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How Gps Works

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I've been curious about how ta heck this gps stuff works anyways.


So here's what I already understand. The satelites all have atomic clocks and send out signals that have a time stamp in them (and the satellite position?). Then the GPS receiver can compare the current time with the time that signal was sent, and determine how far away the satellite is from the receiver. From this the gps can get a radius of possible possitions. With more satellite signals, the receiver can figure out the intersections of all these radii, and determine your position.


So now I'm wondering how does the receiver get an accurate time to use for 'current time' for the comparison? A web site I read said that it gets the time from a gps satellite, but wouldn't that time be inaccurate by the time the receiver got that signal? So like, say, one of the gps satellites sends the 'current time' signal, wouldn't your gps then think you're right on top of the satellite (0 time shift from the satellite and 'current time')?


Thanks for the help,


- bones

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By using a averaging system from multiple satellites it derives the position. One satellite alone is only a clock, two start to show rough location , three give very accurate location (enough for nuclear holocaust)... More satellites, the better the accuracy... Its all in the mathematical algorithm in receiver. Its like anything... More data, more accuracy...



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The GPSr does not need to know the time. It only needs to calculate the time difference between the signals.


If you have two signals, you know what percentage of distance you are between the two satellites. This gives your location on a defined plane. With a 3rd signal, you get a position on a line passing through the center of the earth, thus giving 2D position (no alititude). A 4th signal allows a 3 demensional fix.

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Actually, a GPS receiver must be precisely synchronized with the satellites to operate. It must know the exact time the signal left the satellite, and the time the signal arrived at the receiver, to calculate the time difference and corresponding range to the satellite. By knowing the precise location of the satellite when the signal left, and the range (pseudorange), it knows its exact distance from that satellite (at that point in time). By performing this calculation on multiple

satellites, it can triangulate and calculate its position.


Obviously, timimg is everything. The GPSr must have a precise timing source for this to work. They could install a rubidium or cesium beam oscillator in the GPSr, but this would be a little pricey, use a lot more battery power, and the unit would be a little bulky. Instead, they use a relatively cheap oscillator, and a lot of software finesse. Oscillator compensation data is stored in the unit as a table based upon temperature. When the unit locks on, it measures its internal temperature, calculates the unit's oscillator error and enters a correction factor into this table based upon current internal temperature.

Edited by Sputnik 57
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Some good answers already provided, so this may be redundant.


The Trimble site (www.trimble.com) gives a good GPS tutorial. You're correct that basically all the satellites are sending is information on their own positions (the ephemeris) and a very accurate clock signal. If the GPS receiver had its own highly accurate clock then we'd only need three satellites to get a 3D position. But incorporating an atomic clock in the receiver would be impractical, so we need a fourth satellite so the receiver can solve for the three space dimensions plus the exact time.


Think of a simplified situation if we lived in a one-dimensional world, say all living along a line from LA to NY. Then if we had an accurate clock and got a signal just from a beacon located in LA we'd be able to determine that we're say 1000 miles away from LA and that would poinpoint our location. But without an accurate clock of our own we could use a second beacon from NY. We wouldn't get the distance from either beacon directly, but we could observe that the signals from NY arrive a little later than the ones from LA and therefore determine that we're 500 miles closer to LA than to NY. Knowing the distance between LA and NY (say 2500 miles), then lets us determine our position.


So in a 1D world we need just one signal if we have our own clock, but an additional one if we don't. Similarly in our real 3D world we'd need three signals if we had our own clock but without that clock we need four.

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