See ConfiguringNMEARefclocksDev? for discussion of this topic.

6.1.12. Configuring NMEA Refclocks

Generic NMEA refclocks use the 127.127.20.x driver. The configuration directives required depends on the receiver and its internal configuration. The basic ntp.conf configuration is:

server 127.127.20.0

For configuration details please see the examples listed below or the distribution documentation Generic NMEA GPS Receiver page.

6.1.12.1. Accord NAV2300 GPS Clock

Accord's NAV2300 GPS Clock generates few standard NMEA sentences and a custom NMEA sentence at 9600B. Unlike the standard NMEA sentences which contain UTC time, the custom NMEA sentence has GPS time as shown below:

$GPZDG,HHMMSS.S,DD,MM,YYYY,AA.BB,V*CS<CR><LF>

It contains the GPS timestamp valid for next PPS pulse. Apart from the familiar fields,

• 'AA.BB' denotes the signal strength( should be < 05.00 )
• 'V' denotes the GPS sync status :
  • '0' indicates INVALID time,
  • '1' indicates accuracy of +/-20 ms
  • '2' indicates accuracy of +/-100 ns

Since the NMEA refclock driver was designed to work at 4800B, this GPS Clock cannot be used with NTP. To enable the support, the reference clock driver has been updated with following features:

• Support for 9600B (default is 4800B)
• Support for parsing Accord's custom NMEA sentence (contains GPS Time not UTC)

The unused bits of the mode field have been used for this purpose.

• bit 0 - enables RMC (1)
• bit 1 - enables GGA (2)
• bit 2 - enables GLL (4)
• bit 3 - enables ZGD (8) - Accord GPS Clock's custom sentence with GPS time
• bit 4 - selects the baudrate for serial port :
  • 0 for 4800B (default)
  • 1 for 9600B

Multiple sentences may be selected except when ZDG is selected. In case of incorrect mode selection, the driver falls back to standard NMEA sentences. The clock generates PPS pulse at TTL levels (with 5 ms on time). A sample configuration for this clock is shown below:

server 127.127.20.0 mode 24
fudge 127.127.20.0 flag2 0
fudge 127.127.20.0 flag3 1

-- VenuGopal - 11 Apr 2008

Related Topics: NMEARefclockUsers, Generic NMEA GPS Receiver page