Electronic tracking devices Positioning techniques

Several different techniques have been developed for using the GPS tracker to pinpoint a user’s position, and to refine that positioning information though a combination of GPS derived data and additional signals from a variety of sources. Some of the more popular techniques, such as autonomous positioning, differential positioning and server-assisted positioning, are briefly described below.

Autonomous GPS positioning

Autonomous positioning, also known as single-point positioning, is the most popular positioning technique used today. It is the technique that is commonly thought of when a reference to using the GPS to determine the location of a person, object or address is made. In basic terms, autonomous positioning is the practice of using a single GPS receiver to acquire and track all visible GPS satellites, and calculate a PVT solution. Depending upon the capabilities of the system being used and the number of satellites in view, a user’s latitude, longitude, altitude and velocity may be determined. As mentioned earlier, until May of 2000 this technique was limited in its accuracy for commercial tracking device . However, with the discontinuation of S/A this technique may now be used to determine a user’s location with a degree of accuracy and precision that was previously available only to privileged users.

Differential GPS positioning

The use of differential GPS (DGPS) has become popular among GPS users requiring accuracies not previously achievable with single-point positioning. DGPS effectively eliminated the intentional errors of S/A, as well as errors introduced as the satellite broadcasts pass through the ionosphere and troposphere. Unlike autonomous positioning, DGPS uses two GPS receivers to calculate PVT, one placed at a fixed point with known coordinates (known as the master site), and a second (referred to here as the mobile unit) which can be located anywhere in the vicinity of the master site where an accurate position is desired. For example, the master site could be located on a hill or along the coastline, and the mobile unit could be a GPS receiver mounted in a moving vehicle. This would allow the master site to have a clear view of the maximum number of satellites possible, ensuring that pseudorange corrections for satellites being tracked by the mobile unit in the vicinity would be available.

The master site tracks as many visible satellites as possible, and processes that data to derive the difference between the position calculated based on the SV broadcasts and the known position of the master site. This error between the known position and the calculated position is translated into errors in the pseudorange for each tracked satellite, from which corrections to the measured distance to each satellite are derived. These pseudorange corrections may then be applied to the pseudoranges measured by the mobile unit, effectively eliminating the affects of SA and other timing errors in the received signals.

Corrections to measured pseudoranges at the master site are considered equally applicable to both receivers with minimum error as long as the mobile unit is less than 100km from the master site. This assumption is valid because the distance at which the GPS satellites are orbiting the earth is so much greater than the distance between the master site and the mobile unit that both receivers can effectively be considered to be at the same location relative to their distance from each SV. Therefore, the errors in the pseudorange calculated for a particular satellite by the mobile unit are effectively the same as errors in the same pseudorange at the master site (i.e. the tangent of the angle between the master site and second receiver is negligible.

More electronic tracking devices at http://www.jimilab.com/ .