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     13  1.1.1.2  christos   <body>
     14  1.1.1.2  christos     <h3>Generic NMEA GPS Receiver</h3>
     15  1.1.1.3  christos <p>Last update:
     16  1.1.1.5  christos   <!-- #BeginDate format:En2m -->17-Oct-2020  16:00<!-- #EndDate -->
     17  1.1.1.3  christos   UTC</p>
     18  1.1.1.2  christos     <hr>
     19  1.1.1.2  christos     <h4>Synopsis</h4>
     20  1.1.1.2  christos 
     21  1.1.1.2  christos     <p>
     22  1.1.1.2  christos       Address: 127.127.20.<i>u</i><br>
     23  1.1.1.2  christos       Reference ID: <tt>GPS</tt><br>
     24  1.1.1.2  christos       Driver ID: <tt>GPS_NMEA</tt><br>
     25  1.1.1.2  christos       Serial Port: <tt>/dev/gps<i>u</i></tt>; 4800 - 115200 bps, 8-bits, no parity<br>
     26  1.1.1.2  christos       Serial Port: <tt>/dev/gpspps<i>u</i></tt>; for just the PPS signal (this
     27  1.1.1.2  christos       is tried first for PPS, before <tt>/dev/gps<i>u</i></tt>)<br>
     28  1.1.1.2  christos       Serial Port: <tt>/dev/gps<i>u</i></tt>; symlink to server:port (for nmead)<br>
     29  1.1.1.2  christos       Features: <tt>tty_clk</tt>
     30  1.1.1.2  christos     </p>
     31  1.1.1.2  christos 
     32  1.1.1.2  christos     <h4>Description</h4>
     33  1.1.1.2  christos 
     34  1.1.1.2  christos     <p>
     35  1.1.1.2  christos       This driver supports GPS receivers with
     36  1.1.1.2  christos       the <tt>$GPRMC</tt>, <tt>$GPGLL</tt>, <tt>$GPGGA</tt>, <tt>$GPZDA</tt>
     37  1.1.1.2  christos       and <tt>$GPZDG</tt> NMEA sentences by default.&nbsp; Note that Accord's
     38  1.1.1.2  christos       custom NMEA sentence <tt>$GPZDG</tt> reports using the GPS timescale,
     39  1.1.1.2  christos       while the rest of the sentences report UTC.&nbsp; The difference between
     40  1.1.1.2  christos       the two is a whole number of seconds which increases with each leap
     41  1.1.1.2  christos       second insertion in UTC.&nbsp; To avoid problems mixing UTC and GPS
     42  1.1.1.2  christos       timescales, the driver disables processing of UTC sentences
     43  1.1.1.2  christos       once <tt>$GPZDG</tt> is received.
     44  1.1.1.4  christos       <br>
     45  1.1.1.4  christos       <strong>Caveat:</strong> Please see <a href="#talkerids">Talker
     46  1.1.1.4  christos       IDs</a> when using non-GPS or multi-system receivers.
     47  1.1.1.2  christos     </p>
     48  1.1.1.2  christos     <p>
     49  1.1.1.2  christos       The driver expects the receiver to be set up to transmit at least one
     50  1.1.1.2  christos       supported sentence every second.
     51  1.1.1.2  christos     </p>
     52  1.1.1.2  christos     <p>
     53  1.1.1.2  christos       The accuracy depends on the receiver used. Inexpensive GPS models are
     54  1.1.1.2  christos       available with a claimed PPS signal accuracy of
     55  1.1.1.2  christos       1 &mu;s or better relative to the broadcast
     56  1.1.1.2  christos       signal. However, in most cases the actual accuracy is limited by the
     57  1.1.1.2  christos       precision of the timecode and the latencies of the serial interface and
     58  1.1.1.2  christos       operating system.
     59  1.1.1.2  christos     </p>
     60  1.1.1.2  christos     <p>
     61  1.1.1.2  christos       If the Operating System supports PPSAPI
     62  1.1.1.2  christos       (<a href="http://www.ietf.org/rfc/rfc2783.txt">RFC 2783</a>), fudge flag1
     63  1.1.1.2  christos       1 enables its use.
