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diff --git a/html/drivers/driver20.html b/html/drivers/driver20.html new file mode 100644 index 0000000..6391e86 --- /dev/null +++ b/html/drivers/driver20.html @@ -0,0 +1,432 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> +<html><head> + <meta http-equiv="Content-Type" content="text/html;charset=iso-8859-1"><title>Generic NMEA GPS Receiver</title> + <!-- Changed by: Harlan &, 31-Mar-2014 --> + <link href="scripts/style.css" type="text/css" rel="stylesheet"> + <style type="text/css"> + table.dlstable { font-size:85%; } + td.ttf{ font-family:Courier; font-weight:bold; } + </style></head> + + + + <body> + <h3>Generic NMEA GPS Receiver</h3> +<p>Last update: + <!-- #BeginDate format:En2m -->31-Mar-2014 03:55<!-- #EndDate --> + UTC</p> + <hr> + <h4>Synopsis</h4> + + <p> + Address: 127.127.20.<i>u</i><br> + Reference ID: <tt>GPS</tt><br> + Driver ID: <tt>GPS_NMEA</tt><br> + Serial Port: <tt>/dev/gps<i>u</i></tt>; 4800 - 115200 bps, 8-bits, no parity<br> + Serial Port: <tt>/dev/gpspps<i>u</i></tt>; for just the PPS signal (this + is tried first for PPS, before <tt>/dev/gps<i>u</i></tt>)<br> + Serial Port: <tt>/dev/gps<i>u</i></tt>; symlink to server:port (for nmead)<br> + Features: <tt>tty_clk</tt> + </p> + + <h4>Description</h4> + + <p> + This driver supports GPS receivers with + the <tt>$GPRMC</tt>, <tt>$GPGLL</tt>, <tt>$GPGGA</tt>, <tt>$GPZDA</tt> + and <tt>$GPZDG</tt> NMEA sentences by default. Note that Accord's + custom NMEA sentence <tt>$GPZDG</tt> 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 <tt>$GPZDG</tt> is received. + </p> + <p> + The driver expects the receiver to be set up to transmit at least one + supported sentence every second. + </p> + <p> + The accuracy depends on the receiver used. Inexpensive GPS models are + available with a claimed PPS signal accuracy of + 1 μs or better relative to the broadcast + signal. However, in most cases the actual accuracy is limited by the + precision of the timecode and the latencies of the serial interface and + operating system. + </p> + <p> + If the Operating System supports PPSAPI + (<a href="http://www.ietf.org/rfc/rfc2783.txt">RFC 2783</a>), fudge flag1 + 1 enables its use. + </p> + <p> + The various GPS sentences that this driver recognises look like this:<br> + (others quietly ignored) + </p> + + <p><table class="dlstable" border="1"> + <caption>Accepted NMEA sentences</caption> + <tbody><tr> + <th>Sentence</th> + <th>Vendor</th> + </tr><tr> + <td class="ttf">$GPRMC,UTC,POS_STAT,LAT,LAT_REF,LON,LON_REF,SPD,HDG,DATE,MAG_VAR,MAG_REF*CS<cr><lf></td> + </tr><tr> + <td class="ttf">$GPGLL,LAT,LAT_REF,LON,LON_REF,UTC,POS_STAT*CS<cr><lf></td> + </tr><tr> + <td class="ttf">$GPGGA,UTC,LAT,LAT_REF,LON,LON_REF,FIX_MODE,SAT_USED,HDOP,ALT,ALT_UNIT,GEO,G_UNIT,D_AGE,D_REF*CS<cr><lf></td> + </tr><tr> + <td class="ttf">$GPZDA,UTC,DD,MM,YYYY,TH,TM,*CS<cr><lf></td> + </tr><tr> + <td class="ttf">$GPZDG,GPSTIME,DD,MM,YYYY,AA.BB,V*CS<cr><lf></td> + <td>Accord</td> + </tr> + </tbody></table></p> + + <p><table class="dlstable" border="1"> + <caption>NMEA data items</caption> + <tbody><tr> + <th>Symbol</th> + <th>Meaning and Format</th> + </tr> + + <tr> + <td class="ttf">UTC</td> + <td>Time of day on UTC timescale. Hours, minutes and seconds [fraction (opt.)]