### Generic NMEA GPS Receiver

#### Synopsis

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

#### Description

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)

    $GPRMC,POS_UTC,POS_STAT,LAT,LAT_REF,LON,LON_REF,SPD,HDG,DATE,MAG_VAR,MAG_REF*CC<cr><lf>
    $GPGLL,LAT,LAT_REF,LONG,LONG_REF,POS_UTC,POS_STAT*CC<cr><lf>
    $GPGGA,POS_UTC,LAT,LAT_REF,LONG,LONG_REF,FIX_MODE,SAT_USED,HDOP,ALT,ALT_UNIT,GEO,G_UNIT,D_AGE,D_REF*CC<cr><lf>

      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.

    "$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(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