     64  1.1.1.2  christos     </p>
     65  1.1.1.2  christos     <p>
     66  1.1.1.2  christos       The various GPS sentences that this driver recognises look like this:<br>
     67  1.1.1.2  christos       (others quietly ignored)
     68  1.1.1.2  christos     </p>
     69  1.1.1.2  christos 
     70  1.1.1.2  christos     <p><table class="dlstable" border="1">
     71  1.1.1.2  christos       <caption>Accepted NMEA sentences</caption>
     72  1.1.1.2  christos       <tbody><tr>
     73  1.1.1.2  christos 	<th>Sentence</th>
     74  1.1.1.2  christos 	<th>Vendor</th>
     75  1.1.1.2  christos       </tr><tr>
     76  1.1.1.2  christos 	<td class="ttf">$GPRMC,UTC,POS_STAT,LAT,LAT_REF,LON,LON_REF,SPD,HDG,DATE,MAG_VAR,MAG_REF*CS&lt;cr&gt;&lt;lf&gt;</td>
     77  1.1.1.2  christos       </tr><tr>
     78  1.1.1.2  christos 	<td class="ttf">$GPGLL,LAT,LAT_REF,LON,LON_REF,UTC,POS_STAT*CS&lt;cr&gt;&lt;lf&gt;</td>
     79  1.1.1.2  christos       </tr><tr>
     80  1.1.1.2  christos 	<td class="ttf">$GPGGA,UTC,LAT,LAT_REF,LON,LON_REF,FIX_MODE,SAT_USED,HDOP,ALT,ALT_UNIT,GEO,G_UNIT,D_AGE,D_REF*CS&lt;cr&gt;&lt;lf&gt;</td>
     81  1.1.1.2  christos       </tr><tr>
     82  1.1.1.2  christos 	<td class="ttf">$GPZDA,UTC,DD,MM,YYYY,TH,TM,*CS&lt;cr&gt;&lt;lf&gt;</td>
     83  1.1.1.2  christos       </tr><tr>
     84  1.1.1.2  christos 	<td class="ttf">$GPZDG,GPSTIME,DD,MM,YYYY,AA.BB,V*CS&lt;cr&gt;&lt;lf&gt;</td>
     85  1.1.1.2  christos 	<td>Accord</td>
     86  1.1.1.4  christos       </tr><tr>
     87  1.1.1.4  christos       </tr><tr>
     88  1.1.1.4  christos 	<td class="ttf">$PGRMF,gpsWk,gpsTow,DATE,UTC,LEAPS,LAT,LAT_REF,LON,LON_REF,TYPE,MODE,SPD,HDOP,TDOP*CS&lt;cr&gt;&lt;lf&gt;</td>
     89  1.1.1.4  christos 	<td>Garmin</td>
     90  1.1.1.4  christos       </tr><tr>
     91  1.1.1.4  christos 	  <td class="ttf">$PUBX,04,UTC,DATE,utcTow,utcWk,LEAPS,clkBias,clkDrift,tpGran,*CS&lt;cr&gt;&lt;lf&gt;</td>
     92  1.1.1.4  christos 	  <td>UBLOX</td>
     93  1.1.1.4  christos       </tr>	
     94  1.1.1.2  christos     </tbody></table></p>
     95  1.1.1.2  christos 
     96  1.1.1.2  christos     <p><table class="dlstable" border="1">
     97  1.1.1.2  christos       <caption>NMEA data items</caption>
     98  1.1.1.2  christos       <tbody><tr>
     99  1.1.1.2  christos 	<th>Symbol</th>
    100  1.1.1.2  christos 	<th>Meaning and Format</th>
    101  1.1.1.2  christos       </tr>
    102  1.1.1.2  christos       
    103  1.1.1.2  christos       <tr>
    104  1.1.1.4  christos 	<td class="ttf">ALT</td>
    105  1.1.1.4  christos 	<td>Antenna Altitude</td>
    106  1.1.1.2  christos       </tr><tr>
    107  1.1.1.4  christos 	<td class="ttf">ALT_UNIT</td>
    108  1.1.1.4  christos 	<td>Altitude Units (Metres/Feet)</td>
    109  1.1.1.2  christos       </tr><tr>
    110  1.1.1.4  christos 	<td class="ttf">DATE</td>
    111  1.1.1.4  christos 	<td>Date (ddmmyy)</td>
    112  1.1.1.2  christos       </tr><tr>
    113  1.1.1.4  christos 	<td class="ttf">DD</td>
    114  1.1.1.4  christos 	<td>Day of the month (1-31)</td>
    115  1.1.1.2  christos       </tr><tr>
    116  1.1.1.4  christos 	<td class="ttf">D_AGE</td>
    117  1.1.1.4  christos 	<td>Age of last DGPS Fix</td>
    118  1.1.1.2  christos       </tr><tr>
    119  1.1.1.4  christos 	<td class="ttf">D_REF</td>
    120  1.1.1.4  christos 	<td>Reference ID of DGPS station</td>
    121  1.1.1.2  christos       </tr><tr>
    122  1.1.1.4  christos 	<td class="ttf">FIX_MODE</td>
    123  1.1.1.4  christos 	<td>Position Fix Mode (0 = Invalid, &gt;0 = Valid)</td>
    124  1.1.1.2  christos       </tr><tr>
    125  1.1.1.4  christos 	<td class="ttf">GEO</td>
    126  1.1.1.4  christos 	<td>Geoid/Elipsoid separation</td>
    127  1.1.1.2  christos       </tr><tr>
    128  1.1.1.4  christos 	<td class="ttf">GPSTIME</td>
    129  1.1.1.4  christos 	<td>Time of day on GPS timescale. Hours, minutes and seconds [fraction (opt.)] (hhmmss[.f])</td>
    130  1.1.1.2  christos       </tr><tr>
    131  1.1.1.4  christos 	<td class="ttf">gpsTow</td>
    132  1.1.1.4  christos 	<td>GPS week time, seconds since start of GPS week (0..604799)</td>
    133  1.1.1.2  christos       </tr><tr>
    134  1.1.1.4  christos 	<td class="ttf">gpsWk</td>
    135  1.1.1.4  christos 	<td>Week number in the GPS time scale (may exceed 1024)</td>
    136  1.1.1.2  christos       </tr><tr>
    137  1.1.1.4  christos 	<td class="ttf">G_UNIT</td>
    138  1.1.1.4  christos 	<td>Geoid units (M/F)</td>
    139  1.1.1.2  christos       </tr><tr>
    140  1.1.1.4  christos 	<td class="ttf">HDG</td>
