UNITED STATES NAVAL OBSERVATORY (USNO) AUTOMATED DATA SERVICE (ADS) File GPSSY -- last updated <950605.1837> GLOBAL POSITIONING SYSTEM INFORMATION ------------------------------------- The following Global Positioning System (GPS) policy and system segment information are obtained from the 1992 Federal Radionavigation Plan (FRP), prepared jointly by the Department of Defense (DoD) and the Department of Transportation (DoT) and other sources such as meetings and conferences. GPS POLICY The GPS is a DoD developed, worldwide, satellite-based radionavigation system that will be the DoD's primary radionavigation system well into the next century. The constellation will consist of 24 operational satellites. GPS Full Operational Capability (FOC) will be declared by the Secretary of Defense when 24 operational satellites (Block II/IIA) are functioning in their assigned orbits and when the constellation has successfully completed testing for operational military functionality. Military FOC is planned in 1995. Editor's note: According to the June 1995 issue of GPS WORLD magazine, The U.S. Air Force Space Command (AFSC) has formally declared the GPS satellite constellation as having me the requirement for full operational capability (FOC) as of April 27, 1995. Prior to FOC an Initial Operational Capability (IOC) will be attained when 24 GPS satellites (Block I/II/IIA) are operating in their assigned orbits, are available for navigation use and can provide the levels of service specified below. Editor's note: GPS IOC has been achieved, and a letter dated December 8, 1993 from Secretary of Defense Aspin notified Secretary of Transportation Pena of this event. GPS will provide two levels of service - a Standard Positioning Service (SPS) and a Precise Positioning Service (PPS). SPS POLICY: SPS is a positioning and timing service which will be available to all GPS users on a continuous, worldwide basis with no direct charge. SPS will be provided on the GPS L1 frequency which contains a coarse acquisition (C/A) code and a navigation data message. SPS is planned to provide, on a daily basis, the capability to obtain horizontal positioning accuracy within 100 meters (2 drms, 95 percent probability) and 300 meters (99.99 probability), vertical positioning accuracy within 140 meters (95 percent probability), and timing accuracy within 340 nanoseconds (95 percent probability). The GPS L1 frequency also contains a precision (P) code that is not a part of the SPS. PPS POLICY: PPS is a highly accurate military positioning, velocity, and timing service which will be available on a continuous, worldwide basis to users authorized by the DoD. PPS will be the data transmitted on GPS L1 and L2 frequencies. PPS was designed primarily for U.S. military use. It will be denied to unauthorized users by the use of cryptography. PPS will be made available to U.S. Federal and Allied Government (civil and military) users through special agreements with the DoD. Limited, non-Federal Government, civil use of PPS, both domestic and foreign, will be considered upon request and authorized on a case-by-case basis, provided: * It is in the U.S. national interest to do so. * Specific GPS security requirements can be met by the applicant. * A reasonable alternative to the use of PPS is not available. For questions regarding GPS policy, the user is advised to refer to the regularly appearing FRP. The FRP is published every two years and is available from the National Technical Information Service, Springfield, VA 22161. The latest report number is DOT-VNTSC-RSPA-92-2/DOD-4650.5 for report date 1992. GPS SIGNAL CHARACTERISTICS The satellites transmit on two L-band frequencies: L1 = 1575.42 MHz and L2 = 1227.6 MHz. Three pseudo-random noise (PRN) ranging codes are in use. The coarse/acquisition (C/A) code has a 1.023 MHz chip rate, a period of one millisecond (MS) and is used primarily to acquire the P-code. The precision (P) code has a 10.23 MHz rate, a period of seven days and is the principle navigation ranging code. The Y-code is used in place of the P-code whenever the anti-spoofing (A-S) mode of operation is activated. The C/A code is available on the L1 frequency and the P-code is available on both L1 and L2. The various satellites all transmit on the same frequencies, L1 and L2, but with individual code assignments. Due to the spread spectrum characteristic of the signals, the system provides a large margin of resistance to interference. Each satellite transmits a navigation message containing its orbital elements, clock behavior, system time and status messages. In addition, an almanac is also provided which gives the approximate data for each active