GPS requires extremely accurate time to provide accurate positions over such long distances. Time can be transferred between points using special techniques. A better discussion of this subject is available at the U.S. Naval Observatory.
Coordinated Universal Time (UTC)
Coordinated universal time, sometimes called Greenwich Mean Time, is a uniform time scale with leap seconds inserted to keep it in synch with the earth's rotation. In the US, time is kept by the U.S. Naval Observatory
GPS Time
Internally in the Navstar system, time is kept as GPS time. GPS Time began on January 6, 1980 and is referenced in GPS Weeks and seconds. GPS time is a composite time composed of the times of all available satellite and monitor station clocks. It is monitored by the GPS Operational Control System and by the U.S. Naval Observatory and is steered to keep it within 1 microsecond of UTC. However, leap seconds are not inserted so GPS time lags behind UTC. The exact difference is provided as constants in the GPS navigation message. By using the information given in the NAV message, the user can refer time via GPS directly to UTC (as determined by the USNO) with a precision of better than 340 nanoseconds (95% probability) using the standard positioning service and 100 nanoseconds using the precise positioning service.
- BIH
- The world standards for time and frequency are maintained by the BIH in France.
- U.S. Naval Observatory
- The USNO is the master timekeeper for the United States. The primary mission of the USNO has always been to maintain star charts and timekeeping information in support of navigation and time transfer. Periodically, the USNO time standards are compared to the BIH standards in France, maintaining international compatibility.
- National Institute of Science and Technology
- The NIST is the keeper of frequency standards for the United States. The NIST was formerly known as the National Bureau of Standards (NBS). NIST's primary mission is to promote U.S. economic growth by working with industry to develop and apply technology, measurements, and standards. It was established by Congress to assist industry in the development of technology.
A better and more detailed description of time scales is available from the U.S. Naval Observatory. Much of the information posted here originates from that source.
- Sidereal Time
- Sidereal time has units equal to the period of the earth's rotation with respect to a point nearly fixed with respect to the stars. Sidereal time varies with the rotation of the earth.
- Atomic Time
- Atomic time, with units defined in S.I. (System Internationale) seconds, is defined as the duration of 9,192,631,700 cycles of radiation corresponding to the transition between two hyperfine levels of the ground state of cesium 133. The International Atomic Timescale (TAI) is a statistical timescale composed of measurements from many atomic clocks
- Universal Time
- Universal time has a duration of a mean solar day. It is defined to be as uniform as possible despite minor variations in the rotation of the earth. There are two versions of UT. UT0 is the rotational timescale of a particular place of observation. It is observed as the motion of stars or extraterrestrial radio sources. UT1 is computed by correcting UT0 for the effect of polar motion on the longitude of the observing site. It varies from uniformity because of the irregularities in the Earth's rotation.
- UTC
- UTC, or Coordinated Universal Time is based on the International Atomic Timescale but varies by a number of seconds. UTC is steered to stay within 0.9 seconds of UT1 by the inclusion of leap seconds. Leap seconds are usually added rather than subtracted.
- GPS Time
- GPS time is a composite time composed of measurements from atomic clocks on each of the GPS satellites as well as the master stations. GPS is constant and does not have leap seconds inserted. GPS receivers that supply UTC time as an output do so by adding broadcast time correction factors to the broadcast GPS time. GPS time is measured by the USNO which provides corrections to the NAVSTAR master control station (MCS).
- FAA
- IRIG
- The Inter-Range Instrumentation Group (IRIG) codes were developed to support the transfer of missile test range data between the ranges and industry, educational and government laboratories.
- IRIG-A
- IRIG-B
- IRIG-C
- NASA
Clocks used in telecommunications are generally used as frequency standards rather than time pieces. Long term accuracy is quite important for digital network syncronization. Special terms have developed regarding different degrees of long term accuracy.
- Stratum 1
- A Stratum 1 clock has a long-term accuracy of 10E-11. Stratum 1 clocks are generally used for syncronizing a few master sites in a digital telecommunications network. The syncronized signals propagate the time standard throughout the network.
- Stratum 2
- A Stratum 2 clock has a long-term accuracy of 10E-10
- Stratum 3
- A Stratum 3 clock has a long-term accuracy of 10E-9
- Windows NT Resource Kit
- Want to syncronize the clock on your PC?
I was looking through the Windows NT 3.51 Resource Kit the other day and what did I find but a time syncronization utility. On reading through the documentation, I found that it would accept output from a GPS receiver! The docs indicated that it would accept the output of Trimble and Rockwell receivers.
- Tom Clark's Totally Accurate Clock project
- Tom Clark from NASA's (?) has developed a high quality timing module from a low cost GPS receiver. The Totally Accurate Clock (TAC) project began as a home project, but was found to have applications in the scientific community as well. Incidentally, the name "Totally Accurate Clock" is a take-off on the old Heathkit "Most Accurate Clock" that syncronized itself to WWV signals. The TAC uses a Motorola Oncore Basic equipped with Option A (timing) and a small home built PC board containing additional buffering circuitry and signal amplification.
Tom's comments on performance:
"At the Haystack meeting I showed some preliminary results from the first two prototype TAC's running against two different Hydrogen Masers. Since that time I did a 2 week long run sampling every 5 minutes. In that period there were ZERO spurious readings, and the RMS of fit to a straight line (Maser offset 2.13E-13) was 44.9 nsec on an HP5345 computing counter (which has 2 nsec internal clock)."Details and software for the TAC are available by FTP. People that have additional questions about the TAC can read the FAQ."A few comments about this performance: Each 5 minute sample in this test was the average of 4-5 GPS pulses. In the PVT-6, the 1 PPS output is derived from a ~10 MHz oscillator with no phase resolver on its output. Hence individual pulses are uniformly distributed over a +/- ~50 nsec window. Averaging a few individual pulses in the counter reduces this jitter to a level comparable to the basic instrumental precision."