    141  1.1.1.4  christos 	<td>Heading/track made good (degrees True) (x.x)</td>
    142  1.1.1.2  christos       </tr><tr>
    143  1.1.1.2  christos 	<td class="ttf">HDOP</td>
    144  1.1.1.2  christos 	<td>Horizontal Dilution of Precision</td>
    145  1.1.1.2  christos       </tr><tr>
    146  1.1.1.4  christos 	<td class="ttf">LAT</td>
    147  1.1.1.4  christos 	<td>Latitude (llll.ll)</td>
    148  1.1.1.2  christos       </tr><tr>
    149  1.1.1.4  christos 	<td class="ttf">LAT_REF</td>
    150  1.1.1.4  christos 	<td>Latitude direction (N = North, S = South)</td>
    151  1.1.1.2  christos       </tr><tr>
    152  1.1.1.4  christos 	<td class="ttf">LEAPS</td>
    153  1.1.1.4  christos 	<td>Leap seconds or difference between GPS time scale and UTC</td>
    154  1.1.1.2  christos       </tr><tr>
    155  1.1.1.4  christos 	<td class="ttf">LON</td>
    156  1.1.1.4  christos 	<td>Longitude (yyyyy.yy)</td>
    157  1.1.1.2  christos       </tr><tr>
    158  1.1.1.4  christos 	<td class="ttf">LON_REF</td>
    159  1.1.1.4  christos 	<td>Longitude direction (E = East, W = West)</td>
    160  1.1.1.2  christos       </tr><tr>
    161  1.1.1.4  christos 	<td class="ttf">MAG_REF</td>
    162  1.1.1.4  christos 	<td>Magnetic variation (E = East, W = West)</td>
    163  1.1.1.2  christos       </tr><tr>
    164  1.1.1.4  christos 	<td class="ttf">MAG_VAR</td>
    165  1.1.1.4  christos 	<td>Magnetic variation (degrees) (x.x)</td>
    166  1.1.1.2  christos       </tr><tr>
    167  1.1.1.2  christos 	<td class="ttf">MM</td>
    168  1.1.1.2  christos 	<td>Month of the year (1-12)</td>
    169  1.1.1.2  christos       </tr><tr>
    170  1.1.1.4  christos 	<td class="ttf">POS_STAT</td>
    171  1.1.1.4  christos 	<td>Position status. (A = Data valid, V = Data invalid)</td>
    172  1.1.1.4  christos       </tr><tr>
    173  1.1.1.4  christos 	<td class="ttf">SAT_USED</td>
    174  1.1.1.4  christos 	<td>Number of Satellites used in solution</td>
    175  1.1.1.4  christos       </tr><tr>
    176  1.1.1.4  christos 	<td class="ttf">SPD</td>
    177  1.1.1.4  christos 	<td>Speed over ground. (knots) (x.x)</td>
    178  1.1.1.4  christos       </tr><tr>
    179  1.1.1.4  christos 	<td class="ttf">UTC</td>
    180  1.1.1.4  christos 	<td>Time of day on UTC timescale. Hours, minutes and seconds [fraction (opt.)] (hhmmss[.fff])</td>
    181  1.1.1.4  christos       </tr><tr>
    182  1.1.1.2  christos 	<td class="ttf">YYYY</td>
    183  1.1.1.2  christos 	<td>Year</td>
    184  1.1.1.2  christos       </tr><tr>
    185  1.1.1.4  christos 	<td class="ttf">WEEK</td>
    186  1.1.1.4  christos 	<td>GPS week (0-1023)</td>
    187  1.1.1.4  christos       </tr><tr>
    188  1.1.1.4  christos 	<td class="ttf">WSEC</td>
    189  1.1.1.4  christos 	<td>Seconds since start of week (0-604799)</td>
    190  1.1.1.4  christos       </tr><tr>
    191  1.1.1.4  christos 	<td class="ttf">LEAP</td>
    192  1.1.1.4  christos 	<td>GPS leap seconds, that is, seconds ahead of UTC</td>
    193  1.1.1.4  christos       </tr><tr>
    194  1.1.1.2  christos 	<td class="ttf">AA.BB</td>
    195  1.1.1.2  christos 	<td>Denotes the signal strength (should be &lt; 05.00)</td>
    196  1.1.1.2  christos       </tr><tr>
    197  1.1.1.2  christos 	<td class="ttf">V</td>
    198  1.1.1.2  christos 	<td>GPS sync status<br>
    199  1.1.1.2  christos 	  &nbsp;&nbsp;&nbsp;'0' =&gt; INVALID time,<br>
    200  1.1.1.2  christos 	  &nbsp;&nbsp;&nbsp;'1' =&gt; accuracy of +/- 20ms,<br>
    201  1.1.1.2  christos 	  &nbsp;&nbsp;&nbsp;'2' =&gt; accuracy of +/- 100ns</td>
    202  1.1.1.2  christos       </tr><tr>
    203  1.1.1.2  christos 	<td class="ttf">CS</td>
    204  1.1.1.2  christos 	<td> Checksum</td>
    205  1.1.1.2  christos       </tr><tr>
    206  1.1.1.2  christos 	<td class="ttf">&lt;cr&gt;&lt;lf&gt;</td>
    207  1.1.1.2  christos 	<td>Sentence terminator.</td>
    208  1.1.1.2  christos       </tr>
    209  1.1.1.2  christos     </tbody></table></p>
    210  1.1.1.2  christos 
    211  1.1.1.2  christos 
    212  1.1.1.4  christos     <h4><a name="talkerids"/>NMEA Talker IDs</h4>
    213  1.1.1.4  christos     
    214  1.1.1.4  christos     <p>
    215  1.1.1.4  christos       GNSS receivers use a distinct talker ID for the GNSS they
    216  1.1.1.4  christos       process. Receivers capable of tracking different systems at the same time
    217  1.1.1.4  christos       can emit <tt>$GPRMC</tt> (GPS), <tt>$GLRMC</tt> (GLONASS),
    218  1.1.1.4  christos       <tt>$GARMC</tt> (Galileo), <tt>$GNRMC</tt> (generic/combined) and others
    219  1.1.1.4  christos       all in one data stream.
