Tracking Stations

Figure 1-6.—Satellite orbits. run out. Each two-minute long satellite message is timed so that the end of the 78th bit, which is the last bit of the second synchronization signal, coincides with even 2 minutes of Greenwich mean time (GMT). Thus the satellites can also be used as an accurate time reference by all navigators equipped with a satellite navigation set. Each satellite is designed to receive, sort, and store data transmitted from the ground and to retransmit this data at scheduled intervals as it circles the earth. Each satellite tells users which satellite it is, the time according to the satellite clock, and its present location. With this information, the user’s navigation set can determine exactly where the satellite is, one of the necessary steps toward determining a precise navigational position. Tracking Stations Tracking stations are located in Maine, Minnesota, California, and Hawaii. As each satellite passes within radio line-of-sight (los) of each of these tracking stations, it is tracked to accurately determine its present and future orbits. Just before predicted satellite acquisition, the tracking station’s antenna is pointed toward the satellite to acquire its signals. As the satellite rises above the horizon, the tracking antenna continues to follow the satellite’s predicted path until the radio receiver in the tracking station locks on to the satellite’s transmitted signal. The receiver processor and data processing equipment decode and record the satellite message. The Doppler tracking signal is digitized and sent with the satellite time measurements to the computing center, via a control center, where a refined orbit is calculated. The tracking stations maintain highly stable oscil- lators that are continually compared against a WWV transmitted frequency standard. In addition, the Naval Observatory sends daily messages that give the error in the transmitted standard. The Naval observatory error is then added to the data obtained from the frequency standard, and corrections are made to the station oscillators. The station oscillators are used to drive station clocks, which are compared with the time marks received from the satellite. This time data is transmitted by the tracking stations to the control center, where the satellite clock error is calculated and the necessary time correction bits are added or deleted in the next injection message to the satellite. Computing Center The central computing center continually accepts satellite data inputs from the tracking stations and the Naval Observatory. Periodically, to obtain fixed orbital parameters for a satellite, the central computing center computes an orbit for each satellite that best fits the Doppler curves obtained from all tracking stations. Using this computed orbital shape, the central computing center extrapolates the position of the satellite at each even 2-minutes in universal time for the 12 to 16 hours subsequent to data injection. These various data inputs are supplied to the injection stations via the control center, as is data on the nominal space of the orbits of the other satellites, commands and time correction data for the satellite, and antenna pointing orders for the injection station antennas. Injection Station The injection stations, after receiving and verifying the incoming message from the control center, store the message until it is needed for 1-7