Specifications

CHAPTER 4. THE GPS-BASED CLOCK SYNCHRONIZATION SYSTEM 17
4.2 Description of the method
At each site of the testbed we use one GPS card and several clock cards (one for each
computer). The two signals from the GPS (10MHz and PPS) are distributed to all clock
cards. The slave clock cards are forced to count at the s ame rate by using the 10MHz
output from the GPS clock card — in this way they are automatically synchronized. The
setup inside a site is presented in Figure 4.1
Slave Clock
Card
Slave Clock
Card
Slave Clock
Card
Master Clock
Card Wire for commands from the master card
10 MHz and 1Hz PPS signal from GPS
GPS Card
Figure 4.1: Local synchronization in a testbed site
Our global clock synchronization syste m is based on two key points:
• the ability to reset the counters of all the slave clock cards at the same time
• the ability to force the slave clock c ards to count at exactly the same rate
Resetting the counters at the same time is accomplished with the help of the GPS card
and 1Hz PPS signal. This signal is issued by the card with an accuracy of 250 ns with
respect to the UTC second. The 1 Hz signal is connected directly to all clock cards in a
site. When a counter reset is required, the slave clock cards are put in a special listening
mode by software. In this mo de, the cards are waiting for the next 1 Hz pulse to reset its
counter — when the first pulse comes, all the cards on the site reset their counter to zero.
Because the PPS signal is triggered by the UTC second, it means that it will arrive in the
same time in two remote sites that use different GPS cards. In this way we synchronize all
the clock cards of the testbed.
The procedure is as follows. First the users of the system agree to a certain reference time
at which they want to reset all the clocks. Let’s say this time is 8:00:00 o’clock. After this
the master computer at each site will put all the clock cards in a ”standby” state (using a
software signal, sent via the normal network).
Then the software that runs an all master computers in the system will start querying
continuously the time from the GPS, waiting for the reference time. When the time is 7:59
and 59.5 seconds (0.5 seconds before the reference time), the master card sends a signal to
all the clock cards from the neighboring computers. This signal is sent over a wire that is
linked to every card (all cards being connected in series, in a chain), so this signal is not
affected by network delays.