- Mon Nov 07, 2011 7:25 pm
Although the quality of receiver plays a part, other factors are often much more important in the accuracy of the solution. These include:
"Constellation/Geometry" This is the geometry of the satellite vehicles (SV) in the sky. All the SVs in one quadrant in none in others will give you an awful solution. The orbits of the SVs are designed to minimize this problem, but sometimes the geometry is poor.
"Number of SVs". The more the merrier. Three is bare minimum for a 2D solution. Four for a 3D solution, and five SVs if you want to be able to begin to have an idea of what your errors are. With that extra fifth SV, the problem is over constrained and you can solve the equations multiple ways. This gives you an idea if one range equation should be discarded because it is "out of family".
"Poor horizon". If you live in an urban area where the horizon is obscured by trees and buildings, you will not see SVs that are low on the horizon. These are the ones (if they are more than 10 degrees up) that give you the best horizontal positioning error.
"Multipath" If your receiver is around items that will reflect the GPS signal (buildings, cars, ...) the timing signal from the SVs will arrive a multiple times (think echo). This can play havoc with a solution. If you have a military GPS that uses multiple bands (L1,L2) you can avoid multipath to a large extent.
I have a GPS antenna that is bolted to the roof and has not moved in 4 years. The variability or accuracy of the solution changes on a day by day hour by hour basis. Could be as much as 10 meters off on rainy days with a poor constellation. Could be as little as a meter off on good days. Some days it is perfect (but only due to probability)!