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Because every fraction of a second counts 

Function and Comparison of Accuracy DCF77 and GPS Time Signal Receiver

1. Function DCF77 Transmitter-Receiver

The time code transmitter DCF77 emits a time signal and time code information in the long wave range of 77.5 kHz. The time information is visualised by the lowering of the carrier power to 25% of the standard value (amplitude modulation). The beginning of a second is marked by the beginning of a lowering. The lowering takes 0.1 sec for a logic "0" and 0.2 sec for a logic "1".

During a minute the BCD values for the minutes, hours, day of the week, month and year are transmitted after the 20th second. Being the synchronisation marker the 59th second is not lowered.

The emitted power does not immediately drop to the 25%-value. Because of the resonance quality of the antenna this value is reached after 600-800 µsec.

The inaccuracy of the transmitted carrier frequency

 

As the carrier frequency and the carrier lowering have the same source, the above inaccuracy also applies to the beginning of the lowering of the second markers.

The DCF77-signal is usually received by a an active ferrite antenna and fed to a straight-receiver. Rectification and levelling of the signal turn the decoded DCF77-signal into a pulse data string.

2. Sources of Interference

The time code is transmitted in the longwave range by amplitude modulation, it can therefore be easily disturbed. The many sources of interference include atmospheric disturbances like thunderstorms on the way to the receiver. In case of thunderstorms near the location of the transmitter the transmission is stopped for the length of the storm. This may take up to several hours.

Interferences on the location of the receiver are mainly caused by motors, monitors, displays, corona discharges from high-voltage lines or switching contactors. The place for the antenna must therefore be selected with utmost care. Narrowband receivers are an alternative to suppress interferences.

Please note: Narrowband reception and accuracy exclude each other!!

3. Accuracy DCF77

The short-term and the long-term accuracy of the DCF77-signal show a considerable difference.

The decoded second marker may deviate from the absolute second marker by +5 to +150 msec if standard decoding techniques are used. This is due mainly to the used signal filters and the signal rectification. Narrow-band antennas and very narrow-band crystal filters are used to suppress interferences, which results in a long decay time. The edge is further delayed by the rectification used to obtain the pulse.

The accuracy suffices completely for the every-day use of our clocks where the long-term accuracy matters. After one year the deviation of the second is no more than +5 to +150 msec.

For industrial purposes these deviations are not tolerable. For more accurate second markers both the antenna and the receiver must be wide-band. Values between +5 to +15 msec require bandwidths of about 4 kHz for the antenna which means that the antenna transmits far more noise to the electronics and that the reception electronics often cannot decode the minute cycle. Comparing these to the clocks for every-day use this liability to interferences is mistaken for too little sensitivity.

Basically the following is correct:

Short-term accuracy and high noise immunity exclude each other under DCF77

By changing an amplitude modulated signal to a frequency modulated signal a tolerable accuracy is reached.

During the DCF77 lowering of the second the frequency changes from about 500 Hz to 400 Hz. In the decoding process the pulse width of every frequency oscillation is measured and saved. In case of a change in the pulse width the starting point is traced back and interpreted as the second marker. The accuracy which is achieved ranges around a pulse width, i.e. ± 2 msec. Over a period of one minute the second markers are observed and tendencies are ascertained. If for example the calculated second marker tends to be earlier, on average, than in the previous minute, two control values are deducted:

This process adjusts the crystal frequency to ± 2 ppm inaccuracy for the free-running of the clock.

Further inaccuracies may be caused by travel times from the transmitter to the receiver. In case of just ground-wave reception a constant is included in the calculation if the distance is permanent. In case of just space wave reception the reception side cannot influence the time fluctuations. Time fluctuations are influenced directly by the changing altitude of the reflecting and bending layer of the ionosphere. Similar problems arise where ground and space waves overlap. This field is not constant but changes in the course of the day between 600 to 1200 km from the transmitter position. At fixed locations there may be time fluctuations in the range of some milliseconds.

4. Function GPS

When GPS systems are used as timers world-wide operation at highest accuracy is possible. At an altitude of about 20,000 km satellites circle around the earth on 6 different orbits twice a day. There are 3 satellites on every orbit. Every satellite contains 2 atomic clocks being as accurate as at least 1 x 10 -12.

The satellites constantly transmit their position and the GPS world time at the same point of time at a frequency of 1.57542 GHz. GPS antenna receive the data from the satellites moving in the view range of the antenna. These data are then decoded by a 6 to 12 channel GPS receiver. First the position of the receiving antenna is calculated from these values. Once the position is calculated the travelling times of the transmitter information from the individual satellites can be determined.

The GPS time information and the average travelling times are used to construct the GPS world time (GPS-UTC) achieving an accuracy of ± 1 µsec. The accuracy of the time determination depends above all on how accurately the position has been calculated.

