DCF77 time signal

What it is

DCF77 is a long-wave transmitter at Mainflingen, about 40 km south east of Frankfurt am Main, broadcasting on 77.5 kHz. PTB in Braunschweig is responsible for the programme; Media Broadcast GmbH runs the transmitter on its behalf. The call sign is a code in itself: D for Germany, C for long wave, F for the Frankfurt area and 77 for the frequency.

How it works

Every second the carrier is briefly reduced in strength. A reduction of 0.1 seconds means binary zero, 0.2 seconds means one. Over a minute the receiver collects minute, hour, day, weekday, month and the last two digits of the year, plus flags for summer time and announced leap seconds. The 59th second has no mark, so clocks know where the next minute starts. The signal is steered by atomic clocks on site that PTB controls from Braunschweig. Since November 2006 the first 14 bits of each minute have also carried civil warning data and an encrypted weather forecast supplied by Meteo Time, which needs a licence to decode.

Why it stands out

Long waves follow the curve of the Earth and pass through walls, so a clock needs only a small ferrite antenna inside its case. PTB gives the range as about 2,000 km. Standard frequency transmissions began on 1 January 1959, the 24 hour service in 1970 and the coded time on 5 June 1973; the code has barely changed since. PTB has estimated that about 100 million DCF77 receivers were produced between 2000 and 2008 alone.

Where you meet it

Wall clocks, alarm clocks and wristwatches labelled radio controlled; Deutsche Bahn station clocks; tariff switching clocks of energy suppliers; traffic light controllers; broadcasters' time announcements. In 2021 the operation was extended by ten years, to 2031.

Sources

• PTB: DCF77 time code
• PTB-Mitteilungen: 50 years of time dissemination with DCF77
• VDE: Sendeanlage Mainflingen