the clock error. A clock with an accuracy of one in a million will introduce an error of one millionth of a second each second. This error multiplied by the speed of light gives an error of 300 meters. For a typical satellite constellation this error will increase by about (less if satellites are close together, more if satellites are all near the horizon). If positional calculation was done using this clock and only using three satellites, just standing still the GPS would indicate that you are traveling at a speed in excess of 300 meters per second, (over 1000 km/hour or 600 miles an hour). With only signals from three satellites the GPS receiver would not be able to determine whether the 300m/s was due to clock error or actual movement of the GPS receiver. If the satellites being used are scattered throughout the sky, then the value of geometric dilution of precision (GDOP) is low while if satellites are clustered near each other from the receiver's vantage point the GDOP values are higher. The lower the value of GDOP then the better the ratio of position error to range error computing will be, so GDOP plays an important role in calculating the receiver's position on the surface of the earth using pseudoranges. The larger the number of satellites, the better the value of GDOP will be. See also Pseudo-range multilateration References Peter J. G. Teunissen and Alfred Kleusberg, GPS observation equations and positioning concepts, Lecture Notes in Earth Sciences, 1996, volume 60/1996, pages 175-217 Geodesy Satellite navigation