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     12      1.1    kardel 
     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  1.1.1.3  christos   <!-- #BeginDate format:En2m -->31-Mar-2014  03:55<!-- #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>$GPZDG</tt> is received.
     44  1.1.1.2  christos     </p>
     45  1.1.1.2  christos     <p>
     46  1.1.1.2  christos       The driver expects the receiver to be set up to transmit at least one
     47  1.1.1.2  christos       supported sentence every second.
     48  1.1.1.2  christos     </p>
     49  1.1.1.2  christos     <p>
     50  1.1.1.2  christos       The accuracy depends on the receiver used. Inexpensive GPS models are
     51  1.1.1.2  christos       available with a claimed PPS signal accuracy of
     52  1.1.1.2  christos       1 &mu;s or better relative to the broadcast
     53  1.1.1.2  christos       signal. However, in most cases the actual accuracy is limited by the
     54  1.1.1.2  christos       precision of the timecode and the latencies of the serial interface and
     55  1.1.1.2  christos       operating system.
     56  1.1.1.2  christos     </p>
     57  1.1.1.2  christos     <p>
     58  1.1.1.2  christos       If the Operating System supports PPSAPI
     59  1.1.1.2  christos       (<a href="http://www.ietf.org/rfc/rfc2783.txt">RFC 2783</a>), fudge flag1
     60  1.1.1.2  christos       1 enables its use.
     61  1.1.1.2  christos     </p>
     62  1.1.1.2  christos     <p>
     63  1.1.1.2  christos       The various GPS sentences that this driver recognises look like this:<br>
     64  1.1.1.2  christos       (others quietly ignored)
     65  1.1.1.2  christos     </p>
     66  1.1.1.2  christos 
     67  1.1.1.2  christos     <p><table class="dlstable" border="1">
     68  1.1.1.2  christos       <caption>Accepted NMEA sentences</caption>
     69  1.1.1.2  christos       <tbody><tr>
     70  1.1.1.2  christos 	<th>Sentence</th>
     71  1.1.1.2  christos 	<th>Vendor</th>
     72  1.1.1.2  christos       </tr><tr>
     73  1.1.1.2  christos 	<td class="ttf">$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>
     74  1.1.1.2  christos       </tr><tr>
     75  1.1.1.2  christos 	<td class="ttf">$GPGLL,LAT,LAT_REF,LON,LON_REF,UTC,POS_STAT*CS&lt;cr&gt;&lt;lf&gt;</td>
     76  1.1.1.2  christos       </tr><tr>
     77  1.1.1.2  christos 	<td class="ttf">$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>
     78  1.1.1.2  christos       </tr><tr>
     79  1.1.1.2  christos 	<td class="ttf">$GPZDA,UTC,DD,MM,YYYY,TH,TM,*CS&lt;cr&gt;&lt;lf&gt;</td>
     80  1.1.1.2  christos       </tr><tr>
     81  1.1.1.2  christos 	<td class="ttf">$GPZDG,GPSTIME,DD,MM,YYYY,AA.BB,V*CS&lt;cr&gt;&lt;lf&gt;</td>
     82  1.1.1.2  christos 	<td>Accord</td>
     83  1.1.1.2  christos       </tr>
     84  1.1.1.2  christos     </tbody></table></p>
     85  1.1.1.2  christos 
     86  1.1.1.2  christos     <p><table class="dlstable" border="1">
     87  1.1.1.2  christos       <caption>NMEA data items</caption>
     88  1.1.1.2  christos       <tbody><tr>
     89  1.1.1.2  christos 	<th>Symbol</th>
     90  1.1.1.2  christos 	<th>Meaning and Format</th>
     91  1.1.1.2  christos       </tr>
     92  1.1.1.2  christos       
     93  1.1.1.2  christos       <tr>
     94  1.1.1.2  christos 	<td class="ttf">UTC</td>
