}/\mathrm{s}$. Every relay that “hangs still” sits on this one circle over the equator — there is no other.

Example 27.11 (The navigation constellation)

Satellite-navigation systems fly some thirty satellites at $r \approx 2.66 \times 10^{7}\,\mathrm{m}$ (altitude $\approx 20\,200\,\mathrm{km}$), where Proposition 27.3 gives $T \approx 4.32 \times 10^{4}\,\mathrm{s}$: half a sidereal day, so each satellite retraces its ground track daily. Your receiver compares the arrival times of their clock signals against orbits known to meters.