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Radiotower Realization and dissemination of the legal time of the Federal Republic of Germany
Radiotower Atomic second and atomic clocks
Radiotower Time signal and standard frequency transmitter DCF77
Radiotower Pseudo-random carrier phase shift keying
Radiotower PTB Telephone Time Service
Radiotower Extract from the Time Act of 1978 and from a decree of 1994 of the Federal Government

Realization and dissemination of the legal time of the Federal Republic of Germany

The CS3 and CS4 atomic clocks of the PTB
Atomic clocks CS3 and CS4 (PTB)
The Time Act of 1978 has entrusted the Physikalisch-Technische Bundesanstalt (PTB) with the task of giving the time which determines public life in the Federal Republic of Germany. For this purpose, the PTB designed and constructed tour primary caesium clocks, CS1 to CS4, which am among the most accurate clocks in the world. Within one year, the readings of these clocks differ from one another by less than one millionth of a second.

Everybody receiving the long-wave transmitter DCF77 can use the PTB`s time. This transmitter is located near Frankfurt and disseminates the PTB time in continuous operation. The Time Act refers to these PTB tasks as the "realization and dissemination of Legal Time".

Throughout Germany, the time signals emitted by DCF77 can be used to keep radio clocks in compliance with Legal Time with an accuracy better than one millisecond. The time information given by the broadcasting and television stations, the clocks of the Federal Railways Administration and of the speaking-clock announcement of the Deutsche Telekom AG as well as a great number of tariff counters of power industry, traffic control devices and traffic light equipment are also controlled by DCF77. In industry and science, the time signals emitted by the PTB serve to control and supervise complicated processes. Furthermore, everybody can buy radio-controlled clocks for private use.

The CS1 and CS2 atomic clocks of the PTB
The CS1 and CS2 atomic clocks of the PTB
The unit of time, the second, of the International System of Units (UI) which is based on the oscillation of the caesium-133 atom was defined in 1967. This consistently led to an international atomic time scale related to the second at sea level and to the zero meridian. This scale superseded the "universal time" - also known as "Greenwich Mean Time" obtained from astronomic observations. The time scale used now is called "Universal Time Coordinated" (UTC). Leap seconds intercalated into the UTC time scale about once a year ensure that UTC never deviates from the Universal Time determined by die Position of the sun by more than 0,9 second.

The Legal Time of die Federal Republic of Germany is either die Central European Time CET(D) or the Central European Summer Time CEST(D). Whether or not CEST(D) is adopted is stipulated in advance by a decree issued by die German government.

The following relations are valid between UTC and CET and between UTC and CEST:
CET(D) = UTC(PTB) + 1 h,
CEST(D) = UTC(PTB) +2 h.

Atomic second and atomic clocks

Atomic second

Pursuant to international agreements, the second as the unit of time is defined as follows:

The second is the duration of 9.192.631.770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.

According to this definition, the unit of time is realised with caesium atomic clocks which are manufactured by industry or constructed and operated by research laboratories to meet highest accuracy demands. Only about ten specimen's of the latter exist world-wide.
Principle underlying an atomic clock
Atomic clocks work on the following principle: Atoms occur in different energy states one of which is identified by the symbol (+) and one by the symbol (-). The transition of an atom from the (+) to the (-) state can be stimulated and is connected with the emission of electromagnetic radiation of a characteristic frequency. In the ease of the caesium atom, this frequency, fcs has a value of 9.192.631.770 Hz, corresponding to an oscillation period of (1 / 9.192.631.770) seconds. According to the laws of atomic physics, fcs is equal to the energy difference between the (+) and (-) states divided by the Planck constant h. For the caesium atom in particular, the constancy in time of fcs much better than that of, for example, the oscillation period of a pendulum, the oscillation frequency of a quartz or the rotation period of the earth.

Caesium atoms are evaporated in the vacuum chamber of an atomic clock. The magnet arranged behind the oven separates the atoms such that only atoms in the (+) state enter the cavity resonator. Here exposure to a microwave field stimulates the atoms to pass into the (-) state. The second magnet then directs to the detector only these atoms. The number of atoms at the collector reaches a maximum when the frequency of the irradiation field has the value fcs. A feedback circuit ensures that the microwave oscillator Q is kept at the frequency fcs. By the counting of 9.192.631.770 periods, the time interval of one second is obtained from the oscillator signal.

