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Time and Frequency from A to Z: G
One billion cycles per second (109 Hz).
Global Positioning System (GPS)
A constellation of satellites controlled and operated by the United States Department of Defense (USDOD). The constellation includes at least 24 satellites that orbit the Earth at a height of 20,200 km in six fixed planes inclined 55° from the equator. The orbital period is 11 h 58 m, which means that a satellite will orbit the earth twice per day. By processing signals received from the satellites, a GPS receiver can determine its own position with an uncertainty of < 10 m.
The GPS satellites broadcast on two carrier frequencies: L1, at 1575.42 MHz, and L2, at 1227.6 MHz. Each satellite broadcasts a spread-spectrum waveform, called a pseudo-random noise (PRN) code on L1 and L2, and each satellite is identified by the PRN code it transmits. There are two types of PRN codes. The first type is a coarse acquisition (C/A) code with a chip rate of 1023 chips per millisecond. The second type is a precision (P) code with a chip rate of 10230 chips per millisecond. The C/A code is broadcast on L1, and the P code is broadcast on both L1 and L2. GPS reception is line-of-sight, which means that the antenna must have a clear view of the sky. The signals can be received nearly anywhere on Earth where a clear sky view is available.
The primary purpose of GPS is to serve as a radionavigation system, but it has also become perhaps the dominant system for the distribution of time and frequency. Each satellite carries either rubidium or cesium oscillators, or a combination of both. The on-board oscillators provide the reference for both the carrier and code broadcasts. They are steered from USDOD ground stations and are referenced to Coordinated Universal Time (UTC) maintained by the United States Naval Observatory (USNO). By mutual agreement UTC(USNO) and UTC(NIST) are maintained within 100 ns of each other, and the frequency difference between the two time scales is < 1 x 10-13.
There are several types of time and frequency measurements that involve GPS, including one-way, common-view, and carrier-phase measurements. To view one-way GPS data received at Boulder and compared to UTC(NIST), please visit the GPS data archive.
The Global Navigation Satellite System operated by the Soviet Federation as a space-based navigation system. GLONASS is similar in some ways to GPS, and is sometimes used as a reference or common-view source for time and frequency measurements. The GLONASS satellites use two frequency bands: 1602.5625 to 1615.5 MHz and 1240 to 1260 MHz. The satellites carry on-board cesium oscillators.
An acronym for the Geostationary Operational Environmental Satellites operated by the National Oceanic and Atmospheric Agency (NOAA). These satellites were used to broadcast a NIST time code from 1974 until December 31, 2004, when the service was discontinued. To download an article covering the history of the GOES satellite time code service, click here.
Greenwich Mean Time (GMT)
A 24-hour time keeping system whose hours, minutes, and seconds represent the time-of-day at the Earth's prime meridian (0° longitude) located near Greenwich, England. Technically speaking, GMT no longer exists, since it was replaced by other astronomical time scales many years ago, and those astronomical times scales were subsequently replaced by the atomic time scale UTC. However, the term GMT is still incorrectly used by the general public. When heard today, it should be considered as a synonym for UTC.
In radio transmission, a wave that propagates close to the surface of the Earth. Groundwave propagation is a characteristic of low frequency (LF) radio signals. Since the propagation or path delay of a groundwave signal remains relatively constant, LF signals tend to be a better time and frequency reference than high frequency (HF) signals, which are often dominated by skywave.