     95  1.1.1.2  christos 	<td>Time of day on UTC timescale. Hours, minutes and seconds [fraction (opt.)]. (hhmmss[.fff])</td>
     96  1.1.1.2  christos       </tr><tr>
     97  1.1.1.2  christos 	<td class="ttf">POS_STAT</td>
     98  1.1.1.2  christos 	<td>Position status. (A = Data valid, V = Data invalid)</td>
     99  1.1.1.2  christos       </tr><tr>
    100  1.1.1.2  christos 	<td class="ttf">LAT</td>
    101  1.1.1.2  christos 	<td>Latitude (llll.ll)</td>
    102  1.1.1.2  christos       </tr><tr>
    103  1.1.1.2  christos 	<td class="ttf">LAT_REF</td>
    104  1.1.1.2  christos 	<td>Latitude direction. (N = North, S = South)</td>
    105  1.1.1.2  christos       </tr><tr>
    106  1.1.1.2  christos 	<td class="ttf">LON</td>
    107  1.1.1.2  christos 	<td>Longitude (yyyyy.yy)</td>
    108  1.1.1.2  christos       </tr><tr>
    109  1.1.1.2  christos 	<td class="ttf">LON_REF</td>
    110  1.1.1.2  christos 	<td>Longitude direction (E = East, W = West)</td>
    111  1.1.1.2  christos       </tr><tr>
    112  1.1.1.2  christos 	<td class="ttf">SPD</td>
    113  1.1.1.2  christos 	<td>Speed over ground. (knots) (x.x)</td>
    114  1.1.1.2  christos       </tr><tr>
    115  1.1.1.2  christos 	<td class="ttf">HDG</td>
    116  1.1.1.2  christos 	<td>Heading/track made good (degrees True) (x.x)</td>
    117  1.1.1.2  christos       </tr><tr>
    118  1.1.1.2  christos 	<td class="ttf">DATE</td>
    119  1.1.1.2  christos 	<td>Date (ddmmyy)</td>
    120  1.1.1.2  christos       </tr><tr>
    121  1.1.1.2  christos 	<td class="ttf">MAG_VAR</td>
    122  1.1.1.2  christos 	<td>Magnetic variation (degrees) (x.x)</td>
    123  1.1.1.2  christos       </tr><tr>
    124  1.1.1.2  christos 	<td class="ttf">MAG_REF</td>
    125  1.1.1.2  christos 	<td>Magnetic variation (E = East, W = West)</td>
    126  1.1.1.2  christos       </tr><tr>
    127  1.1.1.2  christos 	<td class="ttf">FIX_MODE</td>
    128  1.1.1.2  christos 	<td>Position Fix Mode (0 = Invalid, &gt;0 = Valid)</td>
    129  1.1.1.2  christos       </tr><tr>
    130  1.1.1.2  christos 	<td class="ttf">SAT_USED</td>
    131  1.1.1.2  christos 	<td>Number of Satellites used in solution</td>
    132  1.1.1.2  christos       </tr><tr>
    133  1.1.1.2  christos 	<td class="ttf">HDOP</td>
    134  1.1.1.2  christos 	<td>Horizontal Dilution of Precision</td>
    135  1.1.1.2  christos       </tr><tr>
    136  1.1.1.2  christos 	<td class="ttf">ALT</td>
    137  1.1.1.2  christos 	<td>Antenna Altitude</td>
    138  1.1.1.2  christos       </tr><tr>
    139  1.1.1.2  christos 	<td class="ttf">ALT_UNIT</td>
    140  1.1.1.2  christos 	<td>Altitude Units (Metres/Feet)</td>
    141  1.1.1.2  christos       </tr><tr>
    142  1.1.1.2  christos 	<td class="ttf">GEO</td>
    143  1.1.1.2  christos 	<td>Geoid/Elipsoid separation</td>
    144  1.1.1.2  christos       </tr><tr>
    145  1.1.1.2  christos 	<td class="ttf">G_UNIT</td>
    146  1.1.1.2  christos 	<td>Geoid units (M/F)</td>
    147  1.1.1.2  christos       </tr><tr>
    148  1.1.1.2  christos 	<td class="ttf">D_AGE</td>
    149  1.1.1.2  christos 	<td>Age of last DGPS Fix</td>
    150  1.1.1.2  christos       </tr><tr>
    151  1.1.1.2  christos 	<td class="ttf">D_REF</td>
    152  1.1.1.2  christos 	<td>Reference ID of DGPS station</td>
