German longwave time signal and standard-frequency radio station

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German longwave time signal and standard-frequency radio station

DCF77 time code transmitter The low frequency T-aerial antennas of DCF77 in Mainflingen Location Mainflingen longwave transmitter, Mainflingen, Germany Coordinates Coordinates: Elevation 113 m (371 ft) Operator Media Broadcast GmbH on behalf of the PTB Frequency 77.5 kHz Power 50 kW Began operation 1 January 1959; continuous date and time information was added in June 1973 Official range 2,000 km (1,243 mi) Website DCF 77

Low cost DCF77 receiver

DCF77 is a German longwave time signal and standard-frequency radio station. It started service as a standard-frequency station on 1 January 1959. In June 1973 date and time information was added. Its primary and backup transmitter are located at in Mainflingen, about 25 km south-east of Frankfurt am Main, Germany. The transmitter generates a nominal power of 50 kW, of which about 30 to 35 kW can be radiated via a T-antenna.

DCF77 is controlled by the Physikalisch-Technische Bundesanstalt (PTB), Germany's national physics laboratory and transmits in continuous operation (24 hours). It is operated by Media Broadcast GmbH (previously a subsidiary of Deutsche Telekom AG), on behalf of the PTB. With Media Broadcast GmbH, a temporal transmission availability of at least 99.7% per year or under 26.28 hours of annual downtime has been agreed upon. Most service interruptions are short-term disconnections of under two minutes. Longer lasting transmission service interruptions are generally caused by strong winds, freezing rain or snow induced T-antenna movement. This manifests itself in electrical detuning of the antenna resonance circuit and hence a measurable phase modulation of the received signal. When the maladjustment is too large, the transmitter is taken out of service temporarily.[1] In the year 2002, almost 99.95% availability, or just over 4.38 hours of downtime, was realized.[2] The timestamp sent is either in Coordinated Universal Time (UTC)+1 or UTC+2 depending on daylight saving time.[3]

The highly accurate 77.5 kHz (approximately 3868.3 m wavelength) carrier signal is generated from local atomic clocks that are linked with the German master clocks at the PTB in Braunschweig. The DCF77 time signal is used for the dissemination of the German national legal time to the public.[4]

Radio clocks and watches have been very popular in Europe since the late 1980s and, in mainland Europe, most of them use the DCF77 signal to set their time automatically. Further industrial time-keeping systems at railway stations, in the field of telecommunication and information technology, at radio and TV stations are radio-controlled by DCF77 as well as tariff change-over clocks of energy supply companies and clocks in traffic-light facilities.[5]

Signal [ edit ]

The low frequency T-aerial antennas of the continuously operated DCF77 signal in Mainflingen at night

Time signal [ edit ]

The DCF77 station signal carries an amplitude-modulated, pulse-width coded 1 bit/s data signal. The same data signal is also phase modulated onto the carrier using a 512 bit long pseudorandom sequence (direct-sequence spread spectrum modulation). The transmitted data signal is repeated every minute.

Experimental civil defence emergency signal [ edit ]

Since 2003, 14 previously unused bits of the time code have been used for civil defence emergency signals. This is an experimental service, aimed to one day replace the German network of civil defence sirens.

Civil protection and weather forecast signal [ edit ]

Since 22 November 2006 the DCF77 transmitter uses bits 1–14 to transmit warning messages and weather information.[7][8] Under responsibility of the German Federal Office of Civil Protection and Disaster Assistance (the German Bundesamt für Bevölkerungsschutz und Katastrophenhilfe, BBK), warnings to the population can be transmitted using these 14 bits. As a further extension of the information content transmitted by DCF77, appropriately equipped radio clocks can provide a four-day weather forecast for 60 different regions in Europe. The forecast data is provided by and under responsibility of the Swiss company Meteo Time GmbH and is transferred in a proprietary transfer protocol.[9][10] The same 14 bits are employed in a way that ensures compatibility with the transmission protocols of the warning messages. For decoding the weather forecast data a license is required.[8][11] Since the bits previously reserved for the PTB are used, older radio clocks should not be affected by the weather data signal.

Future and call sign [ edit ]

The signal distribution contract between the PTB and the DCF77 transmitter operator Media Broadcast GmbH is periodically renewed. After negotiations in 2013 the PTB and Media Broadcast GmbH agreed to continue the dissemination of the German national legal time for the next 8 years. The PTB expressed it will initialize new negotiations if modernization activities at the transmitting station to improve the signal reception reliability throughout Europe by increasing the transmission power before 2021 are deemed necessary.[12]

The call sign DCF77 stands for D = Deutschland (Germany), C = long wave signal, F = the longwave transmitters on the premises of the transmitting station Mainflingen (due to its vicinity to Frankfurt am Main), 77 = frequency: 77.5 kHz.