. (hhmmss[.fff])</td> + </tr><tr> + <td class="ttf">POS_STAT</td> + <td>Position status. (A = Data valid, V = Data invalid)</td> + </tr><tr> + <td class="ttf">LAT</td> + <td>Latitude (llll.ll)</td> + </tr><tr> + <td class="ttf">LAT_REF</td> + <td>Latitude direction. (N = North, S = South)</td> + </tr><tr> + <td class="ttf">LON</td> + <td>Longitude (yyyyy.yy)</td> + </tr><tr> + <td class="ttf">LON_REF</td> + <td>Longitude direction (E = East, W = West)</td> + </tr><tr> + <td class="ttf">SPD</td> + <td>Speed over ground. (knots) (x.x)</td> + </tr><tr> + <td class="ttf">HDG</td> + <td>Heading/track made good (degrees True) (x.x)</td> + </tr><tr> + <td class="ttf">DATE</td> + <td>Date (ddmmyy)</td> + </tr><tr> + <td class="ttf">MAG_VAR</td> + <td>Magnetic variation (degrees) (x.x)</td> + </tr><tr> + <td class="ttf">MAG_REF</td> + <td>Magnetic variation (E = East, W = West)</td> + </tr><tr> + <td class="ttf">FIX_MODE</td> + <td>Position Fix Mode (0 = Invalid, >0 = Valid)</td> + </tr><tr> + <td class="ttf">SAT_USED</td> + <td>Number of Satellites used in solution</td> + </tr><tr> + <td class="ttf">HDOP</td> + <td>Horizontal Dilution of Precision</td> + </tr><tr> + <td class="ttf">ALT</td> + <td>Antenna Altitude</td> + </tr><tr> + <td class="ttf">ALT_UNIT</td> + <td>Altitude Units (Metres/Feet)</td> + </tr><tr> + <td class="ttf">GEO</td> + <td>Geoid/Elipsoid separation</td> + </tr><tr> + <td class="ttf">G_UNIT</td> + <td>Geoid units (M/F)</td> + </tr><tr> + <td class="ttf">D_AGE</td> + <td>Age of last DGPS Fix</td> + </tr><tr> + <td class="ttf">D_REF</td> + <td>Reference ID of DGPS station</td> + </tr><tr> + <td class="ttf">GPSTIME</td> + <td>Time of day on GPS timescale. Hours, minutes and seconds [fraction (opt.)]. (hhmmss[.f])</td> + </tr><tr> + <td class="ttf">DD</td> + <td>Day of the month (1-31)</td> + </tr><tr> + <td class="ttf">MM</td> + <td>Month of the year (1-12)</td> + </tr><tr> + <td class="ttf">YYYY</td> + <td>Year</td> + </tr><tr> + <td class="ttf">AA.BB</td> + <td>Denotes the signal strength (should be < 05.00)</td> + </tr><tr> + <td class="ttf">V</td> + <td>GPS sync status<br> + '0' => INVALID time,<br> + '1' => accuracy of +/- 20ms,<br> + '2' => accuracy of +/- 100ns</td> + </tr><tr> + <td class="ttf">CS</td> + <td> Checksum</td> + </tr><tr> + <td class="ttf"><cr><lf></td> + <td>Sentence terminator.</td> + </tr> + </tbody></table></p> + + + <h4>The 'mode' byte</h4> + + <p> + Specific GPS sentences and bitrates may be selected by setting bits of + the 'mode' in the server configuration line:<br> <tt>server + 127.127.20.x mode X</tt> + </p> + + <table border="1"> + <caption>mode byte bits and bit groups</caption> + <tbody><tr> + <th align="center">Bit</th> + <th align="center">Decimal</th> + <th align="center">Hex</th> + <th align="left">Meaning</th> + </tr> + + <tr> + <td align="center">0</td> + <td align="center">1</td> + <td align="center">1</td> + <td>process <tt>$GPMRC</tt></td> + </tr><tr> + <td align="center">1</td> + <td align="center">2</td> + <td align="center">2</td> + <td>process <tt>$GPGGA</tt></td> + </tr><tr> + <td align="center">2</td> + <td align="center">4</td> + <td align="center">4</td> + <td>process <tt>$GPGLL</tt></td> + </tr><tr> + <td align="center">3</td> + <td align="center">8</td> + <td align="center">8</td> + <td>process <tt>$GPZDA</tt> or <tt>$GPZDG</tt></td> + </tr><tr> + <td rowspan="6" align="center">4-6</td> + <td