Specifications

CHAPTER 4. THE GPS-BASED CLOCK SYNCHRONIZATION SYSTEM 18
When the clock cards receive this signal, they start waiting for the next pulse on the 1Hz
PPS wire. Keep in mind that this pulse comes from the GPS and is perfectly synchronized
with the UTC time. When the pulse comes on the 1Hz wire, it means that the reference
time was reached and all the cards will reset their counters. Because this pulse comes from
the GPS and all the GPS cards are synchronized between them, we know that we obtained
global synchronization and all clock cards in the system will start counting from zero at
the same time.
Agree on a reference time
Put all slave clock cards in the "standby" state
When the time reaches 0.5 seconds before the reference
the masters send an "attention" pulse to all connected slaves
The site masters start polling the GPS cards
and wait for the reference time
When the slaves receive the "attention" signal, they
wait for the next pulse on the PPS wire
When the PPS pulse arrives, it means the reference time
was reached and all slaves cards reset their counters
Figure 4.2: The steps involved in the synchronization procedure
The steps involved in the synchronization are shown in Figure 4.2 and the setup on the
global scale is shown in Figure 4.3.
The slave clock cards are forced to count at the same rate by using the 10 MHz TTL
output from the GPS card. The 10 MHz signal is also connected to all the slave clock cards
inside a site. This signal is generated by an oscillator located on-board the GPS card. The
oscillator is controlled by the GPS receiver to output a precise 10 MHz signal. On other
words, the GPS controller keeps this signal as close as possible to a 10 MHz frequency.
A potential problem is the fact that this signal will be adjusted in different ways by each
GPS card. Among the possible reasons of such behavior are very small differences in the
physical characteristics of the crystals and the accuracy of the synchronization to the UTC.
Because of this, even after the initial reset all clocks are synchronized, we have to make
sure the synchronization is not lost after some time.
The solution involves using the 1 Hz pulse from the GPS card to reset the counters of the
slave cards to a known value at each occurrence of the signal. At the beginning of each
second (as triggered by the PPS pulse) we correct the values of the clock counters to known
values (by adding 40 * 10
6
to the value of the counter at the previous PPS). In this way,
the time difference between two synchronized systems is negligible (less then 500 ns) even
after several days of running freely.