USNO GPS DATA FILES
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 100 meters (95 percent) horizontally and 156 meters (95 percent) vertically and time transfer accuracy to UTC within 340 nanoseconds (95 percent).For questions regarding GPS policy, the user is advised to refer to the regularly appearing FRP. The FRP is published every 2 years and is available from the National Technical Information Service, Springfield, VA 22161. The latest report number is DOT-VNTSC-RSPA-95-1/DOD-4650.5 for report date 1994.
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 at least 22 meters (95 percent) horizontally and 27.7 meters vertically and time transfer accuracy to UTC within 200 nanoseconds (95 percent). PPS will be the data transmitted on the 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. and military and U.S. Federal Government users. 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.
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.
SA 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).
The SPACE segment consists of 24 operational 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 4 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 Schriever 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 USER segment consists of antennas and receiver-processors that provide positioning, velocity, and precise timing to the user.
The latest individual satellite measurements are updated daily. (Data format explanation.)
The best current measure of the difference, UTC(USNO MC) - GPS is based on filtered and smoothed data over the past two days.