Broadly speaking, errors in the position a GPS receiver gives you are due to two factors: the precision with which the distance to each GPS satellite is known, and the geometry of the satellites, i.e. how closely or far apart they’re spaced across the sky. Distance errors can be compensated for by using WAAS, post-processing, averaging, and other techniques, but satellite geometry is a fundamental limiting factor. The maximum position accuracy you can achieve is limited by GPS satellite geometry.
The GPS satellite geometry factor is sometimes represented by a numerical measure known as “Dilution Of Precision”, or DOP. The higher the DOP, the greater the possible error in the accuracy of your position; roughly speaking, your total error is the error due to the uncertainty in satellite distance multiplied by the DOP. Professional-grade GPS receivers often come with software that will tell you what your current DOP is, tell you what it will be at some point in the future, and even prevent you from making a measurement when it’s too high. Consumer-grade GPS models don’t usually show you the DOP but instead show a general indicator of positional uncertainty, whose definition varies among different GPS unit manufacturers.
Two makers of high-end professional-grade GPS units, Magellan and Trimble, have made available free copies of software that can calculate GPS DOP as a function of date and time. The Magellan software is available on this website as part of the GPS Toolkit 0.11, which also installs several other programs. But I won’t talk about further, because IMO it’s not as good as Trimble’s Planning Software.
Download the Planning Software installation package from this page. You should also download a copy of the most recent Ephemeris file (almanac.alm) from the link on the same page (you may have to right-click on the link, and choose “Save Link As” or “Save Target As”, depending on your browser). The Ephemeris file contains the basic orbital parameters for all the GPS satellites, and these can change over time, albeit usually slowly. You will probably want to update this file every few weeks or so.
determining-local-gps-satellite-geometry-effects-on-position-accuracy
