100 Years of Relativity Space-Time Structure: Einstein and Beyond ...

258 N. Ashbyincludes an almanac for the entire satellite constellation, information aboutsatellite vehicle health, and information from which Universal CoordinatedTime as maintained by the U. S. Naval Observatory–UTC(USNO)–can bedetermined.The GPS “Control Segment” includes a number of ground-based monitoringstations, which continually gather information from the satellites.These data are sent to a Master Control Station in Colorado Springs, CO,which analyzes the constellation and predicts the satellite ephemerides andclock behavior for the next few hours. This information is then uploadedinto the satellites for retransmission to users. The “User Segment” consistsof all users who, by receiving signals transmitted from the satellites, are ableto determine their position, velocity, and the time on their local clocks.The GPS is a navigation and timing system that is operated by theUnited States Department of Defense (DOD), and therefore is partly classified.Several organizations monitor GPS signals independently and provideservices from which satellite ephemerides and clock behavior can be obtained.Position accuracies in the neighborhood of 5-10 cm are not unusual.Carrier phase measurements of the transmitted signals are commonly doneto better than a millimeter.GPS signals are received on earth at two carrier frequencies, L1 (154 ×10.23 MHz) and L2 (120×10.23 MHz). The L1 carrier is modulated by twotypes of pseudorandom noise codes, one at 1.023 MHz that repeats everymillisecond–called the Coarse/Acquisition or C/A code–and an encryptedone at 10.23 MHz called the P-code. P-code receivers have access to both L1and L2 frequencies and can correct for ionospheric delays, whereas civilianusers only have access to the C/A code. An additional frequency for civilianusers is being planned. There are thus two levels of positioning serviceavailable in real time, the Precise Positioning Service utilizing P-code, andthe Standard Positioning Service using only C/A code. The DOD has thecapability of dithering the transmitted signal frequencies and other signalcharacteristics, so that C/A code users would be limited in positioningaccuracy to about ±100 meters. This is termed Selective Availability, orSA. SA was turned off by order of President Clinton in May, 2000.Technology developments in the late 1960s and early 1970s that directlybenefited the GPS included the Transit system, developed by theJohns Hopkins Applied Physics Laboratory, the Naval Research Laboratory’sTimation satellites, and the U. S. Air Force Project 621B. 20 TheTransit system relied on the Doppler shift of continuously transmitted tonesfrom satellites in polar orbits to determine user position; development of