Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: bizerl on 13/03/2013 23:37:30
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After skimming through a slightly related post (http://www.thenakedscientists.com/forum/index.php?topic=38146.0), I find myself wondering how GPS satellites (is that a tautology?) know where the lines of lattitude and longitude are. It seems possible for the equator to be the most stable baseline, but how is that measured? And what about lines of longitude? If there is a slow but measureable shifting of all landmassess, how can we establish a permanent reference point?
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I believe there are ground stations that the government uses to beam information up to the satellites which tells them where they are based on the location of the ground stations.
Sorry, I don't know how they connect the dots if continents are continuously moving a few cm per year. I presume they set the longitude and latitude in North America and Europe, and then fudge the latitude in the Atlantic/Pacific a bit. Maps will certainly start going awry in a few million years.
A method to improve the accuracy of a worksite GPS is to install your own ground station. You survey in the exact location of the ground station. Then measure satellite drift with respect to the ground station, and use it as one of your GPS points. I think that gives on sub centimeter accuracy, at least for the XY plane.
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Yes, the satellites "know" their "Ephemeris" (data that they broadcast that enables their orbits to be calculated to an accuracy of about 2 metres). If and when this changes so as to be considered insufficiently accurate, a correction is made from a broadcast from a tracking station. Positions on the ground are determined based on a pseudorandom, but predictable, binary sequence that is broadcast from the satellites and the position of the receiver is calculated from the relative arrival time of these sequences from each of 3 or more satellites. Earth movements are usually small but, unless a ground station shifts significantly, you could say it is the maps that are wrong not the gps positioning (if you see what I mean). In any case adjustments are made reasonably often.
What Clifford is referring to (improved accuracy) is Differential GPS. This can be employed over a wide area (and is) to give better accuracy in a particular region or country, or employed specifically to improve accuracy in a very small area to aid surveying or construction. The latter is typically accurate to a few centimetres.
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The GPS measure altitude compared to a "Geoid": sea level of an idealised, rotating Earth.
It ignores things like low-pressure regions (which move over days) and gravitational anomalies (which move much more slowly).
For more, see:
- http://en.wikipedia.org/wiki/Geoid
- http://en.wikipedia.org/wiki/Global_Positioning_System