DCF77
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DCF77
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| Basic data | |||||||||
| Place: | Mainhausen | ||||||||
| Country: | Hesse | ||||||||
| Country: | Germany | ||||||||
| Altitude : | 113 m above sea level NHN | ||||||||
| Coordinates: 50 ° 0 ′ 57.6 ″ N , 9 ° 0 ′ 36 ″ E | |||||||||
| Use: | Time signal transmitter | ||||||||
| Accessibility: | Transmission system not accessible to the public | ||||||||
| Owner : | Media broadcast | ||||||||
| Data on the transmission system | |||||||||
| Construction time: | until 1958 | ||||||||
| Operating time: | since 1959 | ||||||||
| Waveband : | LW transmitter | ||||||||
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The time signal transmitter DCF77 is a long wave transmitter in Mainflingen near Frankfurt am Main , which supplies most of the radio-controlled clocks in Western Europe with the legal (clock) time applicable in Germany .
The transmission frequency is 77.5 k Hz . The designation DCF77 is the call sign assigned to the transmitter for international identification . It is part of the transmission systems in Mainflingen .
Its time signals , which are sent every second, transmit either Central European Time or Central European Summer Time within a minute , in contrast to other time signal transmitters, they do not transmit the difference dUT1 between Earth's rotation time and atomic time . Other well-known time services are MSF in England (60 kHz), France Inter in France (162 kHz), as well as the transmitter groups RWM in Russia (4.996 MHz, 9.996 MHz and 14.996 MHz), WWV, WWVB, WWVH in the USA (60 kHz; 2.5, 5, 10, 15 and 20 MHz) and until 2011 HBG in Switzerland (75 kHz).
General
The transmitter in Mainflingen works on the transmission frequency 77.5 kHz with an output of 50 kW . The transmitter masts can be seen from the A 3 and A 45 at the Seligenstädter Dreieck .
The Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig has been sending a standard frequency via DCF77 since 1959 and, since 1973, a data signal for the date and time. PTB is obliged to do so by the law on units in metrology and time determination . The systems in Mainflingen are operated by Media Broadcast GmbH, which emerged from a former subsidiary of T-Systems and has belonged to the mobile operator mobilcom-debitel since March 2016 .
A control device developed by PTB with three commercial (somewhat less expensive) atomic clocks serves as the basis for generating the time signal at the transmitter site . This control device is synchronized with the primary atomic clocks of the PTB in Braunschweig (two cesium clocks and two cesium fountains ). The signal obtained in this way has an accuracy at the point of transmission with a relative standard deviation of a maximum of 10 −12 . That corresponds to an error of one second in 30,000 years.
Callsign
The call sign of the transmitter, specified in the international frequency list of the ITU , is "DCF77". It is derived from D for Germany, C for long-wave transmitters , F because of its proximity to Frankfurt, and the number 77 for the carrier frequency 77.5 kHz.
The callsign was previously sent by the DCF77 transmitter, three times an hour (twice in a row) as Morse code during the 20th to 32nd seconds of minutes 19, 39 and 59. Although the callsign was generated electronically without interrupting the time stamp sequence, it was transmitted a deterioration in the signal-to-noise ratio of the corresponding second markers. Since, due to the special signal form of the DCF77 signals, a clear assignment of these signals to the DCF77 transmitter is always possible, the transmission of the call sign has been dispensed with since 2007 in accordance with the provisions of the Implementing Regulations for the radio service .
Reception area
The DCF77 signal can - depending on the time of day and season - be received up to a distance of approx. 2000 km. In the case of overreaches , the range of long waves is considerably greater. There are known cases in which clocks in Canada and the Maldives synchronized.
Legal meaning
In the Time Act of 1978, the Physikalisch-Technische Bundesanstalt (PTB) was commissioned with the dissemination of the legal time in Germany. This task was previously performed by the German Hydrographic Institute (DHI). The time information distributed via DCF77 represents the official time of the Federal Republic of Germany . The Time Act was replaced in 2008 by the Units and Time Act. The task of the PTB to “ represent and disseminate the legal time ” has been taken over.
signal
The carrier signal of 77.5 kHz is synchronized in frequency and phase angle with the controlling primary atomic clock and therefore has only slight deviations from the set frequency. Over a day this is less than relative 2 · 10 −12 , on average over 100 days by less than relative 2 · 10 −13 . It can therefore be used as a calibration frequency for very precise high and low frequency generators without evaluating the time information .
