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