Diagram of an atomic clockDiagram of an atomic clock:
O atomic bem ovenQ microwave oscillator
M sorting magnetA detector
H cavity resonatorR servo control circuit

Horizontal section through the CS2 primary time and frequency standard of the PTB
Horizontal section through the CS2 primary time and frequency standard of the PTB
O caesium beam ovenS sorting magnets
V vacuum recipientM mu-metal shields
H cavity resonatorW beam reversal manipulator
C coil to generate a homogeneous magnatic field (C-field)A detector

Time signal and standard frequency transmitter DCF77


Mainflingen (50░ 01' north, 09░ 00' east), about 25 km south-east of Frankfurt a.M.

The steering signal is derived at the transmitter station from atomic clocks of the PTB and is controlled from Braunschweig.

Carrier frequency

Standard frequency: 77,5 kHz
Relative deviation of the carrier frequency from the nominal value on an average over

1 d: < 1 * 10-12
100 d: < 2 * 10-13

The phase time of DCF77 is adjusted such that, at the transmitting antenna, it is kept in agreement with UTC(PTB) in the limits of approximately ±0,3Ás. Larger phase and frequency variations observed at the receiving place are due to the superposition of ground and sky waves.


Transmitter power: 50 kW
Estimated radiated power: 30 kW
Effective coverage radius: approx. 2000 km


Vertical omnidirectional antenna with top-loading capacity, 150 m in height, or 200 m when standby antenna is used.
Time of transmission
24-hour continuous operation. Short intermissions (of a few minutes) are possible when a change-over to a standby transmitter or a standby antenna is necessary as a result of failures or maintenance work. Thunderstorms may result in longer lasting interruptions.

Time signals

The carrier is amplitude-modulated by means of second markers: At the beginning of each second (with the exception of the 59th second of each minute), the carrier amplitude is reduced to approx. 25% for a duration of 0,1 s or 0,2 s. The beginning of the carrier amplitude reductions indicates the exact beginning of the seconds. The absence of the 59th second marker announces the next minute marker.

The second markers arc phase-synchronous with the carrier.

The uncertainty with which the instants of the second markers can be received is higher than that of the steering atomic clocks This is due to the small bandwidth of the transmitting antenna, sky wave inferences and potential interference's. Nevertheless, for the reception of the second markers at distances of up to several hundred kilometres from the transmitter station, uncertainties of less than 0,1 ms can be attained.

Time code

During each minute, the numbers of the minute, hour, day, day of the week, month and year are transmitted in a BCD code by pulse-width modulation of the second markers. This "telegram" relates to the next following minute. Second markers with a duration of 0,1 s correspond to binary zero and second markers with a duration of 0,2 s to binary one. The assignment of the individual second markers to the time information transmitted is shown in the coding scheme below. The three parity cheek bits P1, P2 and P3 complete the preceding information words (7 bits for the minute, 6 bits for the hour and 22 bits for the date, including the number of the day of the week) In form an even number of binary ones.

Coding schemaCoding schema:
M minute marker (0,1 s)
R second marker No. 15 has a duration of 0,2 s when the standby antenna is used
A1 announcement of a forthcoming change from CET to CEST or vice versa
Z1, Z2 zone time bits
A2 announcement of a leap second
S starting bit of the coded time information (0,2 s)
P1-P3 parity check bits
The second markers No. 17 and 18 indicate the time system to which the transmitted time information is related. When CET is transmitted, the second marker No. 18 is extended to 0,2 s; the second marker No. 17 has a duration of 0,1 s. When CEST is emitted, this order is reversed.

In addition, prior to a forthcoming change from CET to CEST or vice versa, the second marker No. 16 is emitted for one hour as a prolonged marker (0.2 s). For a change from CET to CEST (CEST to CET), this prolongation begins at 01.00.16 hours CET (02.00.16 hours CEST and ends at 01.59.16 CET (02.59.16 CEST).

The second marker No. 19 announces a leap second. It is also emitted as a prolonged marker (0,2 s) for one hour prior to the intercalation of the leap second. Throughout the world, leap seconds are intercalated into the Coordinated Universal Time scale UTC at the same instant, preferably at the end of the last hour of December 31 or June 30. This means that in the Legal Time of the Federal Republic of Germany, leap seconds are intercalated one second before 1 hour CET on January 1 or one second before 2 hours CEST on July 1. When a leap second is intercalated on January 1 (July 1), the prolongation of the second marker No. 19 thus begins at 00.00.19 hour CET (01.00.19 hours CEST) and ends at 00.59.19 hours CET (01.59.19 hours CEST).

When a leap second is inserted, the associated minute has a duration of 61 seconds and the 59th second marker preceding the marker 01.00.00 hour CET or 02.00.00 hours CEST is emitted with a duration of 0,1 s. The marker associated with the intercalated 60th second is transmitted without carrier reduction.