    153  1.1.1.2  christos       </tr><tr>
    154  1.1.1.2  christos 	<td class="ttf">GPSTIME</td>
    155  1.1.1.2  christos 	<td>Time of day on GPS timescale. Hours, minutes and seconds [fraction (opt.)]. (hhmmss[.f])</td>
    156  1.1.1.2  christos       </tr><tr>
    157  1.1.1.2  christos 	<td class="ttf">DD</td>
    158  1.1.1.2  christos 	<td>Day of the month (1-31)</td>
    159  1.1.1.2  christos       </tr><tr>
    160  1.1.1.2  christos 	<td class="ttf">MM</td>
    161  1.1.1.2  christos 	<td>Month of the year (1-12)</td>
    162  1.1.1.2  christos       </tr><tr>
    163  1.1.1.2  christos 	<td class="ttf">YYYY</td>
    164  1.1.1.2  christos 	<td>Year</td>
    165  1.1.1.2  christos       </tr><tr>
    166  1.1.1.2  christos 	<td class="ttf">AA.BB</td>
    167  1.1.1.2  christos 	<td>Denotes the signal strength (should be &lt; 05.00)</td>
    168  1.1.1.2  christos       </tr><tr>
    169  1.1.1.2  christos 	<td class="ttf">V</td>
    170  1.1.1.2  christos 	<td>GPS sync status<br>
    171  1.1.1.2  christos 	  &nbsp;&nbsp;&nbsp;'0' =&gt; INVALID time,<br>
    172  1.1.1.2  christos 	  &nbsp;&nbsp;&nbsp;'1' =&gt; accuracy of +/- 20ms,<br>
    173  1.1.1.2  christos 	  &nbsp;&nbsp;&nbsp;'2' =&gt; accuracy of +/- 100ns</td>
    174  1.1.1.2  christos       </tr><tr>
    175  1.1.1.2  christos 	<td class="ttf">CS</td>
    176  1.1.1.2  christos 	<td> Checksum</td>
    177  1.1.1.2  christos       </tr><tr>
    178  1.1.1.2  christos 	<td class="ttf">&lt;cr&gt;&lt;lf&gt;</td>
    179  1.1.1.2  christos 	<td>Sentence terminator.</td>
    180  1.1.1.2  christos       </tr>
    181  1.1.1.2  christos     </tbody></table></p>
    182  1.1.1.2  christos 
    183  1.1.1.2  christos 
    184  1.1.1.2  christos     <h4>The 'mode' byte</h4>
    185  1.1.1.2  christos     
    186  1.1.1.2  christos     <p>
    187  1.1.1.2  christos       Specific GPS sentences and bitrates may be selected by setting bits of
    188  1.1.1.2  christos       the 'mode' in the server configuration line:<br> &nbsp;&nbsp;<tt>server
    189  1.1.1.2  christos       127.127.20.x mode X</tt>
    190  1.1.1.2  christos     </p>
    191  1.1.1.2  christos 
    192  1.1.1.2  christos     <table border="1">
    193  1.1.1.2  christos       <caption>mode byte bits and bit groups</caption>
    194  1.1.1.2  christos       <tbody><tr>
    195  1.1.1.2  christos 	<th align="center">Bit</th>
    196  1.1.1.3  christos 	<th align="center">Decimal</th>
    197  1.1.1.3  christos 	<th align="center">Hex</th>
    198  1.1.1.2  christos 	<th align="left">Meaning</th>
    199  1.1.1.2  christos       </tr>
    200  1.1.1.2  christos       
    201  1.1.1.2  christos       <tr>
    202  1.1.1.2  christos 	<td align="center">0</td>
    203  1.1.1.2  christos 	<td align="center">1</td>
    204  1.1.1.3  christos 	<td align="center">1</td>
    205  1.1.1.2  christos 	<td>process <tt>$GPMRC</tt></td>
    206  1.1.1.2  christos       </tr><tr>
    207  1.1.1.2  christos 	<td align="center">1</td>
    208  1.1.1.2  christos 	<td align="center">2</td>
    209  1.1.1.3  christos 	<td align="center">2</td>
    210  1.1.1.2  christos 	<td>process <tt>$GPGGA</tt></td>
    211  1.1.1.2  christos       </tr><tr>
    212  1.1.1.2  christos 	<td align="center">2</td>
    213  1.1.1.2  christos 	<td align="center">4</td>
    214  1.1.1.3  christos 	<td align="center">4</td>