    220  1.1.1.4  christos     </p><p>
    221  1.1.1.4  christos       The driver supports this to a certain degree by ignoring the
    222  1.1.1.4  christos       talker ID on the standard sentences RMC, GLL, GGA, ZDA and ZDG. (It
    223  1.1.1.4  christos       possibly should not do that on the latter, but for now, that's the way
    224  1.1.1.4  christos       it is.) So whenever <tt>$GPRMC</tt> is mentioned in this document,
    225  1.1.1.4  christos       substitute any possible talker ID your receiver might emit -- it will
    226  1.1.1.4  christos       still match.
    227  1.1.1.4  christos     </p><p>
    228  1.1.1.4  christos       This approach has a drawback. It is easy to use for single-system
    229  1.1.1.4  christos       receivers, but it cannot separate the data streams for multi-system
    230  1.1.1.4  christos       receiver modules. It is therefore undefined which GNSS actually
    231  1.1.1.4  christos       provides the data, and this can lead to strange behavior. This is
    232  1.1.1.4  christos       especially true if the different GNSS provide very different signal
    233  1.1.1.4  christos       quality to the receiver; the driver is not able to cherry-pick the best
    234  1.1.1.4  christos       source and might actually end up in using the worst available.  It is
    235  1.1.1.4  christos       therefore recommended to set up such a receiver to either use just a
    236  1.1.1.4  christos       single GNSS (which would defeat its purpose) or to emit only the
    237  1.1.1.4  christos       combined data, which usually has the <tt>GN</tt> talker ID defined by
    238  1.1.1.4  christos       the NMEA standard.
    239  1.1.1.4  christos     <p>
    240  1.1.1.4  christos 
    241  1.1.1.4  christos     
    242  1.1.1.2  christos     <h4>The 'mode' byte</h4>
    243  1.1.1.2  christos     
    244  1.1.1.2  christos     <p>
    245  1.1.1.2  christos       Specific GPS sentences and bitrates may be selected by setting bits of
    246  1.1.1.2  christos       the 'mode' in the server configuration line:<br> &nbsp;&nbsp;<tt>server
    247  1.1.1.2  christos       127.127.20.x mode X</tt>
    248  1.1.1.2  christos     </p>
    249  1.1.1.2  christos 
    250  1.1.1.2  christos     <table border="1">
    251  1.1.1.2  christos       <caption>mode byte bits and bit groups</caption>
    252  1.1.1.2  christos       <tbody><tr>
    253  1.1.1.2  christos 	<th align="center">Bit</th>
    254  1.1.1.3  christos 	<th align="center">Decimal</th>
    255  1.1.1.3  christos 	<th align="center">Hex</th>
    256  1.1.1.2  christos 	<th align="left">Meaning</th>
    257  1.1.1.2  christos       </tr>
    258  1.1.1.2  christos       
    259  1.1.1.2  christos       <tr>
    260  1.1.1.2  christos 	<td align="center">0</td>
    261  1.1.1.2  christos 	<td align="center">1</td>
    262  1.1.1.3  christos 	<td align="center">1</td>
    263  1.1.1.4  christos 	<td>process <tt>$GPRMC</tt></td>
    264  1.1.1.2  christos       </tr><tr>
    265  1.1.1.2  christos 	<td align="center">1</td>
    266  1.1.1.2  christos 	<td align="center">2</td>
    267  1.1.1.3  christos 	<td align="center">2</td>
    268  1.1.1.2  christos 	<td>process <tt>$GPGGA</tt></td>
    269  1.1.1.2  christos       </tr><tr>
    270  1.1.1.2  christos 	<td align="center">2</td>
    271  1.1.1.2  christos 	<td align="center">4</td>
    272  1.1.1.3  christos 	<td align="center">4</td>
    273  1.1.1.2  christos 	<td>process <tt>$GPGLL</tt></td>
    274  1.1.1.2  christos       </tr><tr>
    275  1.1.1.2  christos 	<td align="center">3</td>
    276  1.1.1.2  christos 	<td align="center">8</td>
    277  1.1.1.3  christos 	<td align="center">8</td>
    278  1.1.1.2  christos 	<td>process <tt>$GPZDA</tt> or <tt>$GPZDG</tt></td>
    279  1.1.1.2  christos       </tr><tr>
