Re the GPS satellites that tell GPS users where they are. Re the positional information they braodcast, what allows to GPS satellite to know what it’s location is with such accuracy relative to the earth? What GPS’s the GPS satellite?
They ask other GPS satellites where *they *are.
Thus proving that GPS satellites are female.
Turn off the GPS, Luke. Trust the force!
You may be interested to know that the satellites broadcast their locational data (ephemeris) continually, but that this is updated after the fact to a precise ephemeris, with a latency of about 12 days. A better-than-broadcast Rapid Ephemeris is available after around 17 hours and Ultra Rapid data is issued 4 hourly.
Or maybe they know where they are, and don’t need to ask directions, if only the wife would stop badgering it long enough to get its bearings. And no we are not low on gas and am not tailgating the other satellites!
Don’t they broadcast a time-tag in the signal too? I assume my little GPS receiver is interested not only in where the satellite is; but how far away it is. Right? Does it (my rcvr) use some sort of time-since-signal-was-sent calculation to determine the distance? Or am I all wet?
Now another question (sorry). Since these elapsed times are very very small, how do the satellite and and receiver sync up? I have never figured this one out.
I would assume that since GPS satellites are in geosynchronous orbits, they don’t really need to worry about time lag. They’re always in exactly the same place relative to the earth. I’m probably completely wrong, though, for some obscure technical reason I don’t know about.
That is a question requiring a complex answer, pullin . Yes, the satellites include time data in their signals… in fact, timing is the very essence of GPS position fixing. Once your receiver has locked on to three or more satellites, it calculates an extremely accurate time (based on atomic clocks in the satellites) and this is used to determine the distances from the SV’s.
Sorry, but this is wrong. GPS satellites are not in geosynchronous orbits, they move relative to the earth… quite rapidly actually.
GPS satilites are not in geosynchronous orbits.
From Garmin (GPS receiver manufacturer)
They’re not geo-sync. Each of the 24 satellites circles the earth twice a day. When you’re navigating with GPS, your receiver is constantly tracking “inbound” and “outbound” satellites - satellites that are coming into view and leaving view.
Link
They aren’t - read the Wikipedia link provided by AndrewT above. They orbit at about 12,000 miles, circling the Earth twice a day. However, their orbit is predictable, and they can calculate their position quite accurately given periodic corrections to their orbital parameters broadcast by the ground stations that have been mentioned. It’s the ground stations that continually monitor them and keep them accurate.
Which is, apparently what like 15 other people just said.
Man - you guys are quick.
What about a GPS receiver? When you turn mine on (and I presume all others are like this), it shows where the satellites are in the sky. Does it know where they should be by taking into consideration the last position it knew it had, the time, and the orbits of the satellites? (i.e. calculating how much the satellites have moved since the receiver was last turned on)
Garmin has some online manuals in .pdf. From The GPS Guide For Beginners:
This is a good explanation except for the use of the word “triangulation”. The accepted meaning of this is to use bearing (i.e. angle) measurements. But (as the quote says later) a GPS receiver is in fact using time-difference measurements - not bearings or their differences.
Some reasons why GPS satellites are not in geosynchronous orbits:
There is lots of demand for such orbits - space is rather crowded there
Lower orbits are less expensive to reach
A transmitter closer to the earth needs less transmit power
Since the satellites move, a single earth-based control station can talk to all of them
No one else has a humorous response? 