### Generic NMEA GPS Receiver

#### 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 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 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 | Dec. | Meaning
--- | --- | ---
0 | 1 | process `$GPMRC`
1 | 2 | process `$GPGGA`
2 | 4 | process `$GPGLL`
3 | 8 | process `$GPZDA` or `$GPZDG`
4-6 | 0 | linespeed 4800 bps
16 | linespeed 9600 bps
32 | linespeed 19200 bps
48 | linespeed 38400 bps
64 | linespeed 57600 bps
80 | linespeed 115200 bps
7 | 128 | 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.

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.

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>"

#### Monitor Data

The GPS sentence that is used is written to the clockstats file and available with ntpq -c clockvar.

#### 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

Reference Clock Drivers