    280  1.1.1.2  christos 	<td rowspan="6" align="center">4-6</td>
    281  1.1.1.2  christos 	<td align="center">0</td>
    282  1.1.1.3  christos 	<td align="center">0</td>
    283  1.1.1.2  christos 	<td>linespeed 4800 bps</td>
    284  1.1.1.2  christos       </tr><tr>
    285  1.1.1.2  christos 	<td align="center">16</td>
    286  1.1.1.3  christos 	<td align="center">0x10</td>
    287  1.1.1.2  christos 	<td>linespeed 9600 bps</td>
    288  1.1.1.2  christos       </tr><tr>
    289  1.1.1.2  christos 	<td align="center">32</td>
    290  1.1.1.3  christos 	<td align="center">0x20</td>
    291  1.1.1.2  christos 	<td>linespeed 19200 bps</td>
    292  1.1.1.2  christos       </tr><tr>
    293  1.1.1.2  christos 	<td align="center">48</td>
    294  1.1.1.3  christos 	<td align="center">0x30</td>
    295  1.1.1.2  christos 	<td>linespeed 38400 bps</td>
    296  1.1.1.2  christos       </tr><tr>
    297  1.1.1.2  christos 	<td align="center">64</td>
    298  1.1.1.3  christos 	<td align="center">0x40</td>
    299  1.1.1.2  christos 	<td>linespeed 57600 bps</td>
    300  1.1.1.2  christos       </tr><tr>
    301  1.1.1.2  christos 	<td align="center">80</td>
    302  1.1.1.3  christos 	<td align="center">0x50</td>
    303  1.1.1.2  christos 	<td>linespeed 115200 bps</td>
    304  1.1.1.2  christos       </tr><tr>
    305  1.1.1.2  christos 	<td align="center">7</td>
    306  1.1.1.2  christos 	<td align="center">128</td>
    307  1.1.1.3  christos 	<td align="center">0x80</td>
    308  1.1.1.2  christos 	<td>Write the sub-second fraction of the receive time stamp to the
    309  1.1.1.2  christos 	clockstat file for all recognised NMEA sentences. This can be used to
    310  1.1.1.2  christos 	get a useful value for fudge time2.<br><strong>Caveat:</strong> This
    311  1.1.1.2  christos 	will fill your clockstat file rather fast. Use it only temporarily to
    312  1.1.1.2  christos 	get the numbers for the NMEA sentence of your choice.</td>
    313  1.1.1.2  christos       </tr>
    314  1.1.1.3  christos       </tr><tr>
    315  1.1.1.3  christos         <td align="center">8</td>
    316  1.1.1.3  christos         <td align="center">256</td>
    317  1.1.1.3  christos         <td align="center">0x100</td>
    318  1.1.1.3  christos 	<td>process <tt>$PGRMF</tt></td>
    319  1.1.1.3  christos       </tr><tr>
    320  1.1.1.4  christos         <td align="center">9</td>
    321  1.1.1.4  christos         <td align="center">512</td>
    322  1.1.1.4  christos         <td align="center">0x200</td>
    323  1.1.1.4  christos 	<td>process <tt>$PUBX,04</tt></td>
    324  1.1.1.4  christos       </tr><tr>
    325  1.1.1.4  christos         <td align="center">10-15</td>
    326  1.1.1.3  christos         <td align="center"></td>
    327  1.1.1.4  christos         <td align="center">0xFC00</td>
    328  1.1.1.3  christos 	<td>reserved - leave 0</td>
    329  1.1.1.3  christos       </tr><tr>
    330  1.1.1.3  christos         <td align="center">16</td>
    331  1.1.1.3  christos         <td align="center">65536</td>
    332  1.1.1.3  christos         <td align="center">0x10000</td>
    333  1.1.1.3  christos         <td>Append extra statistics to the clockstats line.
    334  1.1.1.3  christos         Details below.</td>
    335  1.1.1.4  christos        </tr><tr>
    336  1.1.1.4  christos         <td align="center">17</td>
    337  1.1.1.4  christos         <td align="center">131072</td>
    338  1.1.1.4  christos         <td align="center">0x20000</td>
    339  1.1.1.4  christos         <td>"Silent PPS" mode. Use the PPS channel (if enabled with
    340  1.1.1.4  christos           fudge flag 1) to get precise receive time stamps.
    341  1.1.1.4  christos           Do <em>not</em> set the PPS flag in the clock status, so the
    342  1.1.1.4  christos           clock is not considered as PPS peer.
