USNO NAVSTAR Global Positioning System Information
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USNO NAVSTAR Global Positioning System
The following Global Positioning System (GPS) information is obtained from the Federal Radionavigation Plan (FRP), prepared jointly by the Department of Defense (DoD) and the Department of Transportation (DoT) and other sources such as conferences, meetings and seminars.
The GPS is a DoD-developed, worldwide, satellite-based radionavigation system that is the DoD's primary radionavigation system. The current constellation consists of up to 32 operational satellites. The U.S. Air Force Space Command (AFSC) formally declared the GPS satellite constellation as having met the requirement for Full Operational Capability (FOC) as of April 27, 1995. Requirements include 24 operational satellites (Block IIR, IIR-M, IIF, and IIIA) functioning in their assigned orbits and successful testing completed for operational military functionality.
Prior to FOC an Initial Operational Capability (IOC) was declared on December 8, 1993 when 24 GPS satellites (Block I and Block II/IIA) were operating in their assigned orbits, available for navigation use and providing the Standard Positioning Service (SPS) levels specified below.
GPS provides two levels of service, Standard Positioning Service and the Precise Positioning Service .
The Standard Positioning Service (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 provides a predictable positioning accuracy of 9 meters (95 percent) horizontally and 15 meters (95 percent) vertically and time transfer accuracy to UTC within 40 nanoseconds (95 percent).
The Precise Positioning Service (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 U.S. P(Y) code capable military user equipment provides a predictable positioning accuracy of 2.7 meters (95 percent) horizontally and 4.9 meters vertically and time transfer accuracy to UTC within 40 nanoseconds or better (95 percent). PPS is the data transmitted on the GPS L1 and L2 frequencies. PPS is designed primarily for U.S. military use. It will be denied to unauthorized users by the use of cryptography. PPS is made available to U.S. and military and U.S. Federal Government users. Limited, non-Federal Government, civil use of PPS, both domestic and foreign, is 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 Federal Radionavigation Plan (FRP). The latest report can be found at the following NAVCEN link: http://www.navcen.uscg.gov/?pageName=pubsMain.
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 1 millisecond (ms) and is used primarily to acquire the P-code.
The precision (P) code has a 10.23 MHz rate, a period of 7 days and is the principal 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 was implemented on Block II at the SPS levels, as soon as each Block II satellite was operational. SA was activated July 4, 1991 at 0400 UT (ref: Notice Advisory to NAVSTAR Users 121-92282 DTG 011354Z JUL 91 ). SA was permanently deactivated May 2, 2000. 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).
GPS SYSTEM SEGMENTS
The GPS consists of three major segments: SPACE, CONTROL and USER.
The SPACE segment consists of 24-32 operational satellites in six orbital planes (4-6 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 approximately 4 minutes earlier each day.
The CONTROL segment consists of 17 Monitor Stations, three Ground Antennas, and a Master Control Station (MCS) located near Colorado Springs. 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 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). The CC or "paper" clock consists of all operational Monitor Station and satellite frequency standards. GPS system time, in turn, is referenced to the Master Clock (MC) at the USNO and steered to UTC(USNO) from which system time will not deviate by more than one microsecond. 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 as maintained by the BIPM, and the exact difference, USNO vs. BIPM is available in near real time.
GPS TIME TRANSFER
GPS is at the present time the premier system for widespread time transfer. 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 (i.e. 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.