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<title>Generic NMEA GPS Receiver
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Generic NMEA GPS Receiver</h3>
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Address: 127.127.20.<i>u</i>
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<br>Reference ID: <tt>GPS</tt>
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<br>Driver ID: <tt>GPS_NMEA</tt>
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<br>Serial Port: <tt>/dev/gps<i>u</i></tt>; 4800 baud, 8-bits, no parity
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<br>Features: <tt>tty_clk</tt>
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This driver supports GPS receivers with the <tt>$GPRMC</tt> NMEA output
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string by default. Alternately the <tt>$GPGGA</tt> or <tt>$GPGLL
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<br>The driver expects the receiver to be set up to transmit a <tt>$GPRMC</tt>
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<p>The accuracy depend on the receiver used. Inexpesive GPS models are
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available with a claimed PPS signal accuracy of 1 <font face="Symbol">m</font>s
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or better relative to the broadcast signal. However, in most cases the
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actual accuracy is limited by the precision of the timecode and the latencies
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of the serial interface and operating system.
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<p>If the Operating System supports the PPSAPI, RFC-2783, it will be used.
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<p>The various GPS sentences that this driver recognises look like this:
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<br>(others quietly ignored)
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<pre><tt>$GPRMC,POS_UTC,POS_STAT,LAT,LAT_REF,LON,LON_REF,SPD,HDG,DATE,MAG_VAR,MAG_REF*CC<cr><lf>
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$GPGLL,LAT,LAT_REF,LONG,LONG_REF,POS_UTC,POS_STAT*CC<cr><lf>
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$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>
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POS_UTC - UTC of position. Hours, minutes and seconds [fraction (opt.)]. (hhmmss[.fff])
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POS_STAT - Position status. (A = Data valid, V = Data invalid)
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LAT - Latitude (llll.ll)
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LAT_REF - Latitude direction. (N = North, S = South)
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LON - Longitude (yyyyy.yy)
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LON_REF - Longitude direction (E = East, W = West)
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SPD - Speed over ground. (knots) (x.x)
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HDG - Heading/track made good (degrees True) (x.x)
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DATE - Date (ddmmyy)
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MAG_VAR - Magnetic variation (degrees) (x.x)
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MAG_REF - Magnetic variation (E = East, W = West)
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FIX_MODE - Position Fix Mode ( 0 = Invalid, >0 = Valid)
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SAT_USED - Number Satellites used in solution
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HDOP - Horizontal Dilution of Precision
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ALT - Antenna Altitude
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ALT_UNIT - Altitude Units (Metres/Feet)
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GEO - Geoid/Elipsoid separation
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G_UNIT - Geoid units (M/F)
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D_AGE - Age of last DGPS Fix
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D_REF - Reference ID of DGPS station
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CC - Checksum (optional)
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<cr><lf> - Sentence terminator.</tt></pre>
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Alternate GPS sentences (other than <tt>$GPRMC</tt> - the default) may
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be enabled by setting the relevent bits of 'mode' in the server configuration
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<br> * server 127.127.20.x mode X
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<br> bit 0 - enables RMC ( value =
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<br> bit 1 - enables GGA ( value =
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<br> bit 2 - enables GLL
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<br>multiple sentences may be selected
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<p>The driver will send a <tt>$PMOTG,RMC,0000*1D<cr><lf></tt> message
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each time a <tt>$GPRMC</tt> string is needed. This is not needed on most
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GPS receivers because they automatically send the <tt>$GPRMC</tt> string
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every second and will only work on GPS receivers that understand the <tt>$PMOTG</tt>
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string. Others will just ignore it.
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Setting up the Garmin GPS-25XL</h4>
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Switch off all output with by sending it the following string.
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<pre>"$PGRMO,,2<cr><lf>"</pre>
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Now switch only $GPRMC on by sending it the following string.
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<pre>"$PGRMO,GPRMC,1<cr><lf>"</pre>
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On some systems the PPS signal isn't switched on by default. It can be
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switched on by sending the following string.
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<pre>"$PGRMC,,,,,,,,,,,,2<cr><lf>"</pre>
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The GPS sentence(s) that is used is written to the clockstats file.
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<tt>time1 <i>time</i></tt></dt>
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Specifies the time offset calibration factor, in seconds and fraction,
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with default 0.0.</dd>
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<tt>time2 <i>time</i></tt></dt>
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Not used by this driver.</dd>
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<tt>stratum <i>number</i></tt></dt>
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Specifies the driver stratum, in decimal from 0 to 15, with default 0.</dd>
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<tt>refid <i>string</i></tt></dt>
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Specifies the driver reference identifier, an ASCII string from one to
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four characters, with default <tt>GPS</tt>.</dd>
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<tt>flag1 0 | 1</tt></dt>
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Not used by this driver.</dd>
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<tt>flag2 0 | 1</tt></dt>
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Specifies the PPS signal on-time edge: 0 for assert (default), 1 for clear.</dd>
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<tt>flag3 0 | 1</tt></dt>
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Controls the kernel PPS discipline: 0 for disable (default), 1 for enable.</dd>
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<tt>flag4 0 | 1</tt></dt>
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Not used by this driver.</dd>
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<p>Additional Information
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<p><a href="refclock.htm">Reference Clock Drivers</a></dl>
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David L. Mills (mills@udel.edu)</address>
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