    343  1.1.1.4  christos 	</td>
    344  1.1.1.4  christos        </tr><tr>
    345  1.1.1.4  christos         <td align="center">18</td>
    346  1.1.1.4  christos         <td align="center">262144</td>
    347  1.1.1.4  christos         <td align="center">0x40000</td>
    348  1.1.1.4  christos         <td>Trust the date delivered via NMEA. Do this only if
    349  1.1.1.4  christos           you <em>really</em> trust the receiver!
    350  1.1.1.4  christos           See <a href="#datetrust">below</a>. <strong>Caveat:</strong>
    351  1.1.1.4  christos           This (hitherto undocumented) bit has moved!
    352  1.1.1.4  christos 	</td>
    353  1.1.1.5  christos        </tr><tr>
    354  1.1.1.5  christos         <td align="center">19</td>
    355  1.1.1.5  christos         <td align="center">524288</td>
    356  1.1.1.5  christos         <td align="center">0x80000</td>
    357  1.1.1.5  christos         <td>Ignore receiver status, assume time is always good as long
    358  1.1.1.5  christos 	  as it is present at all.  A possible option for receivers
    359  1.1.1.5  christos 	  with a good hold-over characteristic, but most likely
    360  1.1.1.5  christos 	  harmful in general. Use at own risk.
    361  1.1.1.5  christos 	</td>
    362  1.1.1.3  christos       </tr>
    363  1.1.1.2  christos     </tbody></table> 
    364  1.1.1.2  christos 
    365  1.1.1.2  christos  
    366  1.1.1.2  christos     <p>
    367  1.1.1.2  christos       The default (mode 0) is to process all supported sentences at a linespeed
    368  1.1.1.2  christos       of 4800 bps, which results in the first one received and recognised in
    369  1.1.1.2  christos       each cycle being used.&nbsp; If only specific sentences should be
    370  1.1.1.2  christos       recognised, then the mode byte must be chosen to enable only the selected
    371  1.1.1.2  christos       ones.&nbsp; Multiple sentences may be selected by adding their mode bit
    372  1.1.1.2  christos       values, but of those enabled still only the first received sentence in a
    373  1.1.1.2  christos       cycle will be used.&nbsp; Using more than one sentence per cycle is
    374  1.1.1.2  christos       impossible, because
    375  1.1.1.2  christos       </p><ul>
    376  1.1.1.2  christos 	<li>there is only <a href="#fudgetime2">fudge time2</a> available to
    377  1.1.1.2  christos 	  compensate for transmission delays but every sentence would need a
    378  1.1.1.2  christos 	  different one and
    379  1.1.1.2  christos 	</li><li>using more than one sentence per cycle overstuffs the internal data
    380  1.1.1.2  christos 	  filters.
    381  1.1.1.2  christos       </li></ul>
    382  1.1.1.2  christos       The driver uses 4800 bits per second by default, but faster bitrates can
    383  1.1.1.2  christos       be selected using bits 4 to 6 of the mode field.
    384  1.1.1.4  christos     </p>
    385  1.1.1.2  christos 
    386  1.1.1.2  christos     <p>
    387  1.1.1.2  christos       <strong>Caveat:</strong> Using higher line speeds does not necessarily
    388  1.1.1.2  christos       increase the precision of the timing device.&nbsp; Higher line speeds are
    389  1.1.1.2  christos       not necessarily helpful for the NMEA driver, either.&nbsp; They can be
    390  1.1.1.2  christos       used to accomodate for an amount of data that does not fit into a
    391  1.1.1.2  christos       1-second cycle at 4800 bps, but high-speed high-volume NMEA data is likely
    392  1.1.1.2  christos       to cause trouble with the serial line driver since NMEA supports no
    393  1.1.1.2  christos       protocol handshake.&nbsp; Any device that is exclusively used for time
    394  1.1.1.2  christos       synchronisation purposes should be configured to transmit the relevant
    395  1.1.1.2  christos       data only, e.g. one <tt>$GPRMC</tt> or <tt>$GPZDA</tt> per second, at a
    396  1.1.1.2  christos       linespeed of 4800 bps or 9600 bps.
    397  1.1.1.2  christos     </p>
    398  1.1.1.2  christos 
    399  1.1.1.4  christos     <h4><a name="datetrust"/>About distrusting NMEA date stamps</h4>
    400  1.1.1.4  christos     <p>
    401  1.1.1.4  christos       Trusting the calendar dates delivered via NMEA is a risky thing, and by
    402  1.1.1.4  christos       default these dates are handled with a huge dose of skepticism.  Many
    403  1.1.1.4  christos       receivers deliver a correct calendar date for a period of just 1024 weeks,
    404  1.1.1.4  christos       with a starting point baked somewhere into their firmware.  Beyond that,
    405  1.1.1.4  christos       they warp back to the begin of their era and simply provide wrong date
    406  1.1.1.4  christos       information.  To battle this widely observed effect, the date delivered is
    407  1.1.1.4  christos       by default reduced to GPS time again and then (re-)mapped according to the
    408  1.1.1.4  christos       base date, either the implicit value or the value set via "tos basedate".
