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ntp_clock(5)                                                      ntp_clock(5)

       ntp_clock - Reference Clock Options

       The NTP Version 4 daemon supports some three dozen different radio, satellite and modem reference clocks plus a
       special pseudo-clock used for backup or when no other clock source is available. Detailed descriptions of indi-
       vidual  device drivers and options can be found in the Reference Clock Drivers page. Additional information can
       be found in the pages linked there, including the Debugging Hints for Reference Clock Drivers and How To  Write
       a  Reference  Clock Driver pages. In addition, support for a PPS signal is available as described in Pulse-per-
       second (PPS) Signal Interfacing page.

       A reference clock will generally (though not always) be a radio timecode receiver which is  synchronized  to  a
       source  of  standard  time  such  as the services offered by the NRC in Canada and NIST and USNO in the US. The
       interface between the computer and the timecode receiver is device dependent, but is usually a serial  port.  A
       device driver specific to each reference clock must be selected and compiled in the distribution; however, most
       common radio, satellite and modem clocks are included by default. Note that an attempt to configure a reference
       clock  when the driver has not been compiled or the hardware port has not been appropriately configured results
       in a scalding remark to the system log file, but is otherwise non hazardous.

       For the purposes of configuration, ntpd treats reference clocks in a manner analogous to normal  NTP  peers  as
       much  as  possible. Reference clocks are identified by a syntactically correct but invalid IP address, in order
       to distinguish them from normal NTP peers. Reference clock addresses are of the form 127.127.t.u, where t is an
       integer  denoting  the clock type and u indicates the unit number in the range 0-3. While it may seem overkill,
       it is in fact sometimes useful to configure multiple reference clocks of the same type, in which case the  unit
       numbers must be unique.

       The  server  command  is used to configure a reference clock, where the address argument in that command is the
       clock address. The key, version and ttl options are not used for reference clock support. The  mode  option  is
       added  for  reference clock support, as described below. The prefer option can be useful to persuade the server
       to cherish a reference clock with somewhat more enthusiasm than other reference clocks or peers. Further infor-
       mation on this option can be found in the Mitigation Rules and the prefer Keyword page. The minpoll and maxpoll
       options have meaning only for selected clock drivers. See the individual clock driver document pages for  addi-
       tional information.

       The  fudge  command is used to provide additional information for individual clock drivers and normally follows
       immediately after the server command. The address argument specifies the clock address. The refid  and  stratum
       options  control  can  be used to override the defaults for the device. There are two optional device-dependent
       time offsets and four flags that can be included in the fudge command as well.

       The stratum number of a reference clock is by default zero. Since the ntpd daemon adds one to  the  stratum  of
       each  peer,  a  primary  server  ordinarily displays an external stratum of one. In order to provide engineered
       backups, it is often useful to specify the reference clock stratum as greater than zero. The stratum option  is
       used  for this purpose. Also, in cases involving both a reference clock and a pulse-per-second (PPS) discipline
       signal, it is useful to specify the reference clock identifier as other than  the  default,  depending  on  the
       driver.  The  refid  option  is  used  for  this  purpose. Except where noted, these options apply to all clock

       server 127.127.t.u [prefer] [mode int] [minpoll int] [maxpoll int]
               This command can be used to configure reference clocks in special ways. The options are interpreted  as

               prefer  Marks  the reference clock as preferred. All other things being equal, this host will be chosen
                       for synchronization among a set of correctly operating hosts. See the Mitigation Rules and  the
                       prefer Keyword page for further information.

               mode int
                       Specifies  a  mode  number  which is interpreted in a device-specific fashion. For instance, it
                       selects a dialing protocol in the ACTS driver and a device subtype in the parse drivers.

               minpoll int

               maxpoll int
                       These options specify the minimum and maximum polling interval for reference clock messages  in
                       seconds,  interpreted as dual logarithms (2 ^ x). For most directly connected reference clocks,
                       both minpoll and maxpoll default to 6 (2^16 =  64  s).  For  modem  reference  clocks,  minpoll
                       defaults  to  10  (2^10 = 1024 s = 17.1 m) and maxpoll defaults to 14 (2^14 = 16384 s = 4.5 h).
                       The allowable range is 4 (16 s) to 17 (36.4 h) inclusive.

       fudge 127.127.t.u [time1 sec] [time2 sec] [stratum int] [refid string] [flag1  0|1]  [flag2  0|1]  [flag3  0|1]
       [flag4 0|1]
               This command can be used to configure reference clocks in special ways. It must immediately follow  the
               server command which configures the driver. Note that the same capability is possible at run time using
               the ntpdc program. The options are interpreted as follows:

               time1 sec
                       Specifies a constant to be added to the time offset produced by the driver, a fixed-point deci-
                       mal number in seconds. This is used as a calibration constant to adjust the nominal time offset
                       of a particular clock to agree with an external standard, such as a precision  PPS  signal.  It
                       also  provides a way to correct a systematic error or bias due to serial port or operating sys-
                       tem latencies, different cable lengths or receiver internal delay. The specified offset  is  in
                       addition  to the propagation delay provided by other means, such as internal DIPswitches. Where
                       a calibration for an individual system and driver is available, an  approximate  correction  is
                       noted  in  the  driver documentation pages.  Note: in order to facilitate calibration when more
                       than one radio clock or PPS signal is supported, a special calibration feature is available. It
                       takes the form of an argument to the enable command described in the Miscellaneous Options page
                       and operates as described in the Reference Clock Drivers page.

               time2 secs
                       Specifies a fixed-point decimal number in seconds, which is interpreted in  a  driver-dependent
                       way. See the descriptions of specific drivers in the reference clock drivers page.

               stratum int
                       Specifies  the  stratum number assigned to the driver, an integer between 0 and 15. This number
                       overrides the default stratum number ordinarily assigned by the driver itself, usually zero.

               refid string
                       Specifies an ASCII string of from one to four characters which defines the reference identifier
                       used  by  the  driver.  This string overrides the default identifier ordinarily assigned by the
                       driver itself.

               flag1 flag2 flag3 flag4
                       These four flags are used for customizing the clock driver. The interpretation of these values,
                       and  whether  they  are  used at all, is a function of the particular clock driver. However, by
                       convention flag4 is used to enable recording monitoring data to the clockstats file  configured
                       with  the filegen command. Further information on the filegen command can be found in the Moni-
                       toring Options page.


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