    215  1.1.1.2  christos 	<td>process <tt>$GPGLL</tt></td>
    216  1.1.1.2  christos       </tr><tr>
    217  1.1.1.2  christos 	<td align="center">3</td>
    218  1.1.1.2  christos 	<td align="center">8</td>
    219  1.1.1.3  christos 	<td align="center">8</td>
    220  1.1.1.2  christos 	<td>process <tt>$GPZDA</tt> or <tt>$GPZDG</tt></td>
    221  1.1.1.2  christos       </tr><tr>
    222  1.1.1.2  christos 	<td rowspan="6" align="center">4-6</td>
    223  1.1.1.2  christos 	<td align="center">0</td>
    224  1.1.1.3  christos 	<td align="center">0</td>
    225  1.1.1.2  christos 	<td>linespeed 4800 bps</td>
    226  1.1.1.2  christos       </tr><tr>
    227  1.1.1.2  christos 	<td align="center">16</td>
    228  1.1.1.3  christos 	<td align="center">0x10</td>
    229  1.1.1.2  christos 	<td>linespeed 9600 bps</td>
    230  1.1.1.2  christos       </tr><tr>
    231  1.1.1.2  christos 	<td align="center">32</td>
    232  1.1.1.3  christos 	<td align="center">0x20</td>
    233  1.1.1.2  christos 	<td>linespeed 19200 bps</td>
    234  1.1.1.2  christos       </tr><tr>
    235  1.1.1.2  christos 	<td align="center">48</td>
    236  1.1.1.3  christos 	<td align="center">0x30</td>
    237  1.1.1.2  christos 	<td>linespeed 38400 bps</td>
    238  1.1.1.2  christos       </tr><tr>
    239  1.1.1.2  christos 	<td align="center">64</td>
    240  1.1.1.3  christos 	<td align="center">0x40</td>
    241  1.1.1.2  christos 	<td>linespeed 57600 bps</td>
    242  1.1.1.2  christos       </tr><tr>
    243  1.1.1.2  christos 	<td align="center">80</td>
    244  1.1.1.3  christos 	<td align="center">0x50</td>
    245  1.1.1.2  christos 	<td>linespeed 115200 bps</td>
    246  1.1.1.2  christos       </tr><tr>
    247  1.1.1.2  christos 	<td align="center">7</td>
    248  1.1.1.2  christos 	<td align="center">128</td>
    249  1.1.1.3  christos 	<td align="center">0x80</td>
    250  1.1.1.2  christos 	<td>Write the sub-second fraction of the receive time stamp to the
    251  1.1.1.2  christos 	clockstat file for all recognised NMEA sentences. This can be used to
    252  1.1.1.2  christos 	get a useful value for fudge time2.<br><strong>Caveat:</strong> This
    253  1.1.1.2  christos 	will fill your clockstat file rather fast. Use it only temporarily to
    254  1.1.1.2  christos 	get the numbers for the NMEA sentence of your choice.</td>
    255  1.1.1.2  christos       </tr>
    256  1.1.1.3  christos       </tr><tr>
    257  1.1.1.3  christos         <td align="center">8</td>
    258  1.1.1.3  christos         <td align="center">256</td>
    259  1.1.1.3  christos         <td align="center">0x100</td>
    260  1.1.1.3  christos 	<td>process <tt>$PGRMF</tt></td>
    261  1.1.1.3  christos       </tr><tr>
    262  1.1.1.3  christos         <td align="center">9-15</td>
    263  1.1.1.3  christos         <td align="center"></td>
    264  1.1.1.3  christos         <td align="center">0xFE00</td>
    265  1.1.1.3  christos 	<td>reserved - leave 0</td>
    266  1.1.1.3  christos       </tr><tr>
    267  1.1.1.3  christos         <td align="center">16</td>
    268  1.1.1.3  christos         <td align="center">65536</td>
    269  1.1.1.3  christos         <td align="center">0x10000</td>
    270  1.1.1.3  christos         <td>Append extra statistics to the clockstats line.