align="center">0</td> + <td align="center">0</td> + <td>linespeed 4800 bps</td> + </tr><tr> + <td align="center">16</td> + <td align="center">0x10</td> + <td>linespeed 9600 bps</td> + </tr><tr> + <td align="center">32</td> + <td align="center">0x20</td> + <td>linespeed 19200 bps</td> + </tr><tr> + <td align="center">48</td> + <td align="center">0x30</td> + <td>linespeed 38400 bps</td> + </tr><tr> + <td align="center">64</td> + <td align="center">0x40</td> + <td>linespeed 57600 bps</td> + </tr><tr> + <td align="center">80</td> + <td align="center">0x50</td> + <td>linespeed 115200 bps</td> + </tr><tr> + <td align="center">7</td> + <td align="center">128</td> + <td align="center">0x80</td> + <td>Write the sub-second fraction of the receive time stamp to the + clockstat file for all recognised NMEA sentences. This can be used to + get a useful value for fudge time2.<br><strong>Caveat:</strong> This + will fill your clockstat file rather fast. Use it only temporarily to + get the numbers for the NMEA sentence of your choice.</td> + </tr> + </tr><tr> + <td align="center">8</td> + <td align="center">256</td> + <td align="center">0x100</td> + <td>process <tt>$PGRMF</tt></td> + </tr><tr> + <td align="center">9-15</td> + <td align="center"></td> + <td align="center">0xFE00</td> + <td>reserved - leave 0</td> + </tr><tr> + <td align="center">16</td> + <td align="center">65536</td> + <td align="center">0x10000</td> + <td>Append extra statistics to the clockstats line. + Details below.</td> + </tr> + </tbody></table> + + + <p> + 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 + </p><ul> + <li>there is only <a href="#fudgetime2">fudge time2</a> available to + compensate for transmission delays but every sentence would need a + different one and + </li><li>using more than one sentence per cycle overstuffs the internal data + filters. + </li></ul> + The driver uses 4800 bits per second by default, but faster bitrates can + be selected using bits 4 to 6 of the mode field. + <p></p> + + <p> + <strong>Caveat:</strong> 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 <tt>$GPRMC</tt> or <tt>$GPZDA</tt> per second, at a + linespeed of 4800 bps or 9600 bps. + </p> + + <h4>Monitor Data</h4> + + <p>The last GPS sentence that is accepted or rejected is written to the + clockstats file and available with <code>ntpq -c clockvar</code>. + (Logging the rejected sentences lets you see/debug why they were rejected.) + Filtered sentences are not logged.</p> + + <p> + If the 0x10000 mode bit is on and clockstats is enabled, several extra + counters will be appended to the NMEA sentence that gets logged. + For example: +<pre> +56299 76876.691 127.127.20.20 $GPGGA,212116.000,3726.0785,N,12212.2605,W,1,05,2.0,17.0,M,-25.7,M,,0000*5C 228 64 0 0 64 0 +</pre> + </p> + + <table border="1"> + <caption>Clockstats</caption> + <tbody><tr> + <th align="center">Column</th> + <th align="center">Sample</th> + <th align="left">Meaning</th> + </tr> + + <tr> + <td align="center">1</td> + <td align="center">56299</td> + <td>MJD</td> + </tr><tr> + <td align="center">2</td> + <td align="center">76876.691</td> + <td>Time of day in seconds</td> + </tr><tr> + <td align="center">3</td> + <td align="center">127.127.20.20</td> + <td>IP Address from server config line</td> + </tr><tr> + <td align="center">4</td> + <td align="center">$GPGGA,...0*5C</td> + <td>NMEA