A problem arises when strong winds move the T-antenna . This manifests itself in a measurable phase modulation of the received signal. In the event of a very strong storm and the antenna moving a lot, which leads to a maladjustment of the antenna, the transmitter must be temporarily shut down.
The transmitter generates a nominal power of 50 kW, of which around 30 to 35 kW are radiated via the antenna.
In the range up to about 600 km, the signal can be received as a bump . From around 1100 km the sky wave component predominates . At a distance of 600 km to 1100 km from the transmitter, the signal can occasionally be canceled if the field strengths of the ground and sky waves are the same ( fading of 15 minutes and more). The target range is 2000 km (see reception area ).
Transmission method
The time information is transmitted both by means of amplitude shift keying , which is the older method, and, since mid-1983 and in parallel, by means of phase modulation . Compared to amplitude shift keying, phase modulation allows an absolute temporal resolution that is more precise by several powers of ten for recognizing the start of a second. The carrier frequency of the transmitter, i.e. 77.5 kHz, also represents the normal frequency.
Amplitude shift keying
The amplitude shift is done by negative modulation of the signal by lowering the carrier amplitude to about 15% every second. Within a minute, the decreases are at the beginning of seconds 0 to 58. With the last second 59 there is no decrease, whereby the following second (here again with decrease) marks the beginning of a minute (second 0) and the receiver is synchronized can. If the minute contains a leap second, the second 59 also contains a decrease and an (additional) second 60 follows without a decrease.
The length of the amplitude decrease at the beginning of the second stands for the value of a binary character : 100 ms decrease stands for the value "0", 200 ms for "1". This means that 59 bits of information are available within one minute .
The bits that start counting at 0 are used as follows:
| bit | Importance of values |
|---|---|
| 0 | "0": start of a new minute |
| 1-14 | up to May 1977: Difference UT1 − UTC as a signed number up to Nov. 2006: Operating information from PTB (mostly all 14 bits zero) since Dec. 2006: Weather information from MeteoTime and information from disaster control |
The second marks 15 to 19 contain information about irregularities in the transmitter operation (call bit to alert PTB employees), about the time zone and announce the beginning and end of summer time and leap seconds:
| bit | Importance of values |
|---|---|
| 15th | Rufbit (spare antenna until mid-2003) |
| 16 | "1": At the end of this hour, CET / CEST is changed. |
| 17th | "01": CET "10": CEST |
| 18th | |
| 19th | "1": A leap second is inserted at the end of this hour. |
Bit 17 and 18 indicate whether the details from bit 20 onwards refer to CET or CEST, and are either "01" or "10".
In seconds 20 to 58, the time information for the following minute is transmitted serially in the form of BCD numbers, starting with the least significant bit. Parity bits are used to protect the data; parity is even. The day of the week is coded in accordance with the ISO 8601 or DIN EN 28601 standard, according to which Monday is day one (binary 001) of a week and Sunday is day seven (binary 111).
From the calendar year only the units and tens are transferred, the year 2004 as 04.
| bit | meaning | |
|---|---|---|
| 20th | "1": Start of time information | |
| 21st | Minute (one) |
Bit for 1 |
| 22nd | Bit for 2 | |
| 23 | Bit for 4 | |
| 24 | Bit for 8 | |
| 25th | Minute (tens) |
Bit for 10 |
| 26th | Bit for 20 | |
| 27 | Bit for 40 | |
| 28 | Parity minute | |
| 29 | Hour (one) |
Bit for 1 |
| 30th | Bit for 2 | |
| 31 | Bit for 4 | |
| 32 | Bit for 8 | |
| 33 | Hour (tens) |
Bit for 10 |
| 34 | Bit for 20 | |
| 35 | Parity hour | |
| 36 | Calendar day (one) |
Bit for 1 |
| 37 | Bit for 2 | |
| 38 | Bit for 4 | |
| 39 | Bit for 8 | |
| 40 | Calendar day (tens) |
Bit for 10 |
| 41 | Bit for 20 | |
| 42 | weekday | Bit for 1 |
| 43 | Bit for 2 | |
| 44 | Bit for 4 | |
| 45 | Month number (one) |
Bit for 1 |
| 46 | Bit for 2 | |
| 47 | Bit for 4 | |
| 48 | Bit for 8 | |
| 49 | Month number (tens) |
Bit for 10 |
| 50 | Year (one) |
Bit for 1 |
| 51 | Bit for 2 | |
| 52 | Bit for 4 | |
| 53 | Bit for 8 | |
| 54 | Year (tens) |
Bit for 10 |
| 55 | Bit for 20 | |
| 56 | Bit for 40 | |
| 57 | Bit for 80 | |
| 58 | Parity date | |
In the case of a minute without a leap second, no second marker is sent at second 59. For a minute with a leap second, a "0" is sent for second 59 and no second mark is sent for second 60.
