National Physical Laboratory

GPS Bulletins

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The GPS Monitoring Service

The GPS monitoring service measures and reports on the time difference between GPS system time and UTC(NPL) for up to eight GPS satellites every sixteen minutes. The raw data is recorded in the BIPM common-view format and is available from NPL's public FTP site. The original data is then processed to give daily averages and published in the GPS bulletin. The bulletin is intended primarily for those operating GPS time and frequency standards in stand-alone mode, and it can be used for the following purposes:

  • To validate the integrity of the GPS timing signals received in the United Kingdom.
  • To quantify the accuracy of GPS timing signals received in the United Kingdom.

In contrast, the original data in the BIPM common-view format is made available for those with more demanding time transfer requirements. Again, the data can be used either for integrity monitoring (at the level of individual satellites every few minutes) or for access to remote high accuracy time and frequency standards. For example, the common-view data can be used to transfer the UTC(NPL) standard to remote users by applying post-processed differential-corrections to the user's own GPS device.

The GPS Monitoring Facility at NPL

The NPL GPS monitoring service is based on the GPS common-view method, using two multichannel GPS receivers following the BIPM Technical Directives for Standardisation of GPS Time Receiver Software (see D W Allan & C Thomas, Metrologia, 1994, 31, pp 69-79).

The routine service is operated using a 3S Navigation R100-40 multi-channel GPS receiver and temperature stabilized antenna. Back-up is provided using an NPL/Time and Frequency Solutions GPS common-view receiver (incorporating a Motorola VP-Oncore engine).

Both receivers have had their internal delays calibrated and their reference coordinates optimized to facilitate a smooth switching from the monitoring to the back-up receiver in the event of a failure. Both receivers generate output data sets in the BIPM common view format, giving UTC(NPL) - GPS time for each satellite every 16 minutes. All data is available on our ftp site; each day data is written to a separate file. Daily averages are included in NPL's monthly GPS bulletin.

Note that values of UTC(NPL) - GPS Time published in NPL's GPS bulletin are given in nanoseconds, while values of UTC(NPL) - GPS Time in the BIPM format files are given in multiples of 0.1 nanoseconds.

The method for calculating the daily values published in the GPS bulletins (TXT file 5 KB) are described in a text file.

Last Updated: 26 Mar 2018
Created: 11 Jan 2011


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