National Physical Laboratory

Droitwich Bulletins

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Droitwich Signal

Droitwich 198 kHz signal is broadcast by a BBC Radio 4 transmitter with a carrier power of 400 kW. The Droitwich station uses rubidium frequency standards and the fractional frequency offset typically observed is less than 10-11 over a one day averaging period.

Droitwich Monitoring

The NPL Droitwich monitoring service is based on two independent 198 kHz receivers which phase-lock their local oscillators to the received 198 kHz signal. The output of each receiver is compared to a 198 kHz reference derived from the UK national time scale, UTC(NPL). The phase difference between the reference and received 198 kHz signals is continuously recorded on a chart recorder.

Note that there are two other 198 kHz transmitters operated in the UK, one at Burghead and the other at Westerglen (both sites are in Scotland). NPL cannot monitor these other 198kHz signals at its site in Teddington and hence is not able to comment on their performance.

Droitwich Bulletin Data

The Droitwich bulletin reports the value of the phase difference between UTC(NPL) and MSF recorded at 12:00 UTC and 15:00 UTC each day. The values are expressed in terms of a time interval (multiplying the phase angle by the period of the carrier). Note that the bulletins attempt to avoid discontinuities associated with cycle ambiguities: hence the readings are valid for calculating frequency offsets but should not be used to calculate absolute propagation delays between Droitwich and Teddington. Discontinuities in either the received signal or the phase are indicated by the letters (a) or (b) respectively. The bulletin data are reported with a resolution of 0.1 ms.

Last Updated: 20 Oct 2011
Created: 11 Jan 2011

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