The time information for the following minute is always transmitted. I.e. The time information for the next minute is transmitted every minute so that it can then be checked with the time stamp of the 0th bit of the next minute and, if there are no errors, can be accepted as time information.
In order to get at least a correct time , this means for the user of a radio clock that the reception must run for at least just over 38 seconds. Two seconds of this time span (second 58 and the gap in second 59) are required so that the receiver can synchronize to the beginning of the new minute, and 36 seconds to receive the time telegram including the parity bit. However, after 120 seconds of interference-free reception at the latest, the watch would have all the necessary information available.
The transmitted parity bits only allow error detection of the information received, no error correction, and cannot guarantee error-free detection in poor reception conditions. In order to obtain reliable time information, additional measures are taken, for example the redundancy of the time information is evaluated in consecutive minutes.
Phase modulation
In addition to the amplitude-modulated time transmission, since June 1983 the information about the beginning of the second has been transmitted via phase modulation of the carrier with a pseudo-random sequence (PZF) with a length of 512 chips in one second, the duration of a useful data bit . The chip sequence carries the same user data information of one bit per second as with amplitude shift keying, depending on the status of the useful data bit, the chip sequence is sent normally or inverted.
Using cross-correlation of the pseudo-random sequence, the exact beginning of the second on the receiving end can be determined more precisely than with amplitude modulation. So that the additional phase modulation does not cause any interference in AM operation, the mean phase position remains unchanged, i.e. H. the chip sequence has 256 time-positive and 256 time-negative phase shifts per second, which in total cancel each other out.
Temporal resolution
With optimized decoding algorithms and no excessive band limitation in the reception filter, the time uncertainty with which the exact start of the amplitude-modulated second markers can be recognized is over 100 µs. Standard household radio-controlled clocks use narrow-band receivers, with the usual bandwidth around 10 Hz, and can therefore determine the beginning of the second with an accuracy of 0.1 s when using amplitude modulation.
With phase modulation using pseudo-random sequence (PZF) and cross-correlation in the receiver, the absolute accuracy is 6.5 μs to 25 μs, depending on the time of day and the season, which means an improvement in the time resolution by a factor of around 1,000 compared to the amplitude-modulated second markers .
Additional use
Alerting
Since 1999 there have been investigations and attempts to trigger additional alarms via the DCF77 transmitter , for example in disaster control or in the event of major damage ( chemical disaster , flood ). On behalf of the Federal Ministry of the Interior , the company HKW-Elektronik GmbH carried out a field test at the end of 2003 together with test participants from disaster control organizations to signal alarms in the second marks 1 to 14 of the signal. According to PTB, this test successfully demonstrated the reliability of such a system nationwide. At the beginning of 2004, with the final report of this test, the PTB advised the Federal Ministry of the Interior to use the DCF77 transmitter as part of a system for warning the population in the long term.
Bit structure of the DCF77 alarm signal of the field test
The structure of the bits for the field test to warn the public in the event of a disaster has the following structure:
| Short block | Short block | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Bit no. | 1 | 2 | 3 | 4th | 5 | 6th | 7th | 8th | 9 | 10 | 11 | 12 | 13 | 14th |
| meaning | A. | D1 | D2 | P1 | D3 | P2 | P3 | A. | D1 | D2 | P1 | D3 | P2 | P3 |
| Long block | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Bit no. | 1 | 2 | 3 | 4th | 5 | 6th | 7th | 8th | 9 | 10 | 11 | 12 | 13 | 14th |
| meaning | D4 | D5 | D6 | D7 | D8 | D9 | P4 | D10 | D11 | D12 | P5 | D13 | P6 | P7 |
- A: Alarm bit, note that address data will be transmitted below
- Dx: data bits
- Px: backup bits, parity bits
The transmitted data was secured twice: parity bits and repetition of the transmission. The short block, broadcast twice in the first minute, contains a rough division of the Federal Republic into three regions. The long block, sent in the second and third minute, contains the finer structure of the region transmitted in the short block down to the district level.