    271  1.1.1.3  christos         Details below.</td>
    272  1.1.1.3  christos       </tr>
    273  1.1.1.2  christos     </tbody></table> 
    274  1.1.1.2  christos 
    275  1.1.1.2  christos  
    276  1.1.1.2  christos     <p>
    277  1.1.1.2  christos       The default (mode 0) is to process all supported sentences at a linespeed
    278  1.1.1.2  christos       of 4800 bps, which results in the first one received and recognised in
    279  1.1.1.2  christos       each cycle being used.&nbsp; If only specific sentences should be
    280  1.1.1.2  christos       recognised, then the mode byte must be chosen to enable only the selected
    281  1.1.1.2  christos       ones.&nbsp; Multiple sentences may be selected by adding their mode bit
    282  1.1.1.2  christos       values, but of those enabled still only the first received sentence in a
    283  1.1.1.2  christos       cycle will be used.&nbsp; Using more than one sentence per cycle is
    284  1.1.1.2  christos       impossible, because
    285  1.1.1.2  christos       </p><ul>
    286  1.1.1.2  christos 	<li>there is only <a href="#fudgetime2">fudge time2</a> available to
    287  1.1.1.2  christos 	  compensate for transmission delays but every sentence would need a
    288  1.1.1.2  christos 	  different one and
    289  1.1.1.2  christos 	</li><li>using more than one sentence per cycle overstuffs the internal data
    290  1.1.1.2  christos 	  filters.
    291  1.1.1.2  christos       </li></ul>
    292  1.1.1.2  christos       The driver uses 4800 bits per second by default, but faster bitrates can
    293  1.1.1.2  christos       be selected using bits 4 to 6 of the mode field.
    294  1.1.1.2  christos     <p></p>
    295  1.1.1.2  christos 
    296  1.1.1.2  christos     <p>
    297  1.1.1.2  christos       <strong>Caveat:</strong> Using higher line speeds does not necessarily
    298  1.1.1.2  christos       increase the precision of the timing device.&nbsp; Higher line speeds are
    299  1.1.1.2  christos       not necessarily helpful for the NMEA driver, either.&nbsp; They can be
    300  1.1.1.2  christos       used to accomodate for an amount of data that does not fit into a
    301  1.1.1.2  christos       1-second cycle at 4800 bps, but high-speed high-volume NMEA data is likely
    302  1.1.1.2  christos       to cause trouble with the serial line driver since NMEA supports no
    303  1.1.1.2  christos       protocol handshake.&nbsp; Any device that is exclusively used for time
    304  1.1.1.2  christos       synchronisation purposes should be configured to transmit the relevant
    305  1.1.1.2  christos       data only, e.g. one <tt>$GPRMC</tt> or <tt>$GPZDA</tt> per second, at a
    306  1.1.1.2  christos       linespeed of 4800 bps or 9600 bps.
    307  1.1.1.2  christos     </p>
    308  1.1.1.2  christos 
    309  1.1.1.3  christos     <h4>Monitor Data</h4>
    310  1.1.1.3  christos 
    311  1.1.1.3  christos     <p>The last GPS sentence that is accepted or rejected is written to the
    312  1.1.1.3  christos     clockstats file and available with <code>ntpq -c clockvar</code>.
    313  1.1.1.3  christos     (Logging the rejected sentences lets you see/debug why they were rejected.)
    314  1.1.1.3  christos     Filtered sentences are not logged.</p>
    315  1.1.1.3  christos 
    316  1.1.1.3  christos     <p>
    317  1.1.1.3  christos       If the 0x10000 mode bit is on and clockstats is enabled, several extra
    318  1.1.1.3  christos       counters will be appended to the NMEA sentence that gets logged.
