### Reference Clock Options

#### Reference Clock Support

The NTP Version 4 daemon supports many 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 individual device drivers and options can be found in the Reference Clock Drivers page. Additional information can be found in the pages referenced there, including the Debugging Hints for Reference Clock Drivers and How To Write a Reference Clock Driver pages. In many drivers, support for a PPS signal is available as described in Pulse-per-second (PPS) Signal Interfacing page. Many drivers support special line discipline/streams modules which can significantly improve the accuracy using the driver. These are described in the Line Disciplines and Streams Drivers 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 U.S. 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 included 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. 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 information 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 additional information.

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 stratum 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 drivers.

#### Reference Clock Commands

`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 follows:

`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 to the power of two. For most directly connected reference clocks, both `minpoll` and `maxpoll` default to 6 (64 s). For modem reference clocks, `minpoll` defaults to 10 (17.1 m) and `maxpoll` defaults to 14 (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] [mode int] [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 decimal 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 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.
`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.
`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.
`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 verbose monitoring data to the `clockstats` file configured with the `filegen` command. Further information on the `filegen` command can be found in the Monitoring Options page.

`pps device [assert|clear] [hardpps]`
  Specifies the name of the serial port device to which the PPS signal is connected. Note, this command replaces use of `fudge flag3`, which was used for the same purpose in NTPv3. Note also that this command should preceed the `server` and `fudge` command for the same device.
`assert` `clear`
  Using `assert` or `clear` specifies if the high going or low going edge of the signal must be used. The default is `assert`.
`hardpps`
  This flag is used to tell the kernel that the signal from this device must be used to drive hardpps().
  The `assert`, `clear` and `hardpps` options are only available if the PPSAPI is used.