Sentence</td> + </tr><tr> + <td align="center">5</td> + <td align="center">228</td> + <td>Number of sentences received</td> + </tr><tr> + <td align="center">6</td> + <td align="center">64</td> + <td>Number of sentences accepted and used for timekeeping</td> + </tr><tr> + <td align="center">7</td> + <td align="center">0</td> + <td>Number of sentences rejected because they were marked invalid (poor signal)</td> + </tr><tr> + <td align="center">8</td> + <td align="center">0</td> + <td>Number of sentences rejected because of bad checksum or invalid date/time</td> + </tr><tr> + <td align="center">9</td> + <td align="center">64</td> + <td>Number of sentences filtered by mode bits or same second</td> + </tr><tr> + <td align="center">10</td> + <td align="center">0</td> + <td>Number of PPS pulses used, overrides NMEA sentences</td> + </tr> + </tbody></table> + + Sentences like $GPGSV that don't contain the time will get + counted in the total but otherwise ignored. + + <p> + <a href="https://support.ntp.org/bin/view/Support/ConfiguringNMEARefclocks">Configuring + NMEA Refclocks</a> might give further useful hints for specific hardware + devices that exhibit strange or curious behaviour. + </p> + + <p> + To make a specific setting, select the corresponding decimal values from + the mode byte table, add them all together and enter the resulting + decimal value into the clock configuration line. + </p> + + <h4>Setting up the Garmin GPS-25XL</h4> + + Switch off all output with by sending it the following string. + <pre>"$PGRMO,,2<cr><lf>"</pre> + <p>Now switch only $GPRMC on by sending it the following string.</p> + <pre>"$PGRMO,GPRMC,1<cr><lf>"</pre> + + <p>On some systems the PPS signal isn't switched on by default. It can be + switched on by sending the following string.</p> + <pre>"$PGRMC,,,,,,,,,,,,2<cr><lf>"</pre> + + <h4>Fudge Factors</h4> + + <dl> + <dt><tt>time1 <i>time</i></tt></dt> + <dd>Specifies the PPS time offset calibration factor, in seconds and fraction, with default 0.0.</dd> + <dt><a name="fudgetime2"><tt>time2 <i>time</i></tt></a></dt> + <dd>Specifies the serial end of line time offset calibration factor, in seconds and fraction, with default + 0.0.</dd> + <dt><tt>stratum <i>number</i></tt></dt> + <dd>Specifies the driver stratum, in decimal from 0 to 15, with default 0.</dd> + <dt><tt>refid <i>string</i></tt></dt> + <dd>Specifies the driver reference identifier, an ASCII string from one to four characters, with + default <tt>GPS</tt>.</dd> + <dt><tt>flag1 0 | 1</tt></dt> + <dd>Disable PPS signal processing if 0 (default); enable PPS signal processing if 1.</dd> + <dt><tt>flag2 0 | 1</tt></dt> + <dd>If PPS signal processing is enabled, capture the pulse on the rising edge if 0 (default); capture on the + falling edge if 1.</dd> + <dt><tt>flag3 0 | 1</tt></dt> + <dd>If PPS signal processing is enabled, use the <tt>ntpd</tt> clock discipline if 0 (default); use the kernel + discipline if 1.</dd> + <dt><tt>flag4 0 | 1</tt></dt> + <dd>Obscures location in timecode: 0 for disable (default), 1 for enable.</dd> + </dl> + + <p>Additional Information</p> + <p><tt>flag1</tt>, <tt>flag2</tt>, and <tt>flag3</tt> are ignored under Windows.</p> + <p><a href="../refclock.html">Reference Clock Drivers</a></p> + <hr> + <script type="text/javascript" language="javascript" src="scripts/footer.txt"></script> + </body></html> |