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Time Laboratory

DLR Time Laboratory and it's separation in rooms and measurement systems

In satellite navigation systems the distances between the satellite antennas and the receiver antenna are calculated out of one way propagation delay measurements of the satellite signals. The signals spread with approximately speed of light c = 2.99762458 e8 m/s. In 1 nanosecond (= 0,000 000 001s = 1*10-9 s) the signal spreads about 30cm. Due to the high requirement to the accuracy of satellite navigation systems of about a few meters a very accurate system time is required. The generation of such very stable system time scales is one topic among others in the time group using DLR’s time laboratory.
The time laboratory is separated in 3 different laboratory rooms. All rooms are air-conditioned and are powered by uninterruptible power supplies. In the clock room the very precise atomic clocks (3 active H-Masers and 2 passive H-Masers) are operated. So the very sensitive H-Masers can be operated without outside influences. In the UTC laboratory 3 Cs atomic clocks and 2 redundant GPS time receivers are operated. One of these Cs clocks represents universal time coordinated UTC(DLR). The signals of all atomic clocks are connected to the signal distributors and measurement systems in the time laboratory.

The time differences of the 1PPS (1 Pulse Per Second, rectangular signal) signals of all clocks are measured versus a reference clock using a time interval counter. A switch provides the 1PPS signals of all atomic clocks to the time interval counter successively (accuracy in sub ns range). The measurement interval for each clock is 100 seconds. The stored data is used than for generation of very stable time scales using data of several atomic clocks together (Ensemble Clock).

"Common View" Method

The phase comparators measure the changes of the phase differences between the 5/10 or 100MHz signals of the atomic clocks versus the reference clock signal. The very precise phase measurements (precision in ps range) can be used for clock stability analysis.
Frequency and time transfer between global distributed atomic clocks can be achieved by the Common View method. The GNSS (Global Navigation Satellite System) time receivers on both ground stations measure the time differences between their connected atomic clock and each satellite clock simultaneously. After the data is exchanged offline the satellite clock error can be eliminated in post-processing by calculating the difference between the measurement data of both stations. So the time difference between the two atomic clocks can be calculated in the range of a few ns.

DLR’s time laboratory can be divided in an experimental and in an operational part. The research activities are done in the experimental part:

• Research concerning Galileo PTF (Precise Timing Facility)
• Research concerning time synchronisation with the use of a Micro Phase Stepper
• Generation of very stable system time scales using several clocks’ data together (Ensemble Clock)
• Verification of system time algorithms (Composite Clock) using real measurement data
• Measurement and calibration of atomic clocks
• Research concerning time transfer

The operational part of the time laboratory is composed of UTC(DLR) and GATE (Galileo Test Environment in Berchtesgaden) system time provision. UTC(DLR) is determined by Common View method using GPS time receivers connected to a Cs clock. The recorded data is provided to BIPM (Bureau International des Poids et Mesures) in Paris which then calculates UTC using Common View data of world wide distributed time laboratories in post-processing. As the offset of each contributing clock is provided in the CircularT by BIPM the time difference of UTC(DLR) to UTC is well known.
The second operational part of the time laboratory consists of system time generation for GATE.