DCF77

The time signal transmitter DCF77 is a long wave transmitter in the district Mainflingen Main Hausen, who supplied most of the radio-controlled clocks in Western Europe with the exact legal regulations in Germany time. The designation is the DCF77 transmitter assigned to the international radio call sign identification. He is part of the transmission facilities in Mainflingen.

His sent every second time signal transmitted Central European Time and Central European Summer Time, but in contrast to other time signal transmitters not DUT1, the difference between Erdrotationszeit and atomic time. Other known time services are MSF in England (60 kHz), France Inter in France (162 kHz), and the transmitter groups RWM in Russia, WWV, WWVB, WWVH in the U.S. and by 2011 HBG in Switzerland (75 kHz).

• 4.1 pseudo-random sequence or phase modulation

• 5.1 alerting 5.1.1 Bit - structure DCF77 alarm signal of the field test

Generally

The transmitter is located in Mainflingen near Frankfurt am Main and works on a frequency of 77.5 kHz with a power of 50 kW. The masts can be seen clearly when driving Seligenstädter triangle on the A3 and A45.

About DCF77 sends the Physikalisch-Technische Bundesanstalt ( PTB) in Braunschweig since 1959 a standard frequency and additionally since 1973 to a data signal for date and time. The PTB is in this Act requires relating to units of measurement and the determination of time. Companies are the investments in Mainflingen by Media Broadcast GmbH, which emerged from a former subordinate company of T-Systems and, since January 2008 is owned by the French network operator TDF.

As a basis for generating the timing signal is used at the transmitter site was developed by the PTB control device with three commercial (slightly less complex ) atomic clocks. This control device is synchronized with the primary atomic clocks of the PTB in Braunschweig ( two cesium clocks and two cesium fountain ). The signal thus obtained has the transmission location accuracy as a relative standard deviation of no more than 10-12. This corresponds to an error of about one second in 30,000 years.

Callsign

The term DCF77 is the callsign of the International Frequency List of IFRB. It is derived from D for Germany, C for long-wave transmitter, F because of the proximity to Frankfurt, and the number 77 for the carrier frequency of 77.5 kHz.

Also, the call sign "DCF77 " was sent earlier from the DCF77 transmitter, namely ( sequentially twice each ) in Morse code during the 20 to 32 seconds of the minutes 19, 39 and 59 Although the call sign production was electronically three times an hour without interruption of the timestamp sequence, caused their emission deterioration of the signal-to- noise ratio of the corresponding second marks. Since, due to the special shape of the signal DCF77 signals an unambiguous assignment of these signals to the transmitter DCF77 is always possible is waived in accordance with the provisions of the Radio Regulations Radio Regulations since 2004 on the transmission of the call sign.

Reception area

The DCF77 signal is - to receive up to a distance of about 2000 km - depending on the day and season. At certain times of the reach of long waves, however, is much higher. There are known cases in which clocks have synchronized even in Canada and the Maldives (see overreach ).

Legal Significance

In Time Act of 1978, the Physikalisch- Technische Bundesanstalt, is charged with the dissemination of legal time in Germany. Before this task by the German Hydrographic Institute ( DHI) was perceived. Thus, the spread over DCF77 time information represents the official time of the Federal Republic of Germany

Signal

The carrier signal of 77.5 kHz is synchronized in frequency and phase with the controlling primary atomic clock and therefore has only minor deviations from the nominal frequency. By using a tag, these are relatively less than 2 · 10-12, on average over 100 days to less than 2 × 10-13 relative. It can thus be used as calibration frequency for highly accurate high-and low -frequency generators without evaluation of the time information.

A problem arises when strong wind moves the T antenna. This manifests itself in a measurable phase modulation of the received signal. With very strong storm and thereby greatly moving antenna, resulting in a mismatch of the antenna, the transmitter must be rendered temporarily inoperative.

The transmitter generates a nominal power of 50 kW, of which approximately 30 to 35 kW can be radiated via the antenna.

In the range up to about 600 km, the signal can be received as a bump. From about 1100 km outweighs the space wave component. At a distance of 600 km to 1100 km from the transmitter, it can occasionally at the same field strengths of ground - and space wave to the extinction of the signal (fading of 15 min duration and more ) can occur. The target range is 2000 km ( cf. reception area ).

