18 User Manual3. When the satellite locations and their distances are known, the GPSreceiver fixes its own position by triangulation:d1d2 d3Fig. 1.3a - The GPS position calculationAs illustrated in the previous figure, the position is calculated as the meeting pointof three spheres which are drawn around the three satellites with diameters d1,d2 and d3.Position Fixing Accuracy: HDOPThe GPS fix accuracy is due to the locations of 3 satellites in the sky. High accuracyis obtainable when the satellites are widely scattered in the sky; on the contrary,accuracy is reduced when the satellites have gathered in a narrow space.In the following figure, in both cases it is possible to obtain the GPS fix, but in theleft case the accuracy will be higher than the right:Fig. 1.3b - HDOPThe index for position-fixing accuracy is called HDOP ("Horizontal Dilution OfPrecision"). The smaller the HDOP value, the more accurately the position can befixed.
