See last page for copyright and document info, File: Reeve_GpsNtp-Pi_Setup.doc, Page 30
11. Provision NTP Server Pool
When the shared memory mode and PPS drivers were setup, the pool statement in the ntp.conf file was
commented out with the # character for testing purposes. If the RPi NTP server will not be used with any
external LAN or WAN NTP servers whatsoever, the pool statement can be left as-is:
# pool us.pool.ntp.org iburst minpoll 6 prefer
However, if internet access is available it is recommended to use backup servers in case of, for example, GPS
receiver or antenna failure or loss of signal because the antenna is in a poor location. Also, other servers can be
used in a peer arrangement to improve overall system performance (peering is beyond the scope of this project
but information can be found at {
NTPOrg
}).
A pool of external servers is accessed by the GpsNtp-Pi NTP server through an internet connection and access to
them is enabled by uncommenting the pool statement in the ntp.conf file:
pool us.pool.ntp.org iburst minpoll 6 prefer
When the pool has been enabled, it is necessary to restart the protocol:
sudo service ntp restart
Wait several minutes after restart and then query the NTP. If the NTP server pool statement is enabled, you
should see additional servers listed (for the US pool, typically 7 or more):
ntpq -p
The character in column 1 indicates the status. In the example above, the pool servers are marked with +
character indicating they have been found to be acceptable. Since these are taken from a pool, the list will
change over time. See {
NTPQ
-1} and {NTPQ-2} for a more complete description of the status indicators and
parameters.
If you experience problems using external time servers, make sure that UDP port 123 is open in both directions
on all firewalls between the GpsNtp-Pi network connection and the remote time servers that are to be used for
synchronization.
