GNSS Time Transfer
One of the main ways laboratories and scientific institutions can obtain time traceable back to UTC(NPL) is via Global Navigation Satellite Systems (GNSS) such as GPS. NPL also uses this technique to contribute to world time, UTC.
Having highly accurate clocks at NPL is not very useful unless that time information is distributed to the public somehow. This introduces the idea of Time Transfer – time information is transferred from one location to the other. One of the ways NPL makes available UTC(NPL) is by our high quality GPS receivers.
The US Global Positioning System (GPS) uses highly accurate atomic clocks to help users determine their position. The GPS satellites broadcast their own timescale, GPS Time, which users on the earth compare against the clocks in their own GPS receivers. The 'time of flight' is multiplied by the speed of light in vacuum to determine the range to the satellite.
Part of this process involves making a measurement between the user's clock and GPS Time. NPL uses GPS receivers that take a time signal from UTC(NPL), and so this measurement gives traceability between GPS Time and UTC. If another user does the same type of measurement with their own clocks, and then subtracts these two numbers, then the user can obtain their time offset from UTC(NPL).
NPL uses this technique to compare its atomic clocks against those of other laboratories around the world; principally with those at the BIPM so that UTC(NPL) can contribute to UTC, but also for research purposes with other laboratories. The fact that the receivers' antennae are stationary allows averaging over time, which lowers the uncertainty significantly. This technique will also work with other GNSS systems, such as Galileo and GLONASS.
As you would expect, atmospheric effects cause noise in the measurements. The principal effect is due to the ionosphere, where solar radiation causes free electrons in the top part of the atmosphere to delay the signal. Other sources of error are the uncertainty in the satellite position, multipath (signals bouncing off objects before reaching the receiver), and the variable delay through the lower half of the atmosphere, called the troposphere.
The uncertainty in UTC(NPL) – GPS Time is around 10 ns (1σ) for a standard receiver. More expensive receivers, essentially modified surveying receivers, can drastically reduce the ionosphere delays by using both of the GPS broadcast frequencies, giving around 3 to 4 ns (1σ) uncertainty.
NPL currently has 2 geodetic-quality GPS receivers. One of them is an Ashtech Z12-T receiver, which also contributes observation data as NPLD to the International GNSS Service (IGS), an international body that provides high-accuracy GPS satellite orbit and site data.
NPL is currently researching how to reduce the uncertainty in GPS Time Transfer by modelling other error sources, as part of a collaboration with University College London. For more information about our activities, please contact Peter Whibberley.