Weather data
Since November 22nd, 2006 the transmitter DCF77 has been transmitting weather data in the second marks 1 to 14 in addition to disaster reports. Correspondingly equipped radio clocks are able to display a four-day weather forecast for 60 regions in Europe . The weather data is provided by HKW-Elektronik GmbH under the “Meteotime” brand. They are transmitted in the proprietary Meteo-Time protocol, for the decryption of which a license is required.
Since the seconds marks 1 to 14 previously reserved for PTB are used, older radio clocks should not be affected by the weather signal.
Network Time Protocol
The identifier .DCFa is used for time servers . for a standard DCF77 receiver as reference time source. The amplitude modulation is evaluated here, the phase modulation evaluation is carried out by .DCFp. indexed.
See also
literature
- Peter Hetzel: Time information and normal frequency . In: telekom praxis , issue 1/1993, pp. 25–36.
- Peter Hetzel: The long wave transmitter DCF77 on 77.5 kHz: 40 years of time signals and normal frequency, 25 years of coded time information . In: PTB-Mitteilungen , Vol. 109, pp. 11-18, 1999.
- Dirk Piester, Peter Hetzel, Andreas Bauch: Time and normal frequency distribution with DCF77 . In: PTB-Mitteilungen , Vol. 114, pp. 345–368, 2004.
Web links
- Official homepage of the DCF77 transmitter PTB
- PTB Division 4.4 - Extensive information on time and frequency
- Article on the suitability of DCF77 for warning notices
- www.dcf77.de (via Archive.org) on DCF77 ( Memento from December 27, 2014 in the Internet Archive )
- R. Heret, T. Losert: Everything about radio clocks and the time signal
- Private website with decoding examples (including leap second)
- Time and normal frequency distribution with DCF77 (PDF; 3.15 MB) PTB
- Website with logged DCF77 data and the transmitted weather data
- Evaluation of the DCF signal with the microcontroller or a decoder chip DCF-RS1
Individual evidence
- ↑ Dirk Piester, Peter Hetzel, Andreas Bauch: PTB Topic : Time and normal frequency distribution with DCF77 . (PDF) In: PTB-Mitteilungen . 114, No. 4, 2004, p. 348. Retrieved July 10, 2012.
- ↑ PTB Braunschweig: How does time transfer work? Retrieved April 30, 2016
- ↑ PTB Braunschweig: DCF77 control device Accessed : January 21, 2013
- ↑ a b Dirk Piester, Peter Hetzel, Andreas Bauch: PTB Topic : Time and normal frequency distribution with DCF77 . (PDF) In: PTB-Mitteilungen . 114, No. 4, 2004, p. 350. Retrieved July 10, 2012.
- ^ PTB Braunschweig: Sender identification ; accessed: April 27, 2017
- ↑ Robert Heret, Thomas Losert: Range of the DCF77 transmitter . In: The Funk-Uhr Homepage . Retrieved December 12, 2010.
- ↑ Law on Units in Metrology and Determination of Time , Section 6, Paragraph 2, Item 2; accessed: November 29, 2017
- ↑ a b Weather data description of the Meteotime system - Version 1.0. (PDF; 579 kB) (No longer available online.) Meteo Time GmbH, October 27, 2006, archived from the original on December 29, 2009 ; Retrieved December 12, 2010 .
- ↑ time code. January 6, 2020, accessed March 29, 2020 .
- ↑ a b DCF77 phase modulation. Retrieved July 13, 2020 .
- ↑ DCF77 fit for the future. PTB time signal transmission by long wave transmitter has been "completely renewed". Physikalisch-Technische Bundesanstalt PTB, December 12, 2006, accessed on April 30, 2016 .
- ↑ Takeover of Meteotime by HKW-Elektronik ; Retrieved January 5, 2017
- ^ The Network Time Protocol (NTP). Check the NTP status. Meinberg Funkuhren, Bad Pyrmont, accessed on August 29, 2011 .