    319  1.1.1.3  christos       For example:
    320  1.1.1.3  christos <pre>
    321  1.1.1.3  christos 56299 76876.691 127.127.20.20 $GPGGA,212116.000,3726.0785,N,12212.2605,W,1,05,2.0,17.0,M,-25.7,M,,0000*5C  228 64 0 0 64 0
    322  1.1.1.3  christos </pre>
    323  1.1.1.3  christos     </p>
    324  1.1.1.3  christos 
    325  1.1.1.3  christos     <table border="1">
    326  1.1.1.3  christos       <caption>Clockstats</caption>
    327  1.1.1.3  christos       <tbody><tr>
    328  1.1.1.3  christos 	<th align="center">Column</th>
    329  1.1.1.3  christos 	<th align="center">Sample</th>
    330  1.1.1.3  christos 	<th align="left">Meaning</th>
    331  1.1.1.3  christos       </tr>
    332  1.1.1.3  christos       
    333  1.1.1.3  christos       <tr>
    334  1.1.1.3  christos 	<td align="center">1</td>
    335  1.1.1.3  christos 	<td align="center">56299</td>
    336  1.1.1.3  christos 	<td>MJD</td>
    337  1.1.1.3  christos       </tr><tr>
    338  1.1.1.3  christos 	<td align="center">2</td>
    339  1.1.1.3  christos 	<td align="center">76876.691</td>
    340  1.1.1.3  christos 	<td>Time of day in seconds</td>
    341  1.1.1.3  christos       </tr><tr>
    342  1.1.1.3  christos 	<td align="center">3</td>
    343  1.1.1.3  christos 	<td align="center">127.127.20.20</td>
    344  1.1.1.3  christos 	<td>IP Address from server config line</td>
    345  1.1.1.3  christos       </tr><tr>
    346  1.1.1.3  christos 	<td align="center">4</td>
    347  1.1.1.3  christos 	<td align="center">$GPGGA,...0*5C</td>
    348  1.1.1.3  christos 	<td>NMEA Sentence</td>
    349  1.1.1.3  christos       </tr><tr>
    350  1.1.1.3  christos         <td align="center">5</td>
    351  1.1.1.3  christos         <td align="center">228</td>
    352  1.1.1.3  christos         <td>Number of sentences received</td>
    353  1.1.1.3  christos       </tr><tr>
    354  1.1.1.3  christos         <td align="center">6</td>
    355  1.1.1.3  christos         <td align="center">64</td>
    356  1.1.1.3  christos         <td>Number of sentences accepted and used for timekeeping</td>
    357  1.1.1.3  christos       </tr><tr>
    358  1.1.1.3  christos         <td align="center">7</td>
    359  1.1.1.3  christos         <td align="center">0</td>
    360  1.1.1.3  christos         <td>Number of sentences rejected because they were marked invalid (poor signal)</td>
    361  1.1.1.3  christos       </tr><tr>
    362  1.1.1.3  christos         <td align="center">8</td>
    363  1.1.1.3  christos         <td align="center">0</td>
    364  1.1.1.3  christos         <td>Number of sentences rejected because of bad checksum or invalid date/time</td>
    365  1.1.1.3  christos       </tr><tr>
    366  1.1.1.3  christos         <td align="center">9</td>
    367  1.1.1.3  christos         <td align="center">64</td>
    368  1.1.1.3  christos         <td>Number of sentences filtered by mode bits or same second</td>
    369  1.1.1.3  christos       </tr><tr>
    370  1.1.1.3  christos         <td align="center">10</td>
    371  1.1.1.3  christos         <td align="center">0</td>
    372  1.1.1.3  christos         <td>Number of PPS pulses used, overrides NMEA sentences</td>
    373  1.1.1.3  christos       </tr>
    374  1.1.1.3  christos     </tbody></table>
    375  1.1.1.3  christos 
    376  1.1.1.3  christos     Sentences like $GPGSV that don't contain the time will get
    377  1.1.1.3  christos     counted in the total but otherwise ignored.
    378  1.1.1.3  christos 
    379  1.1.1.2  christos     <p>
    380  1.1.1.2  christos       <a href="https://support.ntp.org/bin/view/Support/ConfiguringNMEARefclocks">Configuring
    381  1.1.1.2  christos       NMEA Refclocks</a> might give further useful hints for specific hardware
    382  1.1.1.2  christos       devices that exhibit strange or curious behaviour.
