NAME

       radioclkd - decode time from radio clock(s) attached to serial port

SYNOPSIS

       radioclkd [ -tphv ] device

DESCRIPTION

       radioclkd  is  a simple daemon that decodes the time from a radio clock device attached to the DCD and/or
       CTS and/or DSR status lines of serial port of a computer. It is able to decode the DCF77,  MSF  and  WWVB
       time  signals. The received time is then sent to ntpd using the shared memory reference clock driver. The
       type of time signal being received is automatically determined. If you have problems getting the  program
       to work using interrupts, the following command is known to help in many instances. If this fails you can
       always fall back to the polling method.

              stty crtscts < /dev/ttyS0

       Details on a cheap and easy to make device for receiving these time signals can be found at

              http://www.buzzard.org.uk/jonathan/radioclock.html

OPTIONS

       -p, --poll
              Poll the serial port for changes of status  in  the  DCD,  CTS  and  DSR  lines  rather  than  use
              interrupts

       -t, --test
              Enter  test  mode printing the length of each pulse and the decoded time at the end of each minute
              on stdout. The time is not sent to ntpd using the shared memory reference  clock  driver  in  this
              mode.

       -h, --help
              Print a short synopsis of the command line arguments.

       -v, --version
              Print the version number and then exit.

CONFIGURATION

       Configuration  is  very simple. Use server 127.127.28.0 in your ntp.conf file for a clock attached to the
       DCD line, server 127.127.28.1 for a clock attached to the CTS line, and server 127.127.28.2 for  a  clock
       attached  to  the  DSR line. You will also want to use a fudge line on the server to change the displayed
       refid.

CALIBRATION

       Due to delays in the propogation of the radio signal, it's processing  by  the  receiver  board  and  the
       latency of the operating system the time decoded by the receiver will be slightly offset from actual UTC.
       Typically this delay will be less than 20ms, so unless you are very fussy about the time,  or  are  using
       more  than  one  time source, such as a GPS unit, other radio clock or NTP server on the internet you can
       ignore this section.

       The basics of the calibration procedure is to determine the average offset of the radio receiver, and use
       the  time1  fudge  factor  in  ntp.conf  to bring the receiver as close as possible to the real time. The
       easiest way of determining the offset of the radio receivers time is to run it against a reference  clock
       that  does  not suffer from these problems. The best reference clock would be a GPS unit. This might be a
       GPS unit that you don't wish to dedicate to time keeping, or a borrowed unit. If this is not possible you
       could use a stratum 1 server on the internet.

       The  method of calibration is quite simple. We attach the calibration reference clock to the computer and
       fudge the stratum of our radio receiver up to say 5.  This way we can be sure that ntpd  will  lock  onto
       the  calibration reference clock. We need to make sure that ntpd is configured to collect peer statistics
       so make sure we have some lines similar to these in ntp.conf

           statsdir /var/log/ntpstats/
           statistics loopstats peerstats clockstats
           filegen peerstats file peerstats type day enable

       After that we restart ntpd and leave it running for several hours. We can then make a copy the  peerstats
       file.  The  trick  is  to  remove  all  the  entries  before  ntpd has come into close aggrement with the
       calibration reference clock and then run the peer.awk script in the scripts/stats directory  of  the  ntp
       distribution.  This  will  give us a mean offset of our radio receivers in milliseconds. This can them be
       converted into seconds and added to the fudge line in ntp.conf for our receiver.

       The final step is to remove the change in stratum level for our reference clock and restart ntpd. If  you
       move the receiver any significant distance then you will need to repeat this calibration step. Across the
       room or around the current building will be fine, but if you move it to the next town/city then you  will
       need to recalibrate.

IN USE

       The  version  of  ntpd that comes with most Linux distributions does not have the shared memory reference
       clock driver compiled in by default. This can be identified by checking the logs after ntpd  is  started.
       If  the shared memory reference clock driver is not compiled in then the logs will contain warnings about
       the reference clock driver not being recognized. To compile ntpd with the shared memory  reference  clock
       driver you must specify the --enable-SHM option when running configure.

       Neither  radioclkd or ntpd ever mark the shared memory segment for deletion. If you stop using the shared
       memory reference clock driver therefore any shared memory segments  will  persist  until  you  reboot  or
       manually  delete the segment using ipcrm.  The segments can be identified as the one with key 0x4e545030,
       0x4e545031 or 0x4e545032 using the ipcs command.

BUGS

       If you are running a kernel with the PPS kit and have a clock attached to the DCD line you may experience
       lockups.  If you encounter this problem the currently recommended solution is to move the clock to either
       the CTS or DSR lines.

AUTHOR

       This program was written by Jonathan Buzzard <jonathan@buzzard.org.uk>  and  may  be  freely  distributed
       under the terms of the GNU General Public License. There is ABSOLUTELY NO WARRANTY for this program.