Configuring Modem Refclocks


Dave Mills uses:

 server minpoll 12 maxpoll 17 
 phone atdt913034944774 atdt917195676742 

This calls NIST in Boulder CO first, and if that fails USNO in Boulder is called.

The initial 9 is to obtain an outside line.

Dave also says: The phone string is copied verbatim to the modem as per Hayes command set. You can test by sending the same string to the modem using a terminal program. You can also test using the NTP filegen facility and flag 4 set on the fudge command. The modem protocol and command responses are written to the clockstats file.


Dave found the following:

I stumbled on . Apparently, PTB does something similar to NIST. It can measure and correct for the propagation delay. However, while the NIST correction is automatic, the PTB correction must be enabled by a special sequence, as quoted below.

"The communication parameters are: CCITT-V.22 modem, 1200 baud, 8 ASCII data bits, one stop bit, no parity. The change from CR (carriage return) to LF (line feed) indicates the beginning of each transmitted second. The information transmitted before this time marker (leading edge of the start bit of the LF) refers to the next following second.

In order to correct for the propagation delay time, the code transmission is stopped by the command // (two slashes), the incoming CR-LF time markers are echoed back, and the time code generator is set by the command GDM (do generator delay time measurements) into the GDM mode. In this mode, the time code generator measures 8 round-loop delays, calculates the mean value and the standard deviation and advances the time marker for half the mean value determined. If the GDM has been successful, the visible time marker will change from * to #.

The result of the delay time determination is reported in the time protocols."

Related Topics: ModemRefclockUsers
Topic revision: r3 - 13 Nov 2008, HarlanStenn
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