satellite. This allows the user set to find all satellites once the first has been acquired. SELECTIVE AVAILABILITY, ANTI-SPOOFING Selective Availability (SA), the denial of full accuracy, is accomplished by manipulating navigation message orbit data (epsilon) and/or satellite clock frequency (dither). Anti-spoofing (A-S) guards against fake transmissions of satellite data by encrypting the P-code to form the Y-code. SA will not be implemented on the Block I and will be implemented on Block II at the SPS levels, as soon as each Block II satellite is operational. SA was activated July 4, 1991 at 0400 UT (ref: Notice Advisory to NAVSTAR Users 121-92282 DTG 011354Z JUL 91). A-S was exercised intermittently through 1993 and implemented on January 31, 1994 (ref: Notice Advisory to NAVSTAR Users 050-94042, DTG 112054Z FEB 94). A-S does not apply to the Block I satellites. Requests for an "SA/A-S free vehicle" have been denied. GPS SYSTEM SEGMENTS The GPS consists of three major segments: SPACE, CONTROL and USER. The SPACE segment, when fully operational, will be composed of 24 satellites in six orbital planes, (four satellites in each plane). The satellites operate in circular 20,200 km (10,900 nm) orbits at an inclination angle of 55 degrees and with a 12-hour period. The position is therefore the same at the same sidereal time each day, i.e. the satellites appear four minutes earlier each day. The CONTROL segment consists of five Monitor Stations (Hawaii, Kwajalein, Ascension Island, Diego Garcia, Colorado Springs), three Ground Antennas, (Ascension Island, Diego Garcia, Kwajalein), and a Master Control Station (MCS) located at Falcon AFB in Colorado. The monitor stations passively track all satellites in view, accumulating ranging data. This information is processed at the MCS to determine satellite orbits and to update each satellite's navigation message. Updated information is transmitted to each satellite via the Ground Antennas. The Monitor Station coordinates have been precisely surveyed with respect to the World Geodetic System 1972, otherwise referred to as WGS-72. The new standard is the WGS-84 and the transition took place on January 10, 1987. The USER segment consists of antennas and receiver-processors that provide positioning, velocity and precise timing to the user. GPS SYSTEM TIME GPS system time is given by its Composite Clock (CC), that was implemented on June 17, 1990 at 0000 UT. The CC or "paper" clock consists of all operational Monitor Station and satellite frequency standards. The system time, in turn, is referenced to the Master Clock at the USNO. The standard tolerance is one microsecond but during the last several years has been within a few hundred nanoseconds. The exact difference is contained in the navigation message in the form of two constants, A0 and A1, giving the time difference and rate of system time against UTC(USNO,MC). UTC(USNO) itself is kept very close to the international benchmark UTC(BIPM), the exact difference is available in near real time on the ADS of the USNO or can be obtained via return email from the ADS Postmaster. For details see email from the main menu. GPS TIME TRANSFER GPS is at the present time the most competent system for the distribution of Precise Time and Time Interval (PTTI). The system uses time of arrival (TOA) measurements for the determination of user position. A precisely timed clock is not essential for the user because time is obtained in addition to position by the measurement of TOA of FOUR satellites simultaneously in view. If altitude is known (e.g., for a surface user), then THREE satellites are sufficient. If time is being kept by a stable clock (say, since the last complete coverage), then TWO satellites in view are sufficient for a fix at known altitude. If the user is, in addition, stationary or has a known speed then, in principle, the position can be obtained by the observation of a complete pass of a SINGLE satellite. This could be called the "transit" mode, because the old TRANSIT system uses this method. In the case of GPS, however, the apparent motion of the satellite is much slower, requiring much more stability of the user clock. CURRENT GPS CONSTELLATION (BLOCK I/II/IIA) The current GPS constellation consists of 25 satellites. Eleven Block I satellites have been launched and one remains in full service. Block I reflects various stages of system development and are not identical with the operational satellites (Block II). The first Block II satellite was launched in February 1989; twenty-four Block II/IIA satellites have been launched to date. The remaining Block I satellite remains incorporated with the present constellation.