Time information

The time information is transmitted as a digital signal in addition to the normal frequency ( the carrier frequency of the transmitter, that is 77.5 kHz). This is done by negative modulation of the signal ( lowering the carrier amplitude to about 15 %) in the second.

The beginning of the reduction is in each case at the start of 0-58 seconds in a minute. At the last second no reduction takes place, whereby the subsequent second marker marks the beginning of a minute, and the receiver can be synchronized. The length of the amplitude reductions at the beginning of each second represents the value of a binary character 100 ms are available for reducing the value of "0", 200 ms for "1". Thus, there are 59 bits of information available within a minute.

The bits whose number starts from 0, are used as follows:

To November 2006: operating information of the PTB (usually every 14 bits zero)

Since the end of 2006: Weather Information Company Meteotime as well as information on civil protection

The second marks 15 to 19 contain information on irregularities of the transmitter operation ( call bit to alert the PTB staff ), about the time zone and announce the beginning and end of daylight saving time and leap seconds to:

From 20 to 58 seconds, the time information for each subsequent minute is transmitted serially in the form of BCD numbers, starting with the least significant bit, respectively. To protect the data parity bits are used, this is an even parity. The encoding of the week day is done according to the standard ISO 8601 or DIN EN 28601 according to which the Monday, the day one (binary 001 ) is one week and Sunday is the day seven (binary 111).

In order to obtain at least a correct time, this means for the user of a radio clock that the reception must run at least just over 38 seconds. From this period two seconds ( 58 seconds, and the gap of 59 seconds ) are needed to enable the receiver to synchronize with the start of new minute and 36 seconds for the reception of the data string including the parity bit. But at the latest after 120 seconds interference-free reception, the clock would have all the necessary information.

The transmitted parity bits allow only an error detection on the received information, no error correction, and reception is poor and can not guarantee error-free detection. To obtain a reliable time information takes one additional measures, for example by the redundancy of the time information is analyzed in consecutive minutes.

Accuracy: Detection of seconds start

Even with optimized decoding algorithms and none too strong band limitation in the reception filter is the temporal uncertainty associated with the exact start of the amplitude-modulated second marks can be detected, at least about 100 microseconds. Common household radio clocks set narrowband receiver ( with 10 Hz bandwidth) and can therefore detect the beginning of a second only accurate to 0.1 s.

Pseudo-random sequence or phase modulation

In addition to amplitude-modulated transmission time is from June 1983, this information transferred to a length of 512 bits and a phase modulation of the carrier with a pseudo-random sequence ( PAE ). By means of cross-correlation can be determined much better with the reproduced signal on the receiving end of the exact beginning of the second markers. The PTB Braunschweig indicates the still to be accepted inaccuracies with 6.5 to 25 microseconds, depending on day and season.

Additional Terms

Alerting

Since 1999, investigations and experiments, in addition to trigger alarms via the transmitter DCF77, for example in disaster or major incidents ( chemical disaster, flood ). On behalf of the Federal Ministry of the Interior HKW -Elektronik GmbH resulted in the end of 2003 together with test participants from civil protection organizations through a field test for signaling alarms in the second marks 0 to 14 of the signal. According to PTB this test proved successful nationwide the reliability of such a system. Beginning of 2004, the PTB with the final report of this test the Interior Ministry close, long-term use the DCF77 transmitter as part of a system for warning the population.

Bit structure DCF77 alarm signal of the field test

The structure of the bits for the field test of population warning in the event of a disaster has the following structure

These data were backed up twice parity bits and repeat the transmission. The short block, sent twice in the first minute, contains a rough division of the Federal Republic in three regions. The long block is transmitted in the second and third minute, contains the finer structure of the transmitted block in the short region down to the district level.

Weather information

Since 22 November 2006, in addition to disaster messages and weather data are transmitted via the transmitter DCF77 in the second marks 1 to 14. Appropriately equipped clocks are thus able to display a four-day weather forecast for 60 regions in Europe. Weather data provided by the Swiss company Meteo Time GmbH. They are transmitted in the proprietary Meteo- time protocol, for which a license is required for decryption.

As previously reserved for the PTB seconds trademarks are used 1 to 14, older radio clocks of the weather signal should not be affected. In fact, however, some radio alarm clock, station clocks and watches were equipped in public places with faulty decode, so these clocks stopped after some time.

Network Time Protocol

In time servers is the identifier. DCFA. for a standard DCF77 receiver as reference time source.