As discussed above, there are several external sources which introduce errors into a GPS position. While the errors discussed above always affect accuracy, another major factor in determining positional accuracy is the alignment, or geometry, of the group of satellites (constellation) from which signals are being received. The geometry of the constellation is evaluated for several factors, all of which fall into the category of Dilution Of Precision, or DOP.
DOP is an indicator of the quality of the geometry of the satellite constellation. Your computed position can vary depending on which satellites you use for the measurement. Different satellite geometries can magnify or lessen the errors in the error budget described above. A greater angle between the satellites lowers the DOP, and provides a better measurement. A higher DOP indicates poor satellite geometry, and an inferior measurement cofiguration.
Some GPS receivers can analyze the positions of the satellites available, based upon the almanac, and choose those satellites with the best geometry in order to make the DOP as low as possible. Another important GPS receiver feature is to be able to ignore or eliminate GPS readings with DOP values that exceed user-defined limits. Other GPS receivers may have the ability to use all of the satellites in view, thus minimizing the DOP as much as possible.
Go back to the previous chapter: The GPS Error Budget
Go on to the next chapter: Using Differential GPS to Increase Accuracy
Return to the Table of Contents
| 413 SW Jefferson Ave. Corvallis, OR 97333 USA info@cmtinc.com |
Tel: 541/752-5456 Fax: 541/752-4117 BBS: 541/752-7206 |
As powerful as GPS is, +/-50 - 100 meters of uncertainty is not acceptable in many applications. How can we obtain higher accuracies?
A technique called differential correction is necessary to get accuracies within 1 -5 meters, or even better, with advanced equipment. Differential correction requires a second GPS receiver, a base station, collecting data at a stationary position on a precisely known point (typically it is a surveyed benchmark). Because the physical location of the base station is known, a correction factor can be computed by comparing the known location with the GPS location determined by using the satellites.
The differential correction process takes this correction factor and applies it to the GPS data collected by a GPS receiver in the field. Differential correction eliminates most of the errors listed in the GPS Error Budget discussed earlier. After differential correction, the GPS Error Budget changes as follows:
Source Uncorrected With Differential Ionosphere 0-30 meters Mostly Removed Troposphere 0-30 meters All Removed Signal Noise 0-10 meters All Removed Ephemeris Data 1-5 meters All Removed Clock Drift 0-1.5 meters All Removed Multipath 0-1 meters Not Removed SA 0-70 meters All Removed
By eliminating many of the above errors, differential correction allows GPS positions to be computed at a much higher level of accuracy.
Go back to the previous chapter: Measuring GPS Accuracy
Go on to the next chapter: Levels of GPS Accuracy
Return to the Table of Contents
| 413 SW Jefferson Ave. Corvallis, OR 97333 USA info@cmtinc.com |
Tel: 541/752-5456 Fax: 541/752-4117 BBS: 541/752-7206 |