    409  1.1.1.4  christos       If the receiver can <em>really</em> be trusted to deliver the right date
    410  1.1.1.4  christos       (which is not impossible, just more expensive for the manufacturer), then
    411  1.1.1.4  christos       mode bit 18 can be used to bypass the era mapping.  Setting this bit is
    412  1.1.1.4  christos       not needed under most circumstances, and setting it with an unreliable
    413  1.1.1.4  christos       receiver can have severe effects. Handle with care.
    414  1.1.1.4  christos     </p><p>
    415  1.1.1.4  christos       <strong>Note:</strong> This functionality was available for some time as
    416  1.1.1.4  christos       undocumented feature, with a different bit value.  It was moved in the
    417  1.1.1.4  christos       process of becoming officially acknowledged to avoid excessive scattering
    418  1.1.1.4  christos       of the mode bit mask.
    419  1.1.1.4  christos     </p>
    420  1.1.1.4  christos     
    421  1.1.1.4  christos     
    422  1.1.1.3  christos     <h4>Monitor Data</h4>
    423  1.1.1.3  christos 
    424  1.1.1.3  christos     <p>The last GPS sentence that is accepted or rejected is written to the
    425  1.1.1.3  christos     clockstats file and available with <code>ntpq -c clockvar</code>.
    426  1.1.1.3  christos     (Logging the rejected sentences lets you see/debug why they were rejected.)
    427  1.1.1.3  christos     Filtered sentences are not logged.</p>
    428  1.1.1.3  christos 
    429  1.1.1.3  christos     <p>
    430  1.1.1.3  christos       If the 0x10000 mode bit is on and clockstats is enabled, several extra
    431  1.1.1.3  christos       counters will be appended to the NMEA sentence that gets logged.
    432  1.1.1.3  christos       For example:
    433  1.1.1.3  christos <pre>
    434  1.1.1.3  christos 56299 76876.691 127.127.20.20 $GPGGA,212116.000,3726.0785,N,12212.2605,W,1,05,2.0,17.0,M,-25.7,M,,0000*5C  228 64 0 0 64 0
    435  1.1.1.3  christos </pre>
    436  1.1.1.3  christos     </p>
    437  1.1.1.3  christos 
    438  1.1.1.3  christos     <table border="1">
    439  1.1.1.3  christos       <caption>Clockstats</caption>
    440  1.1.1.3  christos       <tbody><tr>
    441  1.1.1.3  christos 	<th align="center">Column</th>
    442  1.1.1.3  christos 	<th align="center">Sample</th>
    443  1.1.1.3  christos 	<th align="left">Meaning</th>
    444  1.1.1.3  christos       </tr>
    445  1.1.1.3  christos       
    446  1.1.1.3  christos       <tr>
    447  1.1.1.3  christos 	<td align="center">1</td>
    448  1.1.1.3  christos 	<td align="center">56299</td>
    449  1.1.1.3  christos 	<td>MJD</td>
    450  1.1.1.3  christos       </tr><tr>
    451  1.1.1.3  christos 	<td align="center">2</td>
    452  1.1.1.3  christos 	<td align="center">76876.691</td>
    453  1.1.1.3  christos 	<td>Time of day in seconds</td>
    454  1.1.1.3  christos       </tr><tr>
    455  1.1.1.3  christos 	<td align="center">3</td>
    456  1.1.1.3  christos 	<td align="center">127.127.20.20</td>
    457  1.1.1.3  christos 	<td>IP Address from server config line</td>
    458  1.1.1.3  christos       </tr><tr>
    459  1.1.1.3  christos 	<td align="center">4</td>
    460  1.1.1.3  christos 	<td align="center">$GPGGA,...0*5C</td>
    461  1.1.1.3  christos 	<td>NMEA Sentence</td>
    462  1.1.1.3  christos       </tr><tr>
    463  1.1.1.3  christos         <td align="center">5</td>
    464  1.1.1.3  christos         <td align="center">228</td>
    465  1.1.1.3  christos         <td>Number of sentences received</td>
    466  1.1.1.3  christos       </tr><tr>
    467  1.1.1.3  christos         <td align="center">6</td>
    468  1.1.1.3  christos         <td align="center">64</td>
    469  1.1.1.3  christos         <td>Number of sentences accepted and used for timekeeping</td>
    470  1.1.1.3  christos       </tr><tr>
    471  1.1.1.3  christos         <td align="center">7</td>
    472  1.1.1.3  christos         <td align="center">0</td>
    473  1.1.1.3  christos         <td>Number of sentences rejected because they were marked invalid (poor signal)</td>
    474  1.1.1.3  christos       </tr><tr>
    475  1.1.1.3  christos         <td align="center">8</td>
    476  1.1.1.3  christos         <td align="center">0</td>
    477  1.1.1.3  christos         <td>Number of sentences rejected because of bad checksum or invalid date/time</td>
    478  1.1.1.3  christos       </tr><tr>
    479  1.1.1.3  christos         <td align="center">9</td>
    480  1.1.1.3  christos         <td align="center">64</td>
    481  1.1.1.3  christos         <td>Number of sentences filtered by mode bits or same second</td>
    482  1.1.1.3  christos       </tr><tr>
    483  1.1.1.3  christos         <td align="center">10</td>
    484  1.1.1.3  christos         <td align="center">0</td>
    485  1.1.1.3  christos         <td>Number of PPS pulses used, overrides NMEA sentences</td>
    486  1.1.1.3  christos       </tr>
    487  1.1.1.3  christos     </tbody></table>
    488  1.1.1.3  christos 
    489  1.1.1.3  christos     Sentences like $GPGSV that don't contain the time will get
    490  1.1.1.3  christos     counted in the total but otherwise ignored.
