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Wow, this is incredible. Why the GPS satellites are far away from the planet, compared to the other ones?
by 101008 3y ago
Wow, this is incredible. Why the GPS satellites are far away from the planet, compared to the other ones?
- numbol 3y agohttps://en.wikipedia.org/wiki/Geostationary_orbit https://en.wikipedia.org/wiki/Geostationary_orbit
- renjimen 3y agoI thought the same, but they're actually below geostationary orbit. They have 12 hour orbits. Also, they don't orbit the equator, which is required for a geostationary orbit. See my comment above with more details.
- numbol 3y agowow that is cool! Thank you for that clarification looks like geostationary used for stuff like https://en.wikipedia.org/wiki/Quasi-Zenith_Satellite_System https://en.wikipedia.org/wiki/Quasi-Zenith_Satellite_System
- mardifoufs 3y agoI think this article, that most here have probably read or heard about considering how popular it was on HN, explains why quite well! It is a very long article though (which is awesome, imo!). The whole thing is an incredible primer on GPS .
- crote 3y agoGeostationary orbit wouldn't work for a GNSS: all satellites would be in the same plane, so it would be impossible to distinguish between the northern and southern hemisphere. This would render it inconvenient in most countries and unusable anywhere near the equator.
- danw1979 3y agoRelated: an inclined geosynchronous orbit uses similar altitudes to GEO, allowing focussed coverage on specific longitudes, as seen with QZSS - https://en.m.wikipedia.org/wiki/Quasi-Zenith_Satellite_System https://en.m.wikipedia.org/wiki/Quasi-Zenith_Satellite_Syste... (albeit these are slightly elliptical)
- renjimen 3y agoI thought the answer to this would be simple but it's pretty complex! From a Quora answer: > 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-geostationary-orbit-Is-it-due-to-the-need-of-a-stronger-comm-system https://www.quora.com/Why-are-GPS-satellites-placed-below-ge...
- alsaffar 3y agoAs far as I know, the GNSS (GPS, GLONASS, etc) satellites needed to have more of Earth visible at all times, so they are farther away. Not an expert, just very interested.
- disillusioned 3y agoNo better explanation on the planet than Bartosz Chiechanowski's: https://ciechanow.ski/gps/ https://ciechanow.ski/gps/
- throwaway2037 3y agoThis guy! Nerd sniped again.