[ntp:questions] NMEA serial driver using 9600 baud rate

Dave Hart davehart at gmail.com
Sun Apr 18 22:48:28 UTC 2010

On Sat, Apr 17, 2010 at 5:37 PM, Chaofu Chen wrote:
> Hi all, I'm confused that ntp does not support NMEA serial driver using
> 9600 baud rate. According to the doc, this should done by specifing
> "mode 18"(GGA sentence at speed of 9600bps) in ntp.conf. while it
> seems does not work in my case(FreeBSD 7.2R with NTP v4.2.4p5).

You must be looking at documentation for a newer revision of the NMEA
driver.  In 4.2.4p5 it supports 4800 only unless you hack the source:

	fd = refclock_open(device, SPEED232, LDISC_CLK);


	fd = refclock_open(device, B9600, LDISC_CLK);

Alternatively, use a newer ntpd built from a ntp.org tarball.  Here's
the text of the driver20.html for 4.2.4p5:

Generic NMEA GPS Receiver


Address: 127.127.20.u
Reference ID: GPS
Serial Port: /dev/gpsu; 4800 baud, 8-bits, no parity
Serial Port: /dev/gpsu; symlink to server:port (for nmead) Features: tty_clk

This driver supports GPS receivers with the $GPRMC NMEA output string
by default.  Alternately the $GPGGA or $GPGLL may be selected.

The driver expects the receiver to be set up to transmit a $GPRMC
message every second.

The accuracy depend on the receiver used. Inexpesive GPS models are
available with a claimed PPS signal accuracy of 1 ms or better
relative to the broadcast signal. However, in most cases the actual
accuracy is limited by the precision of the timecode and the latencies
of the serial interface and operating system.

If the Operating System supports the PPSAPI, RFC-2783, it will be used.

The various GPS sentences that this driver recognises look like this:
(others quietly ignored)


  POS_UTC  - UTC of position. Hours, minutes and seconds [fraction
(opt.)]. (hhmmss[.fff])
  POS_STAT - Position status. (A = Data valid, V = Data invalid)
  LAT      - Latitude (llll.ll)
  LAT_REF  - Latitude direction. (N = North, S = South)
  LON      - Longitude (yyyyy.yy)
  LON_REF  - Longitude direction (E = East, W = West)
  SPD      - Speed over ground. (knots) (x.x)
  HDG      - Heading/track made good (degrees True) (x.x)
  DATE     - Date (ddmmyy)
  MAG_VAR  - Magnetic variation (degrees) (x.x)
  MAG_REF  - Magnetic variation (E = East, W = West)
  FIX_MODE - Position Fix Mode ( 0 = Invalid, >0 = Valid)
  SAT_USED - Number Satellites used in solution
  HDOP     - Horizontal Dilution of Precision
  ALT      - Antenna Altitude
  ALT_UNIT - Altitude Units (Metres/Feet)
  GEO      - Geoid/Elipsoid separation
  G_UNIT   - Geoid units (M/F)
  D_AGE    - Age of last DGPS Fix
  D_REF    - Reference ID of DGPS station
  CC       - Checksum (optional)
  <cr><lf> - Sentence terminator.
Alternate GPS sentences (other than $GPRMC - the default) may be
enabled by setting the relevent bits of 'mode' in the server
configuration line
 * server 127.127.20.x mode X
    bit 0 - enables RMC    ( value = 1)
    bit 1 - enables GGA    ( value = 2)
    bit 2 - enables GLL      ( value = 4)
multiple sentences may be selected

The driver will send a $PMOTG,RMC,0000*1D<cr><lf> message each time a
$GPRMC string is needed. This is not needed on most GPS receivers
because they automatically send the $GPRMC string every second and
will only work on GPS receivers that understand the $PMOTG string.
Others will just ignore it.

Setting up the Garmin GPS-25XL
Switch off all output with by sending it the following string.
Now switch only $GPRMC on by sending it the following string.

On some systems the PPS signal isn't switched on by default. It can be
switched on by sending the following string.

Monitor Data
The GPS sentence(s) that is used is written to the clockstats file.

Fudge Factors
time1 time
Specifies the time offset calibration factor, in seconds and fraction,
with default 0.0.
time2 time
Not used by this driver.
stratum number
Specifies the driver stratum, in decimal from 0 to 15, with default 0.
refid string
Specifies the driver reference identifier, an ASCII string from one to
four characters, with default GPS.
flag1 0 | 1
Not used by this driver.
flag2 0 | 1
Specifies the PPS signal on-time edge: 0 for assert (default), 1 for clear.
flag3 0 | 1
Controls the kernel PPS discipline: 0 for disable (default), 1 for enable.
flag4 0 | 1
Not used by this driver.
Additional Information

Reference Clock Drivers

Dave Hart

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