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DCF77

Från http://www.eecis.udel.edu/~mills/ntp/dcf77.html

Time and Standard Frequency Station DCF77 (Germany) Home | NTP Home Page | NTP Project Page | NTP Documentation | NTP Public Servers

(Original in German available from the address below. Translation errors courtesy of Peter Lamb, Swiss Fed. Inst. of Technology)

Physikalisch-Technische Bundesanstalt (PTB) Braunschweig, Febuar 1984 Lab 1.21 Bundesalle 100 D-3300 Braunschweig

The 1978 law on time standards defines legal time in Germany on the basis of Coordinated World Time (UTC) and gives the PTB responsibility for the keeping and broadcasting of legal time. As well as this, the time standards law empowers the Federal government to issue regulations for the introduction of Summer Time. Legal time in the FRG is either Middle European Time (MEZ - German abbreviation) or, in case of its introduction Middle European Summer Time (MESZ). The following relationships hold between UTC and MEZ and MESZ. MEZ(D) = UTC(PTB) + 1h MESZ(D) = UTC(PTB) + 2h Legal time is generated in the PTB Atomic Clock Building in Braunschweig and it is broadcast mainly through the LF transmitter DCF77 which the PTB rents from the German Post Office (DBP). The PTB has sole responsibility for the control of DCF77, while the DBP has responsibility for the transmitter and antennas. Queries should be directed to the above address or by telephone to 0531/592 1212 or 0531/592 1210, or by telex to 952822 ptb d. DCF77 Specifications Location: Mainflingen transmitter complex, (50:01N, 09:00E), about 25km south-east of Frankfurt a. Main. Carrier Frequency: Standard frequency 77.5kHZ, derived from the PTB atomic clocks. Relative deviation of the carrier from specifications: averaged over 1d: <1e-12 averaged over 100d: <2e-13 The carrier phase is controlled so that deviations relative to UTC(PTB) are never greater than +-0.3us. Larger phase and frequency variation observed at the receiver are due to summation of ground and space waves. Power output: Transmitter power 50kw, estimated emitted power approx. 25kW. Antenna: 150m high (backup antenna 200m high) vertical omnidirectional antenna with top capacitance. Transmission times: 24-hour continuous service. Short interruptions (of a few minutes) are possible if, because of technical problems or servicing, the service must be switched to a backup transmitter or antenna. Thunderstorms can cause longer interruptions to the service. Time signal: The carrier is amplitude-modulated with second marks. At the beginning of each second (with the exception of the 59th second of each minute), the carrier amplitude is reduced to 25% for the duration of either 0.1 or 0.2 seconds. The start of the carrier reduction marks the precise beginning of the second. The minute is marked by the absence of the previous second mark. The second marks are phase-synchronous with the carrier. There is a relatively large uncertainty possible in the time of the second mark which depends on the receiver position. The causes are the relatively low bandwidth of the antenna, space wave and other interference sources. Despite this, it is possible to achieve accuracy better than 1ms at distances of several hundred kilometers. Time code: The transmission of the numerical values for minute, hour, day, weekday, month and year are BCD-encoded through the pulse duration modulation of the second marks. A second mark with duration 0.1s encodes a binary 0 and a duration of 0.2s encodes 1. The order of encoding is shown in the following diagram [replaced by a table in this translation]. The three test bits P1, P2 and P3 extend the 3 major sections of the time code (7 bits for minutes, 6 bits for the hour and 22 bits for the date, including the week day number) to maintain an even count of 1’s. The second marks No. 17 and 18 indicate the time system for the transmitted time codes. In the case of transmission of MEZ, mark 18 has a duration of 0.2s and mark 17 a duration of 0.1s. If MESZ is being transmitted, this is reversed. Furthermore, an approaching transition from MEZ to MESZ or back is announced by extending mark 16 from 0.1 to 0.2s for one hour prior to the changeover. Encoding Scheme [diagram in original] Mark number(s) Encodes (01.s=0, 0.2s=1) 0 Minute, always 0 (0.1s) 1-14 Reserved 15 0=Normal antenna, 1=backup antenna 16 1=Approaching change from MEZ to MESZ or back 17,18 Time zone 0,1=MEZ; 1,0=MESZ 19 The leap second is encoded in this bit one hour prior to occurrence. 20 Start bit for encoded time, always 1 21-27 1, 2, 4, 8, 10, 20, 40 Minutes (mark 21=1 minute) 28 P1 maintains even parity for marks 21-28 29-34 1,2,4,8,10,20 Hours (mark 29=1 hour) 35 P2 maintains even parity for marks 29-35 36-41 Day in month (1, 2, 4, 8, 10, 20) 42-44 Day in week (1,2,4) 45-49 Month number (1, 2, 4, 8, 10) 50-57 Year (1, 2, 4, 8, 10, 20, 40, 80) 58 P3 maintains even parity for marks 36-58 There is no mark transmitted for the 59th second. Literature P. Hetzel, “Die Zeitsignal- und Normalfrequenzaussendungen der PTB ueber den Sender DCF77: Stand 1982” [The PTB time signal and standard frequency transmissions from DCF77: Status 1982] in “Funkuhren” [Radio clocks], W. Hilberg, Oldenburg Publishers, Munich & Vienna 1983, pp 42- 57. G Becker & P. Hetzel, “Vortraege ueber DCF77” [Lectures: DCF77], PTB Reports, PTB-Me-23 (1979), pp 185-253. Braunschweig 1984 Additional information: DCF77 Since July 1983, the DCF77 carrier has been phase modulated in a test configuration. The phase modulation is a pseudorandom binary sequence sent twice each second. The clock frequency of the binary sequence is 645.833...Hz and the phase shift \Delta\tau about 3% of the period (\^{=} 10\deg). Equal numbers of shifts of +\Delta\tau and -\Delta\tau are always sent, so that the mean frequency remains unchanged, and the use of DCF77 as a frequency standard is unaffected. The timecode is encoded in the sequence by inverting the sequence or not. Not inverted sequence corresponds to a 0 bit. The sequence is alleged to be generated by a 9 bit shift register which is coupled back on positions 5 and 9. The polynomial might be: x^9 + x^4 + 1. Because the pseudo-random bitstring has a strictly deterministic nature, the correlation analysis at the receiver end leads to a correlation function with triangular form, and thereby to timing information. Early test results show that the time information received with the help of pseudo-random phase modulation is more resistant to interference and more accurate (standard deviation \approx 10\mu s during the day and \approx 25\mu s at night) than the conventional method using amplitude modulated second marks. Since this new modulation method is compatible with previous usage of DCF77, and that the users have made no difficulties known to us, the tests have been extended. The transmission of the pseudo-random phase distortion still has experimental status, and should not be seen as a permanent commitment. Further information will be made available in the future. Announcement bit for a leap second The DCF77 control unit is currently being modified so that in future an announcement bit for a leap second can be sent. It is expected that for the first time on 1st July 1985 the second mark Nr. 19 will be extended to a length of 0.2s for one hour prior to the introduction of a leap second. Intelligent receivers will then be able to recognise the discontinuity and maintain correct indicated time in spite of a 61s minute.

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