Greenwich Time Signal

The Greenwich Time Signal (GTS), popularly known as the pips, is a series of six short tones (or "pips") broadcast at one-second intervals by many BBC Radio stations. The pips were introduced in 1924 and have been generated by the BBC since 1990 to mark the precise start of each hour. Their utility in calibration is diminishing as digital broadcasting entails time lags. There are six pips (short beeps) in total, which occur on each of the 5 seconds leading up to the hour and on the hour itself. Each pip is a 1 kHz tone (about a fifth of a semitone above musical B5) the first five of which last a tenth of a second each, while the final pip lasts half a second. The actual moment when the hour changes – the "on-time marker" – is at the very beginning of the last pip. When a leap second occurs (exactly one second before midnight UTC), it is indicated by a seventh pip. In this case the first pip occurs at 23:59:55 (as usual) and there is a sixth short pip at 23:59:60 (the leap second) followed by the long pip at 00:00:00. The possibility of an extra pip for the leap second thus justifies the final pip being longer than the others, so that it is always clear which pip is on the hour. Before leap seconds were conceived, the final pip was the same length as the others. Although "negative" leap seconds can also be used to make the year shorter, this has never happened in practice. Although normally broadcast only on the hour by BBC domestic radio, BBC World Service uses the signal at other times as well. The signal is generated at each quarter-hour and has on occasion been broadcast in error. Until 1972, the pips were of equal length and confusion arose as to which was the final pip, hence the last pip is now of extended length. The pips are available to BBC radio stations every fifteen minutes, but, except in rare cases, they are only broadcast on the hour, usually before news bulletins or news programmes. Normally, BBC Radio 4 broadcast the pips every hour except at 18:00 and 00:00, and at 22:00 on Sundays (at the start of the Westminster Hour) when they are replaced by the striking of Big Ben at the Palace of Westminster.