How GPS brings time to the world
Knowing the correct time is something we take for granted but who is it that decides what the correct time is? How do they determine it? And where does GPS fit in to the story?
The short answer: the International Bureau of Weights and Measures (BIPM, Paris), as well as telling us the length of 1 metre also tell us what time it is. To determine the time, they rely on the contributions from a worldwide collaboration of timing laboratories who each maintain their own measure of time and compare it with GPS time.
One clock to rule them all
Timing labs employ precise clocks, with Cesium atomic clocks and Hydrogen masers being among the most popular. Although these clocks are very reliable (accurate to about 2 nanoseconds per day) there are still small variations. At BIPM in Paris, they compare the performance of clocks in timing labs around the world and, by using a weighted average of all contributions, calculate what is known as UTC (Coordinated Universal Time). Labs with better performing or more stable clocks are given more weight in the calculation of UTC.
This means that real-time UTC is only an approximation albeit a very accurate one, with the more precise calculation determined retrospectively. The Circular-T journal, published monthly by BIPM contains the small corrections to be applied to UTC during the previous month.
Where do GPS receivers fit in?
Each timing lab contributing to UTC measures its own version of UTC for example, UTCBrussels is the Belgian measure of UTC. So how does BIPM compare the performance of all these different clocks? The answer is that it uses the GPS receivers or more accurately, GNSS (Global Navigation Satellite System) receivers which besides GPS, also track constellations such as GLONASS, Galileo, BeiDou and IRNSS.
The precise measurement of time is at the heart of every GPS receiver. The distances between satellite and receiver, used to calculate position, are determined by measuring the transit times of the satellite signals to the receiver. An error of 1 nanosecond in the transit time translates into an error of 30cm in the distance. The GPS satellite constellation uses its own precise measure of time called GPS time with each satellite having its own, on-board set of atomic clocks. Satellites can thus be viewed as very accurate flying clocks.
By tracking a GPS satellite, a receiver can record the time differences between its own receiver clock and the satellite clock, e.g. UTCBrussels - GPS time. These time differences, along with other information, are collected in a data format called CGGTTS and sent to BIPM. Using CGGTTS and other data, BIPM can compare a clock in Brussels with a clock in New York by subtracting the individual differences with GPS time: a technique known as "common view".
UTCBrussels - UTCNew York = (UTCBrussels - GPS time) - (UTCNew York - GPS time)
The two GPS time terms above cancel each other out leaving the difference between UTCBrussels and UTCNew York.
Setting up a timing laboratory
To compare the atomic clocks used in timing labs around the world, they need to be connected to a GPS timing receiver. This is a special type of receiver that can use an external atomic clock instead of its own clock which is does using two output signals from the atomic clock:
- a pulse every second synchronised to UTC (PPS IN) and
- a 10 MHz frequency reference that is essentially a sine wave (REF IN)
The basic ingredients of a timing laboratory are shown in Figure 3. However, to reach the nanosecond accuracies required, a great deal of expertise and preparation are also needed. Signal delays in all elements in the setup should be accurately calibrated and for this, BIPM maintains a set of pre-calibrated travelling receivers as calibration references. As well as providing 1/3 of the timing receivers used for the calculation of UTC, Septentrio also provides BIPM with timing receivers for calibration.
Pushing the boundaries of science
Beyond defining and disseminating UTC, recent years have also witnessed GPS timing receivers staking their place at the forefront of science. In the case of the T2K experiment for example, by measuring precisely the transit time of neutrinos between two locations, limits can be placed on their mass thus shedding more light on the nature of these elusive particles.
At the other end of the size spectrum, the technique of VLBI (Very-Long-Baseline Interferometry) uses radio telescopes at distant locations which are linked together in networks by time-synching their observations using GPS common view. The resulting resolution is far in excess of anything that can be achieved by any single telescope on its own.
From the relatively mundane activity of time-stamping banking transactions to the truly extraordinary worlds of astronomy and high-energy physics, GPS technology continues to find new ways to improve our world and advance our knowledge of it.