The world time UTC is calculated by deducting the leap second from GPS-UTC. The leap seconds offer the chance to level out the inaccuracy of the speed of the earth rotation. The adjustment can be done automatically because the satellites include the difference in their transmitted information.

The local time can now be calculated precisely by adding or subtracting a time offset from UTC.

5. Sources of Interference

The GPS signal is nearly disturbance-proof due to the high transmission frequency of 1.57542 GHz. Very narrow-banded antennas and filters can be used to decode the signal, because the information is transmitted in the phase modulation at constant amplitude.

There are no atmospheric disturbances at great heights. The transmission between layers in the atmosphere can cause time offsets, but only in terms of picoseconds. The data from 4 satellites are used for the calculation of the 3D position. If the antenna has a clear view to the horizon an average of 7 to 9 satellites are visible. That means the time information is 100% available. Even if half the horizon is covered the availability still reaches 90 to 95%. Due to the low transmission power of the satellites and the high frequency the cables between the antenna and the electronics must be short, otherwise the signal cannot be filtered from the noise.

Military ground control stations may interfere with the accuracy of the position calculation for a time. Then some satellites transmit wrong orbit data, from which the travelling time of the data is calculated wrongly. The error in the calculation of the time may amount to some µsec.

6. Accuracy under GPS

The accuracy of the individual second marker is, other than with DCF77, the same at every location. It is about ± 1 µsec using standard GPS-receivers and decoding of the time marker. This allows the standard crystals for the free-running characteristics of the clock to be adjusted to ± 0.1 ppm. Also a far better adjustment control of the second marker is possible. Even better free-running characteristics are achieved when oven and temperature stabilised crystals are used, i.e. values between 0.1 and 2.0 ppm.

The time marker of plain GPS position receivers, used for private purposes like sailing, walking etc. is not more accurate than DCF77.

With GPS a high short-term and long-term accuracy is achieved.

7. Future Development

In future GPS will replace DCF77 systems in all the industrial fields where very precise time markers matter. The fast development of the world-wide use of GPS had the effect that the prices for GPS receivers with time decoding have dropped from 20,000 $/piece to less than 1,000 $/piece since 1990.

As GPS has gained importance in the car sector and the American industry cannot be imagined without it any more, the military use of the system has been pushed back to 2nd place. The system is not likely to be removed without being appropriately substituted within the next 20-30 years.

8. Features of GPS and DCF77

8.1 Place of Use

GPS: world-wide
DCF77: within a 2000 km radius around Frankfurt

8.2 Antenna

GPS: Only outdoor antennas possible. This requires more complicated equipment for lightning protection. Also the antenna circuit should be potential free, because in case of lightning protection the antenna coat is earthed which may cause earth loops. The standard antenna cable must not exceed 25 m because of the high frequency and the low reception power. Cable lengths up to 200 m can be reached with special cables and power amplifiers.
DCF77: Outdoor and indoor antenna are possible. Standard cables can bridge distances of up to 500 m between the antenna and the electronics. Outdoor antennas also require lightning protection and potential free antenna circuits.

8.3 Decoding

GPS: The decoding of the high frequency and the low reception power cannot yet be covered by standard equipment. High quality devices and high computing power is necessary. World-wide use make a broad and cheap supply very likely in the near future.
DCF77: The decoding can be done by simple standard units. Low-cost clocks for every-day use are available for less than 50 Euro.

8.4 Noise Immunity

GPS: The high frequency and the phase modulation of the signal guarantee high noise immunity. It is difficult to simulate this signal.
DCF77: Due to the low frequency and amplitude modulation the signal is liable to many interferences from atmospheric, magnetic and electric sources. It is easy to simulate the signal.

8.5 Accuracy

GPS: With time decoding programme high short-term accuracy of ± 1 µsec. Good control characteristics free running crystals (standard ± 0.1 ppm).
DCF77: Bad short-term accuracy, as a rule +5 to +25 msec, pretty bad characteristics for free-running crystals achievable ± 2 ppm.

8.6 Terms

DCF77: German time signal transmitter transmission frequency 77.5 kHz
GPS: Global Positioning System, navigation system supported by satellites, transmission frequency 1.57542 GHz for commercial purposes
UTC: Universal Time Co-ordinated, co-ordinated global time, previously GMT
GPS-UTC: continuous global time without correction by leap second
GHz: 1 billion Hertz
ppb: part per billion = 1 * 10 -9 e.g. time error 1 ppb = 0.0864 msec per day
ppm: part per million = 1 * 10 -6 e.g. time error 1 ppm = 86.4 msec per day
msec: 1 thousandth of a second
µsec: 1 millionth of a second
3D: three dimensional calculation of the position, longitude, latitude and altitude

Author: B. Rega
Company: hopf Elektronik GmbH