    491  1.1.1.3  christos 
    492  1.1.1.2  christos     <p>
    493  1.1.1.2  christos       <a href="https://support.ntp.org/bin/view/Support/ConfiguringNMEARefclocks">Configuring
    494  1.1.1.2  christos       NMEA Refclocks</a> might give further useful hints for specific hardware
    495  1.1.1.2  christos       devices that exhibit strange or curious behaviour.
    496  1.1.1.2  christos     </p>
    497  1.1.1.2  christos 
    498  1.1.1.2  christos     <p>
    499  1.1.1.2  christos       To make a specific setting, select the corresponding decimal values from
    500  1.1.1.2  christos       the mode byte table, add them all together and enter the resulting
    501  1.1.1.2  christos       decimal value into the clock configuration line.
    502  1.1.1.2  christos     </p>
    503  1.1.1.2  christos 
    504  1.1.1.2  christos     <h4>Setting up the Garmin GPS-25XL</h4>
    505  1.1.1.2  christos 
    506  1.1.1.2  christos     Switch off all output with by sending it the following string.
    507  1.1.1.2  christos     <pre>"$PGRMO,,2&lt;cr&gt;&lt;lf&gt;"</pre>
    508  1.1.1.2  christos     <p>Now switch only $GPRMC on by sending it the following string.</p>
    509  1.1.1.2  christos     <pre>"$PGRMO,GPRMC,1&lt;cr&gt;&lt;lf&gt;"</pre>
    510  1.1.1.2  christos 
    511  1.1.1.2  christos     <p>On some systems the PPS signal isn't switched on by default. It can be
    512  1.1.1.2  christos     switched on by sending the following string.</p>
    513  1.1.1.2  christos     <pre>"$PGRMC,,,,,,,,,,,,2&lt;cr&gt;&lt;lf&gt;"</pre>
    514  1.1.1.2  christos 
    515  1.1.1.2  christos     <h4>Fudge Factors</h4>
    516  1.1.1.2  christos 
    517  1.1.1.2  christos     <dl>
    518  1.1.1.2  christos       <dt><tt>time1 <i>time</i></tt></dt>
    519  1.1.1.2  christos       <dd>Specifies the PPS time offset calibration factor, in seconds and fraction, with default 0.0.</dd>
    520  1.1.1.2  christos       <dt><a name="fudgetime2"><tt>time2 <i>time</i></tt></a></dt>
    521  1.1.1.2  christos       <dd>Specifies the serial end of line time offset calibration factor, in seconds and fraction, with default
    522  1.1.1.2  christos 	0.0.</dd>
    523  1.1.1.2  christos       <dt><tt>stratum <i>number</i></tt></dt>
    524  1.1.1.2  christos       <dd>Specifies the driver stratum, in decimal from 0 to 15, with default 0.</dd>
    525  1.1.1.2  christos       <dt><tt>refid <i>string</i></tt></dt>
    526  1.1.1.2  christos       <dd>Specifies the driver reference identifier, an ASCII string from one to four characters, with
    527  1.1.1.2  christos 	default <tt>GPS</tt>.</dd>
    528  1.1.1.2  christos       <dt><tt>flag1 0 | 1</tt></dt>
    529  1.1.1.2  christos       <dd>Disable PPS signal processing if 0 (default); enable PPS signal processing if 1.</dd>
    530  1.1.1.2  christos       <dt><tt>flag2 0 | 1</tt></dt>
    531  1.1.1.2  christos       <dd>If PPS signal processing is enabled, capture the pulse on the rising edge if 0 (default); capture on the
    532  1.1.1.2  christos 	falling edge if 1.</dd>
    533  1.1.1.2  christos       <dt><tt>flag3 0 | 1</tt></dt>
    534  1.1.1.2  christos       <dd>If PPS signal processing is enabled, use the <tt>ntpd</tt> clock discipline if 0 (default); use the kernel
    535  1.1.1.2  christos 	discipline if 1.</dd>
    536  1.1.1.2  christos       <dt><tt>flag4 0 | 1</tt></dt>
    537  1.1.1.2  christos       <dd>Obscures location in timecode: 0 for disable (default), 1 for enable.</dd>
    538  1.1.1.2  christos     </dl>
    539  1.1.1.2  christos 
    540  1.1.1.2  christos     <p>Additional Information</p>
    541  1.1.1.3  christos     <p><tt>flag1</tt>, <tt>flag2</tt>, and <tt>flag3</tt> are ignored under Windows.</p>
    542  1.1.1.2  christos     <p><a href="../refclock.html">Reference Clock Drivers</a></p>
    543  1.1.1.2  christos     <hr>
    544  1.1.1.2  christos     <script type="text/javascript" language="javascript" src="scripts/footer.txt"></script>
    545  1.1.1.2  christos   </body></html>
    546