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1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> 2 <html><head> 3 <meta http-equiv="Content-Type" content="text/html;charset=iso-8859-1"><title>Generic NMEA GPS Receiver</title> 4 <!-- Changed by: Harlan &, 31-Mar-2014 --> 5 <link href="scripts/style.css" type="text/css" rel="stylesheet"> 6 <style type="text/css"> 7 table.dlstable { font-size:85%; } 8 td.ttf{ font-family:Courier; font-weight:bold; } 9 </style></head> 10 11 12 13 <body> 14 <h3>Generic NMEA GPS Receiver</h3> 15 <p>Last update: 16 <!-- #BeginDate format:En2m -->31-Mar-2014 03:55<!-- #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>$GPZDG</tt> is received. 44 </p> 45 <p> 46 The driver expects the receiver to be set up to transmit at least one 47 supported sentence every second. 48 </p> 49 <p> 50 The accuracy depends on the receiver used. Inexpensive GPS models are 51 available with a claimed PPS signal accuracy of 52 1 μs or better relative to the broadcast 53 signal. However, in most cases the actual accuracy is limited by the 54 precision of the timecode and the latencies of the serial interface and 55 operating system. 56 </p> 57 <p> 58 If the Operating System supports PPSAPI 59 (<a href="http://www.ietf.org/rfc/rfc2783.txt">RFC 2783</a>), fudge flag1 60 1 enables its use. 61 </p> 62 <p> 63 The various GPS sentences that this driver recognises look like this:<br> 64 (others quietly ignored) 65 </p> 66 67 <p><table class="dlstable" border="1"> 68 <caption>Accepted NMEA sentences</caption> 69 <tbody><tr> 70 <th>Sentence</th> 71 <th>Vendor</th> 72 </tr><tr> 73 <td class="ttf">$GPRMC,UTC,POS_STAT,LAT,LAT_REF,LON,LON_REF,SPD,HDG,DATE,MAG_VAR,MAG_REF*CS<cr><lf></td> 74 </tr><tr> 75 <td class="ttf">$GPGLL,LAT,LAT_REF,LON,LON_REF,UTC,POS_STAT*CS<cr><lf></td> 76 </tr><tr> 77 <td class="ttf">$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> 78 </tr><tr> 79 <td class="ttf">$GPZDA,UTC,DD,MM,YYYY,TH,TM,*CS<cr><lf></td> 80 </tr><tr> 81 <td class="ttf">$GPZDG,GPSTIME,DD,MM,YYYY,AA.BB,V*CS<cr><lf></td> 82 <td>Accord</td> 83 </tr> 84 </tbody></table></p> 85 86 <p><table class="dlstable" border="1"> 87 <caption>NMEA data items</caption> 88 <tbody><tr> 89 <th>Symbol</th> 90 <th>Meaning and Format</th> 91 </tr> 92 93 <tr> 94 <td class="ttf">UTC</td> 95 <td>Time of day on UTC timescale. Hours, minutes and seconds [fraction (opt.)]. (hhmmss[.fff])</td> 96 </tr><tr> 97 <td class="ttf">POS_STAT</td> 98 <td>Position status. (A = Data valid, V = Data invalid)</td> 99 </tr><tr> 100 <td class="ttf">LAT</td> 101 <td>Latitude (llll.ll)</td> 102 </tr><tr> 103 <td class="ttf">LAT_REF</td> 104 <td>Latitude direction. (N = North, S = South)</td> 105 </tr><tr> 106 <td class="ttf">LON</td> 107 <td>Longitude (yyyyy.yy)</td> 108 </tr><tr> 109 <td class="ttf">LON_REF</td> 110 <td>Longitude direction (E = East, W = West)</td> 111 </tr><tr> 112 <td class="ttf">SPD</td> 113 <td>Speed over ground. (knots) (x.x)</td> 114 </tr><tr> 115 <td class="ttf">HDG</td> 116 <td>Heading/track made good (degrees True) (x.x)</td> 117 </tr><tr> 118 <td class="ttf">DATE</td> 119 <td>Date (ddmmyy)</td> 120 </tr><tr> 121 <td class="ttf">MAG_VAR</td> 122 <td>Magnetic variation (degrees) (x.x)</td> 123 </tr><tr> 124 <td class="ttf">MAG_REF</td> 125 <td>Magnetic variation (E = East, W = West)</td> 126 </tr><tr> 