
How GPS Receivers Work - noheartanthony
http://electronics.howstuffworks.com/gadgets/travel/gps.htm
======
cnvogel
Oh boy. What a strange article. While they try to explain all intricate
details, their funny use of analogies turns most things they tell either wrong
or at least wildly off the mark.

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.

------
defen
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.

~~~
cnvogel
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.

~~~
defen
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>

------
asmithmd1
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>

------
profquail
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#Comparison_of_Accuracy)

