A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Richard B. Langley
Date: 2018 Sep 25, 15:05 -0700
"And in any case, the GNSS constellations don't live in geosynchronous orbit! They are in MEO."
Not trying to be too pedantic, but SOME GNSS orbits ARE geosynchronous. In the satellite game we distinguish between geosynchronous and geostationary. Geostationary orbits are a subset of geosynchronous orbits (with 24 sidereal hour period) with zero-degree inclination; i.e., they are in the equatorial plane. Geosynchronous orbits, in general, have a 24-hour orbital period but can have any orbital inclination and, if non zero, are called inclined geosynchronous orbits or IGSOs and have a figure-of-eight groundtrack. The Japanese Quasi-Zenith Satellite System satellites are IGSO satellites as are some of the Chinese BeiDou and Indian Regional Navigation Satellite System (IRNSS) satellites. Some of the BeiDou and IRNSS satellites are actually in geostationary orbit. GPS satellites, given their 12 sidereal hour period, are sometimes referred to as being in semi-synchronous orbits (completing almost exactly two orbits per sideral day). More info here:
-- Richard Langley