How GPS Receivers Work(electronics.howstuffworks.com)
electronics.howstuffworks.com
How GPS Receivers Work
http://electronics.howstuffworks.com/gadgets/travel/gps.htm
4 comments
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They left out one of the more interesting aspects of GPS calculations, which is the corrections that must be made for special & general relativistic effects.
Those corrections are not actually essential for GPS to function. In first order it's only the satellites clocks that have to be run at a slightly elevated frequency (or is it the other way round? Don't know). The actual GPS receivers don't have to mess with relativity at all.
The satellite clocks run at a slightly lower frequency than the desired frequency on Earth: 10.22999999543 MHz instead of 10.23 MHz.
http://en.wikipedia.org/wiki/Gps#Relativity
http://en.wikipedia.org/wiki/Gps#Relativity
This tutorial from Trimble is a little more detailed:
http://www.trimble.com/gps/index.shtml
The really interesting innovation in GPS is the way they use a priori information about each satellite's signal to "amplify" the signal by processing: http://www.trimble.com/gps/sub_amplify.shtml
The really interesting innovation in GPS is the way they use a priori information about each satellite's signal to "amplify" the signal by processing: http://www.trimble.com/gps/sub_amplify.shtml
If you want to compare the precision of various positioning methods:
http://en.wikipedia.org/wiki/Wide_Area_Augmentation_System#C...
http://en.wikipedia.org/wiki/Wide_Area_Augmentation_System#C...
A GPS receiver only receives at a single frequency (leaving out doppler for now) but correlates several pseudorandom codes in simultaneously. It needs four sattelites no to increase the accuracy, but to solve for the fourth unknown, time. Also it doesn't use a quartz clock that resets "lets say at midnight" but measures the relative state of it's correlators' counters (that's what's often called "pseudorange"). And DGPS doesn't correct for incorrectly sent out almanacs. It only corrects for unknown propagation properties of our atmosphere (rather oddly described as slowing down of electromagnetic energy). DGPS also doesn't help you with multipath reception due to reflections at skyscrapers.