Time code details [ edit ]

Like most longwave time transmitters (akin to the 162 kHz 2 MW TDF time signal broadcast from France), DCF77 marks seconds by reducing carrier power for an interval beginning on the second. The duration of the reduction is varied to convey one bit of time code per second, repeating every minute. The carrier is synchronized so the rising zero-crossing occurs on the second. All modulation changes also occur at rising zero-crossings.

Amplitude modulation [ edit ]

Amplitude modulated signal of DCF77 as a function of time

The DCF77 signal uses amplitude-shift keying to transmit digitally coded time information by reducing the amplitude of the carrier to 15% of normal (−16½ dB) for 0.1 or 0.2 seconds at the beginning of each second. A 0.1 second reduction (7750 cycles of the 77500 Hz carrier amplitude) denotes a binary 0; a 0.2 second reduction denotes a binary 1. As a special case, the last second of every minute is marked with no carrier power reduction.

There was also a Morse code station identification until 2006, sent during minutes 19, 39 and 59 of each hour, however this was discontinued as the station is easily identifiable by the characteristic signal.[13] A 250 Hz tone was generated by square wave modulating the carrier between 100% and 85% power, and that tone was used to send one letter per second, between the second marks. During seconds 20–32, the call sign "DCF77" was transmitted twice.

Phase modulation [ edit ]

In addition, for 793 ms beginning at 200 ms, each time code bit is transmitted using direct-sequence spread spectrum. The bit is mixed with a 512-bit pseudo-random chip sequence and encoded on the carrier using ±13° phase-shift keying.[14] The chip sequence contains equal amounts of each phase, so the average phase remains unchanged. Each chip spans 120 cycles of the carrier, so the exact duration is cycles 15500 through 76940 out of 77500. The last 560 cycles (7.22 ms) of each second are not phase-modulated.[15]

The chip sequence is generated by a 9-bit linear feedback shift register (LFSR), repeats every second, and begins with 00000100011000010011100101010110000….

A software implementation of a Galois LFSR can generate the full chip sequence:

unsigned int i , lfsr ; lfsr = 0 ; for ( i = 0 ; i < 512 ; i ++ ) { unsigned int chip ; chip = lfsr & 1 ; output_chip ( chip ); lfsr >>= 1 ; if ( chip || ! lfsr ) lfsr ^= 0x110 ; }

Each time code bit to be transmitted is exclusive-ored with the LFSR output. The final chipped sequence is used to modulate the transmitter phase. During 0 chips the carrier is transmitted with a +13° phase advance, while during 1 chips it is transmitted with a −13° phase lag.

In lieu of the special minute marker used in the amplitude code, bit 59 is transmitted as an ordinary 0-bit, and the first 10 bits (seconds 0–9) are transmitted as binary 1.

When compared to amplitude modulation, phase modulation makes better use of the available frequency spectrum and results in a more precise low frequency time distribution with less sensitivity to interferences. Phase modulation is however not used by many DCF77 receivers. The reason for this is the worldwide availability of the (precise time reference) signals transmitted by global navigation satellite systems like the Global Positioning System (GPS) and GLONASS. Due to the GPS signal structure and the larger bandwidth available, the GPS reception would, in principle, achieve an uncertainty of the time transmission that is lower by at least one order of magnitude than the uncertainty that can be achieved with DCF77 phase modulation receiving hardware (GPS time is accurate to about ± 10 to 30 nanoseconds[16][17]).

Time code interpretation [ edit ]

The time is represented in binary-coded decimal. It represents civil time, including summer time adjustments. The time transmitted is the time of the following minute; e.g. during December 31 23:59, the transmitted time encodes January 1 00:00.[18]

The first 20 seconds are special flags. The minutes are encoded in seconds 21–28, hours during seconds 29–34, and the date during seconds 36–58.

Two flags warn of changes to occur at the end of the current hour: a change of time zones, and a leap second insertion. These flags are set during the hour up to the event. This includes the last minute before the event, during which the other time code bits (including the time zone indicator bits) encode the time of the first minute after the event.

DCF77 time code Bit Weight Meaning Bit Weight Meaning Bit Weight Meaning PM AM PM AM PM AM :00…
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