Generic NMEA GPS Receiver

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

Synopsis

Address: 127.127.20.u

Reference ID: GPS

Driver ID: GPS_NMEA

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

Description

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

$GPRMC,UTC,POS_STAT,LAT,LAT_REF,LON,LON_REF,SPD,HDG,DATE,MAG_VAR,MAG_REF*CS<cr><lf>

$GPGLL,LAT,LAT_REF,LON,LON_REF,UTC,POS_STAT*CS<cr><lf>

$GPGGA,UTC,LAT,LAT_REF,LON,LON_REF,FIX_MODE,SAT_USED,HDOP,ALT,ALT_UNIT,GEO,G_UNIT,D_AGE,D_REF*CS<cr><lf>

$GPZDA,UTC,DD,MM,YYYY,TH,TM,*CS<cr><lf>

$GPZDG,GPSTIME,DD,MM,YYYY,AA.BB,V*CS<cr><lf>

Accord

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)

YYYY

Year

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

there is only fudge time2 available to
compensate for transmission delays but every sentence would need a
different one and

using more than one sentence per cycle overstuffs the internal data
filters.

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 76876.691 127.127.20.20 $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

Clockstats

Column

Sample

Meaning

1

56299

MJD

2

76876.691

Time of day in seconds

3

127.127.20.20

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.

"$PGRMO,,2<cr><lf>"

Now switch only $GPRMC on by sending it the following string.

"$PGRMO,GPRMC,1<cr><lf>"

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

"$PGRMC,,,,,,,,,,,,2<cr><lf>"

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

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