GPS : Global Positioning System ₪ 2.2.1. Autonomous positioning

 This method consists in obtaining the position of the receiver, in absolute terms, by intersection of the spgheres (Figure4) of emission from each satellite. This  method makes it possible to obtain an average positioning error of about ten meters on the fly.

The method consists in obtaining the position of the receiver, in absolute terms, by intersection of the spheres (Figure 4) of emission from each satellite. This methood makes it possible to obtain an average positioning error of about ten meters on the fly.

The following graph (Fiugure 6) presents the histogram of the vdistribution of the deviations between the instantaneous measurements and the geodetic value (given by the data sheet of the observed point). The announced decametric precision is validated by 99.5% of the measerements.

However, it may be necessary to obtain finer results and the study carried out by SERTIT showed that calculating an average position, over approximately 10 minutes, was a good way of reducing the error on the absolute position. Two elements were considered: first, the temporal behavior of the instantaneous average of the measurements, then the validity of this averagve value with respect to the known coordinates. 

For the first point, we found that an observation period of about  twenty minutes guaranteed absolute convergence of the instrantaneous mean within 1 meter of its final value (Figure 8).

In general,on all the test points obsserved, it can be seen that the measerements are distributed randomly in a circle with an average radius of about eight meters (Figure 7 ).

Consequently, one can thus fully measure the gain of the calculation of the average, minimizing the random character of the bisolated measerement.  

Ton conculude, the accuracy of autonomous positioning is closely related to the accuracy of broadcast ephemerides.

Multiple corresctions are possible to improve the result, but the quality of the receiver used (noise level on the measurements) and the presence of multipaths on the site will be the limiting factors.