    383  1.1.1.2  christos     </p>
    384  1.1.1.2  christos 
    385  1.1.1.2  christos     <p>
    386  1.1.1.2  christos       To make a specific setting, select the corresponding decimal values from
    387  1.1.1.2  christos       the mode byte table, add them all together and enter the resulting
    388  1.1.1.2  christos       decimal value into the clock configuration line.
    389  1.1.1.2  christos     </p>
    390  1.1.1.2  christos 
    391  1.1.1.2  christos     <h4>Setting up the Garmin GPS-25XL</h4>
    392  1.1.1.2  christos 
    393  1.1.1.2  christos     Switch off all output with by sending it the following string.
    394  1.1.1.2  christos     <pre>"$PGRMO,,2&lt;cr&gt;&lt;lf&gt;"</pre>
    395  1.1.1.2  christos     <p>Now switch only $GPRMC on by sending it the following string.</p>
    396  1.1.1.2  christos     <pre>"$PGRMO,GPRMC,1&lt;cr&gt;&lt;lf&gt;"</pre>
    397  1.1.1.2  christos 
    398  1.1.1.2  christos     <p>On some systems the PPS signal isn't switched on by default. It can be
    399  1.1.1.2  christos     switched on by sending the following string.</p>
    400  1.1.1.2  christos     <pre>"$PGRMC,,,,,,,,,,,,2&lt;cr&gt;&lt;lf&gt;"</pre>
    401  1.1.1.2  christos 
    402  1.1.1.2  christos     <h4>Fudge Factors</h4>
    403  1.1.1.2  christos 
    404  1.1.1.2  christos     <dl>
    405  1.1.1.2  christos       <dt><tt>time1 <i>time</i></tt></dt>
    406  1.1.1.2  christos       <dd>Specifies the PPS time offset calibration factor, in seconds and fraction, with default 0.0.</dd>
    407  1.1.1.2  christos       <dt><a name="fudgetime2"><tt>time2 <i>time</i></tt></a></dt>
    408  1.1.1.2  christos       <dd>Specifies the serial end of line time offset calibration factor, in seconds and fraction, with default
    409  1.1.1.2  christos 	0.0.</dd>
    410  1.1.1.2  christos       <dt><tt>stratum <i>number</i></tt></dt>
    411  1.1.1.2  christos       <dd>Specifies the driver stratum, in decimal from 0 to 15, with default 0.</dd>
    412  1.1.1.2  christos       <dt><tt>refid <i>string</i></tt></dt>
    413  1.1.1.2  christos       <dd>Specifies the driver reference identifier, an ASCII string from one to four characters, with
    414  1.1.1.2  christos 	default <tt>GPS</tt>.</dd>
    415  1.1.1.2  christos       <dt><tt>flag1 0 | 1</tt></dt>
    416  1.1.1.2  christos       <dd>Disable PPS signal processing if 0 (default); enable PPS signal processing if 1.</dd>
    417  1.1.1.2  christos       <dt><tt>flag2 0 | 1</tt></dt>
    418  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
    419  1.1.1.2  christos 	falling edge if 1.</dd>
    420  1.1.1.2  christos       <dt><tt>flag3 0 | 1</tt></dt>
    421  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
    422  1.1.1.2  christos 	discipline if 1.</dd>
    423  1.1.1.2  christos       <dt><tt>flag4 0 | 1</tt></dt>
    424  1.1.1.2  christos       <dd>Obscures location in timecode: 0 for disable (default), 1 for enable.</dd>
    425  1.1.1.2  christos     </dl>
    426  1.1.1.2  christos 
    427  1.1.1.2  christos     <p>Additional Information</p>
    428  1.1.1.3  christos     <p><tt>flag1</tt>, <tt>flag2</tt>, and <tt>flag3</tt> are ignored under Windows.</p>
    429  1.1.1.2  christos     <p><a href="../refclock.html">Reference Clock Drivers</a></p>
    430  1.1.1.2  christos     <hr>
    431  1.1.1.2  christos     <script type="text/javascript" language="javascript" src="scripts/footer.txt"></script>
    432  1.1.1.2  christos   </body></html>
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