Danoff
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- Mile High City
This is the ask danoff questions about GPS thread.
I get the feeling that there is a lot of misunderstanding about GPS out there.
Could it get overloaded?
One big misunderstanding is that many people seem to think that the GPS system could get over crowded like cell phone systems. This is incorrect. The GPS system broadcasts signals down to earth, the receivers use those signals to figure out where they are. There is no return broadcast. The satellites dont receive any information from you, so they dont care if there are two or two billion users. Incidentally, that means that the satellites dont know where you are, only your receiver does. So thoughts of the government tracking you with these satellites are misplaced.
Could it be jammed by the enemy?
Yes, but its hard, especially on military frequencies (they have their own GPS frequencies that are better than ours).
How does that little black box figure out where I am?
Basically there are 4 unknowns. X position, Y position, Z position and time - your little receiver doesnt know what time it is accurately enough on its own, the satellites have atomic clocks. So you need 4 signals which basically say when they were broadcast. From that, you can figure out how long it took the signal to get to you (even though you dont quite know what time it is), and figure out the distance between you and the 4 satellites. Thats 4 equations and 4 unknowns. So you solve it. Of course there are dependencies in the math so you have to use a least squares iterative technique to zero in on the solution. But thats basically how its solved.
How accurate is it?
Depends heavily on the way you are using it and what axis youre talking about. Horizontally I think its like 10-20 feet accurate for civilians. Altitude is more like 50 feet (thats because the satellites are above you and you have less resolution in that direction).
Ive heard that it can give sub-centimeter accuracy?
Yup, people have devised a method to get an incredible amount of accuracy out of only the carrier signal itself. Its some pretty cool stuff. You have to maintain lock on the satellites though and you have to have some cool equipment and probably a post processing unit for the raw data.
So who cares about GPS, all I get is my position?
Actually you also get time once it has solved the 4 equations. That means that your little GPS receiver gives you time accurate to within an atomic clock!!! For the price of a few hundred bucks thats a big deal if youre a research institute. Think about the ability to coordinate with that kind of time precision around the world. Position is important too if you want to know where you plane, car, experiment, or missile is.
So what questions (if any) do you have about GPS that I can try to answer? Did you find this interesting at all?
I get the feeling that there is a lot of misunderstanding about GPS out there.
Could it get overloaded?
One big misunderstanding is that many people seem to think that the GPS system could get over crowded like cell phone systems. This is incorrect. The GPS system broadcasts signals down to earth, the receivers use those signals to figure out where they are. There is no return broadcast. The satellites dont receive any information from you, so they dont care if there are two or two billion users. Incidentally, that means that the satellites dont know where you are, only your receiver does. So thoughts of the government tracking you with these satellites are misplaced.
Could it be jammed by the enemy?
Yes, but its hard, especially on military frequencies (they have their own GPS frequencies that are better than ours).
How does that little black box figure out where I am?
Basically there are 4 unknowns. X position, Y position, Z position and time - your little receiver doesnt know what time it is accurately enough on its own, the satellites have atomic clocks. So you need 4 signals which basically say when they were broadcast. From that, you can figure out how long it took the signal to get to you (even though you dont quite know what time it is), and figure out the distance between you and the 4 satellites. Thats 4 equations and 4 unknowns. So you solve it. Of course there are dependencies in the math so you have to use a least squares iterative technique to zero in on the solution. But thats basically how its solved.
How accurate is it?
Depends heavily on the way you are using it and what axis youre talking about. Horizontally I think its like 10-20 feet accurate for civilians. Altitude is more like 50 feet (thats because the satellites are above you and you have less resolution in that direction).
Ive heard that it can give sub-centimeter accuracy?
Yup, people have devised a method to get an incredible amount of accuracy out of only the carrier signal itself. Its some pretty cool stuff. You have to maintain lock on the satellites though and you have to have some cool equipment and probably a post processing unit for the raw data.
So who cares about GPS, all I get is my position?
Actually you also get time once it has solved the 4 equations. That means that your little GPS receiver gives you time accurate to within an atomic clock!!! For the price of a few hundred bucks thats a big deal if youre a research institute. Think about the ability to coordinate with that kind of time precision around the world. Position is important too if you want to know where you plane, car, experiment, or missile is.
So what questions (if any) do you have about GPS that I can try to answer? Did you find this interesting at all?
