![]() Advertisements for quartz watches of the 1970s touted its accuracy, the best at the time. The pendulum used mechanical checks, the quartz clock depended on electronics to control the vibrations of a crystal, keeping accurate time to the thousandths of a second. Image courtesy of the National Physical Laboratory.Īt the heart of all clocks is the ability to produce consistent oscillations for time units. The world's first cesium atomic clock, developed in 1955. Rooted in quantum physics and based on the difference in energy states of the outer electron of the cesium-133 atom, the definition of a second became "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium 133 atom." No longer tied to the Earth's movement, this second was based on an element of the Earth itself. a number of cycles of the cesium-133 atom: 9,192,631,770 (1967, the atomic second)Īt the 1967 General Conference on Weights and Measures held in Paris, delegates from 36 countries agreed to redefine a second based on the oscillations of radiation emitted in the outer electron layer of a cesium atom.a fraction of an average solar day: 1/86,000 of a mean solar day (1940s, the mean solar second).The definition of the second, the fundamental time unit of basic physical systems, changed starting from the 1300s when the day was declared to be 24 equal hours by King Charles of France, to more precise units as mechanical clocks, pendulums, quartz clocks, and atomic clocks developed. Timekeeping has been provided by various means as science and engineering progressed. Figure 1 shows a timeline.įigure 1. The time standard in the USA has been set first by the National Bureau of Standards, now known as the National Institute of Standards and Technology (NIST). GPS time is counted in Cycles, Weeks, Days, and Seconds.įor the curious, you can see a live comparison of local, UTC, GPS, Loran and TAI times here. From its start on midnight between January 5th and 6th of 1980, GPS time has been a continuous count of the seconds since that date.Īs shown in Table 1, when GPS time was initiated in 1980, UTC and GPS time were the same, growing further apart as leap seconds accumulated through the years. Satellites have no need for a "leap" second or other corrections. GPS time, by comparison, does not need to reflect the Earth's movements. Our calendars have regularly scheduled "leap years" and occasionally a " leap second" is inserted (the last one in December 2016). We are all familiar with the corrections needed for the Earth's revolutions. UTC must account for the Earth's passage through the seasons and years. "GPS time" differs from Earth-based time systems like Coordinated Universal Time (UTC). GPS as we know it could not exist without the atomic clock. Space-based timing led to a new generation of GPS, with high precision atomic clocks placed on each satellite. In 1964, Roger Easton realized that by putting a clock on satellites (first launched in the late 1950s) a single source could transmit time to both transmitter and receiver. This required a precision timescale to measure and synchronize the transmitted and received signals. Satellite tracking systems transmitted a continuous wave from a ground-based transmitter and detected echoes from passing satellites. The Minitrack system, as it was called, compared different angles of incoming radio signals at paired antennas. The first global positioning system developed by Guier and Weiffenbach was based on the Doppler shift, determining position based on the frequency changes of the satellite's broadcast signals. The US Navy traditionally used navigation angles in reference to the stars. Position location and satellite tracking systems did not always rely on the precise timing of the atomic clocks. GPS time is used to synchronize wireless communications and timestamp financial transactions it's used by digital broadcasters, Doppler radars, and many scheduling apps. In addition to positioning data, GPS atomic clocks are so precise that GPS has become the time standard for many applications. Multiplied by the speed of light, c, the distance from the receiver to the satellite is determined. That delay becomes the travel time from the satellite. The GPS receiver finds a signal, syncs to it, and then uses its own oscillator to determine the delay in reception. GPS receivers with specialized software and mapping applications determine distances used to triangulate the receiver location. The US Global Positioning System (GPS) provides position, navigation, and timing (PNT) signals that broadcast 3D positions (longitude, latitude, altitude) and time for each satellite. This article looks at the importance of timing for GPS and the clocks that provide it. GPS as we know it requires the precision of atomic clocks.
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