National Physical Laboratory

Time & Frequency Analysis

GSTB-V2/B in Orbit
GSTB-V2/B in orbit, artist's impression
(Image courtesy of ESA)

The Time group is currently involved in producing timing algorithms for two Galileo contracts and on improving our own UK UTC(NPL) timescale. Galileo will be Europe’s own Global Navigation Satellite System (GNSS), and more accurate than the current US GPS system. The two Galileo projects that the Time group is working on are the Precise Time Facility (PTF) and the Time Service Provider (TSP).

Galileo Precise Time Facility algorithm work

The Precise Time Facility (PTF) is the facility that will hold the atomic reference clocks for Galileo and form Galileo System Time (analogous to USNO and GPS system time). The PTF will form one of two reference clocks for the Galileo satellite navigation system, which in turn form part of the Galileo ground infrastructure. A minimum of four high performance caesium clocks and two active hydrogen masers with make up the PTF clock ensemble. The Time group is contributing algorithm prototypes for use in the development of one of the PTFs.

The Time group is designing and prototyping four algorithms:

  • An ensemble algorithm which will provide an internal 'average' highly stable software clock for use within the PTF.
  • A steering algorithm for steering the master clock and back-up clock in the PTF to UTC (steers provided from the TSP) to form a physical realisation of Galileo System Time (GST).
  • A second steering algorithm for steering the second back-up PTF to that of the master PTF.
  • An algorithm for computing the time offset between GPS Time and GST.

Galileo Time Service Provider algorithm work

Galileo Constellation
Galileo constellation
(Image courtesy of ESA)

The Time Service Provider (TSP) will provide the link between UTC and Galileo System Time (GST). Its function is to provide daily steering corrections to GST so that GST does not drift further than 50 ns from UTC within an uncertainty of 25 ns, and the normalised frequency offset must not exceed 5.5 × 10-14 per day. These requirements depend on the performance of both the PTF and TSP. The TSP will achieve these requirements by receiving timing data daily from four main European UTC(k) laboratories (including NPL) and constructing a free running timescale and steered timescale. Since UTC is computed up to 50 days in arrears by BIPM, the TSP will predict UTC and use this to compute the daily steering correction to GST.

The Time group is designing and prototyping the following algorithms:

  • An algorithm for pre-processing the UTC(k) laboratory atomic clock data.
  • An algorithm for pre-processing the Two Way Satellite Time and Frequency Transfer (TWSTFT) data.
  • An algorithm for pre-processing the GPS common view data.
  • An algorithm for computing the prediction and steering of GST to UTC.

Algorithm work on the UTC(NPL) timescale

Building on our experience of timing algorithms the Time group is currently writing algorithms to be used on and enhance the performance of our own UTC(NPL) timescale. Three algorithms are being developed:

  • A clock ensemble algorithm.
  • A clock steering algorithm.

The aim of these two algorithms is to have the best UTC(k) timescale in Europe, support the optical clock and fountain clocks in NPL and to support high accuracy customers.

  • A time transfer (TWSTFT and GPS common view) combining algorithm where we aim to improve our time transfer by combining different techniques.
Last Updated: 17 Jun 2013
Created: 13 Jun 2007

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