3.7.2 Allow 2D Fix for NTP
This checkbox will allow the TM2000B to start serving time once a 2D lock is achieved by the GPS
receiver. It is generally better to get a 3D lock as timing accuracy is improved and more stable. The
GPS module maker doesn't guarantee accuracy until a 3D lock is achieved, but this option exists if
needed. If the TM2000B is capable of getting a 3D lock, then this option should generally be un-
checked.
3.7.3 Disable Holdover Limit
When checked, this option allows the holdover limit of the device to extend forever. The TM2000B
requires a GPS lock to initially set its time, but by checking this option, it can then be run without a
GPS lock. The time WILL drift off of standard at a rate that is dependent on the accuracy of the OCXO
of the device, but this option is included for cases where exact accuracy isn't required, just
synchronization of devices withing a closed system. Drift has been measured as high as 0.1 seconds
per week. This setting also governs the holdover behavior of the PTP modes.
3.7.4 NTP Holdover Time (Minutes)
This setting is used to set how long holdover is allowed GPS lock is lost. The device will change to
Stratum 16 when the set holdover time is expired and GPS lock is not achieved.
3.7.5 Remote NTP Servers Enable
Enabling this option, and setting a list of remote NTP servers will allow the TM2000B to get its time
from another NTP server using the NTP protocol. When operating in this mode, the TM2000B can
begin serving time shortly after it has booted up because it doesn't have to wait for the GPS to lock. It
will also allow the TM2000B to operate without a GPS at all if desired. If at some point after boot up,
GPS lock is obtained, then the GPS becomes the time source and Peer NTP lookups are stopped. Once
the TM2000B has obtained a lock, if it is lost it will not return to the Peer NTP servers for time
maintenance, even after the holdover period has expired.
3.7.6 Remote NTP Servers List
The NTP Servers list allows setting of up to three different time sources. The TM2000B runs a version
of the Linux NTP daemon that will select the “best” source for setting its time, but will also keep tabs
on all of the time sources. If authentication is being used, a key values, 1-65535 can be entered into the
key field on the same line as the server.
3.7.7 Serve Time Only With Authentication
Checking this option will set the server into an authenticated client only mode. Normally, if a ntp.keys
file has been uploaded, and the client sends authentication credentials, the TM2000B will check and
respond with authentication. If a client does not send an NTP request with authentication information,
then the TM2000B will respond with a non-authenticated packet. However, if this mode is selected,
the TM2000B will only respond to clients that have valid authentication included in the NTP request.
3.7.8 Authentication-Upload Key List
Starting is version 0.3.4, the NTP server of the TM2000B supports authentication. Both MD5 and
SHA1 are supported. While these are not considered cryptographically secure in this modern age of
Installation and Operations Manual | Page 13
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