Generic NMEA GPS Receiver

Last update: 31-Mar-2014 03:55 UTC


Address: 127.127.20.u
Reference ID: GPS
Serial Port: /dev/gpsu; 4800 - 115200 bps, 8-bits, no parity
Serial Port: /dev/gpsppsu; for just the PPS signal (this is tried first for PPS, before /dev/gpsu)
Serial Port: /dev/gpsu; symlink to server:port (for nmead)
Features: tty_clk


This driver supports GPS receivers with the $GPRMC, $GPGLL, $GPGGA, $GPZDA and $GPZDG NMEA sentences by default.  Note that Accord's custom NMEA sentence $GPZDG reports using the GPS timescale, while the rest of the sentences report UTC.  The difference between the two is a whole number of seconds which increases with each leap second insertion in UTC.  To avoid problems mixing UTC and GPS timescales, the driver disables processing of UTC sentences once $GPZDG is received.

The driver expects the receiver to be set up to transmit at least one supported sentence every second.

The accuracy depends on the receiver used. Inexpensive GPS models are available with a claimed PPS signal accuracy of 1 μs 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 PPSAPI (RFC 2783), fudge flag1 1 enables its use.

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

Accepted NMEA sentences
Sentence Vendor

NMEA data items
Symbol Meaning and Format
UTC Time of day on UTC timescale. 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 of 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
GPSTIME Time of day on GPS timescale. Hours, minutes and seconds [fraction (opt.)]. (hhmmss[.f])
DD Day of the month (1-31)
MM Month of the year (1-12)
AA.BB Denotes the signal strength (should be < 05.00)
V GPS sync status
   '0' => INVALID time,
   '1' => accuracy of +/- 20ms,
   '2' => accuracy of +/- 100ns
CS Checksum
<cr><lf> Sentence terminator.

The 'mode' byte

Specific GPS sentences and bitrates may be selected by setting bits of the 'mode' in the server configuration line:
  server 127.127.20.x mode X

mode byte bits and bit groups
Bit Decimal Hex Meaning
0 1 1 process $GPMRC
1 2 2 process $GPGGA
2 4 4 process $GPGLL
3 8 8 process $GPZDA or $GPZDG
4-6 0 0 linespeed 4800 bps
16 0x10 linespeed 9600 bps
32 0x20 linespeed 19200 bps
48 0x30 linespeed 38400 bps
64 0x40 linespeed 57600 bps
80 0x50 linespeed 115200 bps
7 128 0x80 Write the sub-second fraction of the receive time stamp to the clockstat file for all recognised NMEA sentences. This can be used to get a useful value for fudge time2.
Caveat: This will fill your clockstat file rather fast. Use it only temporarily to get the numbers for the NMEA sentence of your choice.
8 256 0x100 process $PGRMF
9-15 0xFE00 reserved - leave 0
16 65536 0x10000 Append extra statistics to the clockstats line. Details below.

The default (mode 0) is to process all supported sentences at a linespeed of 4800 bps, which results in the first one received and recognised in each cycle being used.  If only specific sentences should be recognised, then the mode byte must be chosen to enable only the selected ones.  Multiple sentences may be selected by adding their mode bit values, but of those enabled still only the first received sentence in a cycle will be used.  Using more than one sentence per cycle is impossible, because

The driver uses 4800 bits per second by default, but faster bitrates can be selected using bits 4 to 6 of the mode field.

Caveat: Using higher line speeds does not necessarily increase the precision of the timing device.  Higher line speeds are not necessarily helpful for the NMEA driver, either.  They can be used to accomodate for an amount of data that does not fit into a 1-second cycle at 4800 bps, but high-speed high-volume NMEA data is likely to cause trouble with the serial line driver since NMEA supports no protocol handshake.  Any device that is exclusively used for time synchronisation purposes should be configured to transmit the relevant data only, e.g. one $GPRMC or $GPZDA per second, at a linespeed of 4800 bps or 9600 bps.

Monitor Data

The last GPS sentence that is accepted or rejected is written to the clockstats file and available with ntpq -c clockvar. (Logging the rejected sentences lets you see/debug why they were rejected.) Filtered sentences are not logged.

If the 0x10000 mode bit is on and clockstats is enabled, several extra counters will be appended to the NMEA sentence that gets logged. For example:

56299 76876.691 $GPGGA,212116.000,3726.0785,N,12212.2605,W,1,05,2.0,17.0,M,-25.7,M,,0000*5C  228 64 0 0 64 0

Column Sample Meaning
1 56299 MJD
2 76876.691 Time of day in seconds
3 IP Address from server config line
4 $GPGGA,...0*5C NMEA Sentence
5 228 Number of sentences received
6 64 Number of sentences accepted and used for timekeeping
7 0 Number of sentences rejected because they were marked invalid (poor signal)
8 0 Number of sentences rejected because of bad checksum or invalid date/time
9 64 Number of sentences filtered by mode bits or same second
10 0 Number of PPS pulses used, overrides NMEA sentences
Sentences like $GPGSV that don't contain the time will get counted in the total but otherwise ignored.

Configuring NMEA Refclocks might give further useful hints for specific hardware devices that exhibit strange or curious behaviour.

To make a specific setting, select the corresponding decimal values from the mode byte table, add them all together and enter the resulting decimal value into the clock configuration line.

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.


Fudge Factors

time1 time
Specifies the PPS time offset calibration factor, in seconds and fraction, with default 0.0.
time2 time
Specifies the serial end of line time offset calibration factor, in seconds and fraction, with default 0.0.
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
Disable PPS signal processing if 0 (default); enable PPS signal processing if 1.
flag2 0 | 1
If PPS signal processing is enabled, capture the pulse on the rising edge if 0 (default); capture on the falling edge if 1.
flag3 0 | 1
If PPS signal processing is enabled, use the ntpd clock discipline if 0 (default); use the kernel discipline if 1.
flag4 0 | 1
Obscures location in timecode: 0 for disable (default), 1 for enable.

Additional Information

flag1, flag2, and flag3 are ignored under Windows.

Reference Clock Drivers