Clock Error and Pseudo Range
We assumed in the previous discussion that both the satellite and the UE set were generating identical pseudo codes at exactly the same time. Practically speaking, this is not the case. Each satellite carries an atomic clock accurate to 10-9 seconds. Achieving maximum accuracy in synchronizing the codes would require all users to carry atomic clocks with comparable accuracies, significantly increasing both the size and cost of each receiver set. As a compromise, each UE set is equipped with a quartz crystal clock.
Since the accuracy of a quartz crystal clock cannot approach that of an atomic clock, there is a difference between satellite GPS system time and UE set time. As a result, the generation of the two pseudo codes is not perfectly synchronized and a ranging error is induced. Instead of determining actual range, we measure the apparent, or pseudo range, to the satellite. This particular problem area is known as clock bias. Clock bias affects all range measurements equally. The problem is determining the amount of bias error. Using three satellites allows us to determine our position in three dimensions. By using a fourth satellite and comparing pseudo codes, the UE set internally determines the amount of adjustment necessary to make all of the measurements agree.
Satellite Clock Error
It might be safe to assume that since each satellite carries an atomic clock, it would keep extremely accurate time. Since the compact dimensions of an orbiting satellite limit the clock size, its accuracy does not approach that of ground based atomic clocks. As a consequence, there is some error in each satellite’s clock when compared with master GPS system time. The satellite’s generation of the pseudo code is slightly out of synch and some ranging error is induced. This problem is known as satellite clock error. To compensate for this type of error, the GPS control segment comes into play. Monitor stations evaluate the accuracy of the satellite’s clock and its pseudo code generation. This information is then relayed to the master control station where the necessary corrections to the satellite’s transmissions are computed. Updated information is then uploaded to the satellite via the ground antennas.
Ephemeris is the ability to determine the location of a celestial body (in this case a satellite) at regular time intervals. Ephemeris error then is caused by the satellite not being exactly where we thought it was. By using estimation theory techniques, the computers at the master control station predict what the satellite’s position should be at a specific time. This predicted position is then compared with the actual position as determined by the monitor stations. Updated information on the satellite’s future position is then uploaded to each satellite on a regular basis via the ground antennas. Each satellite then continuously transmits these corrections to all users. In this way, ranging error caused by uncertainty as to the satellite’s exact position is minimized.
Atmospheric Propagation Error
We assumed that the satellite’s RF signal traveled at the speed of light, as it does in a vacuum like space. But just as light is refracted through a prism, the RF signal is bent and slowed down as it enters the ionosphere. The degree to which the signal is affected depends on the atmospheric conditions between the satellite and receiver and on the signal’s angle as it passes through the ionosphere. Atmospheric propagation error can cause position uncertainties up to 40 meters. By noting the time delay between the two L-Band signals, much of the effect caused by atmospheric propagation can be removed internally by the UE set. Since only the military is capable of simultaneously monitoring both of the frequencies, civilian users are forced to live with this error.