Pseudo-random carrier phase shift keying

Since 1983, in addition to the AM, a pseudo-random phase noise has been modulated on the carrier of DCF77. To archive this, the phase is keyed according to a random binary sequence (phase-angle deviation: +-12%), the mean value of the carrier phase remaining unchanged.

The pseudo-random sequence is generated by a nine-stage shift register whose outputs 5 and 9 are fed back to the shift register input via an Exclusive-OR gate. 0,2 s after every beginning of a second, the shift register is started from the zero state and stopped after a complete cycle, i. e. approx. 7 ms prior to the next second marker. The clock frequency of 645,8333333 Hz is a subharmonic (77 500/120) of the carrier frequency. The duration of a noise cycle is 793 ms. With every noise cycle 1 bit is transmitted, an inverted pseudo-random sequence corresponding to data state 1. Except for the minute marker identification where a binary zero is transmitted, the binary information transmitted by sequence inversion keying (SIK) of the phase noise is the same as that transmitted by amplitude modulation. To indicate the minute marker with SIK, ten inverted pseudo-random sequences are transmitted in the seconds 0 to 9 in the place of omitting the 59th second marker in the case of amplitude modulation.

At the receiver end, the pseudo-random sequence used can be reproduced as a search signal and cross-correlated with the phase noise received. Cross correlation, combined with pseudo-random phase noise, allows the instants of arrival of the time signals received to be determined more accurately. Receiving of the amplitude-modulated time signals is not disturbed by the phase noise; neither am the properties of DCF77 as a standard frequency transmitter considerably influenced.

PTB Telephone Time Service

Since 1995, the PTB has been offering a telephone time service, phone number +49 531 51 20 38. It allows a computer to receive traceable time in formation from the PTB atomic clocks via the public telephone network and standard telephone modems. For the transmission of the time protocols an ASCII-character code is used which operates with most of the standard modems and computer systems. The following time information is transmitted: legal time and date including the numbers of the day of the week, of the calendar week and of the day of the year, the Coordinated Universal Time UTC, the date of a forthcoming change from Central European Time (CET) to Central European Summer Time (CEST) or vice versa, the date for the insertion of a leap second and the difference DUT1 between UTC and the astronomical time UT1.

In addition to the transmission of coded time information, the telephone time service is designed to determine the time delay along the propagation path. At the receiver end the arriving signal has to be echoed back to the transmitting time code generator in which the round-loop delay is measured. By advancing the transmitted signal by half the delay time, the system thus can correct the arrival time for the signal propagation, if the telephone link has approximate reciprocity. Uncertainties of a few milliseconds are achievable will this technique.

The communication parameters are: CCITT-V.22 modem, 1200 baud, 8 ASCII data bits, one stop bit, no parity. The change from CR (carriage return) to LF (line feed) indicates the beginning of each transmitted second. The information transmitted before this time marker (leading edge of the start bit of the LF) refers to the next
following second.

In order to correct for the propagation delay time, the code transmission is stopped by the command "//" (two slashes), the incoming CR-LF time markers are echoed back, and the time code generator is set by the command "GDM" (do generator delay time measurements) into the GDM mode. In this mode, the time code generator measures 8 round-loop delays, calculates the mean value and the standard deviation and advances the time marker for half the mean value determined. If the GDM has been successful, the visible time marker will change from "*" to "#".

The result of the delay time determination is reported in the time protocols.

Extract from the Time Act of 1978 and from a decree of 1994 of the Federal Government

1. Legal Time is realised and disseminated by the Physikalisch-Technische Bundesanstalt.

2. The Federal Government is authorised to introduce by decree Central European Summer Time for a period between March 1 and October 31 in order to better use Daylight and to adjust time metering to that of neighbouring states.

3. Central European Summer Time shall begin and end on a Sunday. In the decree according to paragraph 1, the Federal Government shall determine the day and the time when Central European Summer Time will begin and end as well as the designation of the hour appearing twice at the end of Central European Summer Time.

4. Central European Summer Time shall begin in

* 1996 on Sunday, March 31 and
* 1997 on Sunday, March 30

at 2 hours.

At the instant of the beginning of summer time, clocks shall be put forward by one hour from 2 hours to 3 hours.

5. Central European Summer Time shall end in

* 1996 on Sunday, October 27 and
* 1997 on Sunday, October 26

at 3 hours Central European Summer Time

At the instant of the end of summer time, clocks shall he put back by one hour from 3 hours to 2 hours.

6. Of the hour from 2 hours to 3 hours appearing twice at the end of Summer time on

* October 27, 1996 and
* October 26, 1997,

the first hour shall be referred to as 2A and the second hour as 2B.
Many thanks to the PTB for this informations.
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Last Update: 09.09.1999