09-06-2016, 10:49 AM
quote:
Originally posted by HereOnThePrimalEdge
Are GPS signals then handed off from one satellite to another like cell phone calls from one tower to another?
Do GPS satellites circle the earth closer than a geosynchronous orbit in order to provide a stronger signal to automotive and small handheld GPS devices? Or for another reason?
I could tell you more than you ever wanted to know about the GPS system. Essentially they have atomic clocks in them. Each satellite broadcasts a pseudo-random (PNR) code specific to that satellite. They are all at the same frequency and receivers "despread" or decode the timing from each satellite depending on the PN code.
The receiver measures the time difference between the received signals from at least 4 satellites (for 3-d solution) and uses a built in atlas of each satellites position in orbit to compute the receivers location.
The satellites are constantly monitored for errors and timing corrections are broadcast from the ground station to each individual satellite. The most interesting correction that needs to be made include time offsets for relativity. See this for a map of stations providing timing corrections: http://www.gps.gov/systems/gps/control/
In addition there are ground stations that broadcast additional GPS corrections for higher accuracy like around airports (for planes). This is called DGPS or differential GPS https://en.wikipedia.org/wiki/Differential_GPS
Also there are very high end GPS systems that can get sub-centimeter accuracy using a technique called Carrier-Phase GPS. They can take 15-30 minutes to acquire this fine of a position, but it is often use in surveys.
The satellites are in LEO or low earth orbit.
quote:
Another issue we wrestled with is which satellite orbits to use. We did not want to be in geostationary or geosynchronous orbits. The reason was these alternatives would force us to deploy ground stations on the other side of the globe, whereas, by putting them in some orbit that periodically passed across the United States, you could update the knowledge of where they were and what time it was on the satellite, then store that information in the satellite and continue to broadcast as it went around the Earth. That is the fundamental way we ended up with twelve-hour orbits. We also wanted to be reasonably high because we didn’t want the orbits significantly disturbed by the atmospheric drag. At the same time, by going high you had more visibility, more coverage on the Earth. So with an Earth coverage antenna and suitable power densities on the Earth, you ended up with the ability of twenty-four satellites to provide very solid, total Earth coverage.