127 <td class="ttf">FIX_MODE</td> 128 <td>Position Fix Mode (0 = Invalid, >0 = Valid)</td> 129 </tr><tr> 130 <td class="ttf">SAT_USED</td> 131 <td>Number of Satellites used in solution</td> 132 </tr><tr> 133 <td class="ttf">HDOP</td> 134 <td>Horizontal Dilution of Precision</td> 135 </tr><tr> 136 <td class="ttf">ALT</td> 137 <td>Antenna Altitude</td> 138 </tr><tr> 139 <td class="ttf">ALT_UNIT</td> 140 <td>Altitude Units (Metres/Feet)</td> 141 </tr><tr> 142 <td class="ttf">GEO</td> 143 <td>Geoid/Elipsoid separation</td> 144 </tr><tr> 145 <td class="ttf">G_UNIT</td> 146 <td>Geoid units (M/F)</td> 147 </tr><tr> 148 <td class="ttf">D_AGE</td> 149 <td>Age of last DGPS Fix</td> 150 </tr><tr> 151 <td class="ttf">D_REF</td> 152 <td>Reference ID of DGPS station</td> 153 </tr><tr> 154 <td class="ttf">GPSTIME</td> 155 <td>Time of day on GPS timescale. Hours, minutes and seconds [fraction (opt.)]. (hhmmss[.f])</td> 156 </tr><tr> 157 <td class="ttf">DD</td> 158 <td>Day of the month (1-31)</td> 159 </tr><tr> 160 <td class="ttf">MM</td> 161 <td>Month of the year (1-12)</td> 162 </tr><tr> 163 <td class="ttf">YYYY</td> 164 <td>Year</td> 165 </tr><tr> 166 <td class="ttf">AA.BB</td> 167 <td>Denotes the signal strength (should be < 05.00)</td> 168 </tr><tr> 169 <td class="ttf">V</td> 170 <td>GPS sync status<br> 171 '0' => INVALID time,<br> 172 '1' => accuracy of +/- 20ms,<br> 173 '2' => accuracy of +/- 100ns</td> 174 </tr><tr> 175 <td class="ttf">CS</td> 176 <td> Checksum</td> 177 </tr><tr> 178 <td class="ttf"><cr><lf></td> 179 <td>Sentence terminator.</td> 180 </tr> 181 </tbody></table></p> 182 183 184 <h4>The 'mode' byte</h4> 185 186 <p> 187 Specific GPS sentences and bitrates may be selected by setting bits of 188 the 'mode' in the server configuration line:<br> <tt>server 189 127.127.20.x mode X</tt> 190 </p> 191 192 <table border="1"> 193 <caption>mode byte bits and bit groups</caption> 194 <tbody><tr> 195 <th align="center">Bit</th> 196 <th align="center">Decimal</th> 197 <th align="center">Hex</th> 198 <th align="left">Meaning</th> 199 </tr> 200 201 <tr> 202 <td align="center">0</td> 203 <td align="center">1</td> 204 <td align="center">1</td> 205 <td>process <tt>$GPMRC</tt></td> 206 </tr><tr> 207 <td align="center">1</td> 208 <td align="center">2</td> 209 <td align="center">2</td> 210 <td>process <tt>$GPGGA</tt></td> 211 </tr><tr> 212 <td align="center">2</td> 213 <td align="center">4</td> 214 <td align="center">4</td> 215 <td>process <tt>$GPGLL</tt></td> 216 </tr><tr> 217 <td align="center">3</td> 218 <td align="center">8</td> 219 <td align="center">8</td> 220 <td>process <tt>$GPZDA</tt> or <tt>$GPZDG</tt></td> 221 </tr><tr> 222 <td rowspan="6" align="center">4-6</td> 223 <td align="center">0</td> 224 <td align="center">0</td> 225 <td>linespeed 4800 bps</td> 226 </tr><tr> 227 <td align="center">16</td> 228 <td align="center">0x10</td> 229 <td>linespeed 9600 bps</td> 230 </tr><tr> 231 <td align="center">32</td> 232 <td align="center">0x20</td> 233 <td>linespeed 19200 bps</td> 234 </tr><tr> 235 <td align="center">48</td> 236 <td align="center">0x30</td> 237 <td>linespeed 38400 bps</td> 238 </tr><tr> 239 <td align="center">64</td> 240 <td align="center">0x40</td> 241 <td>linespeed 57600 bps</td> 242 </tr><tr> 243 <td align="center">80</td> 244 <td align="center">0x50</td> 245 <td>linespeed 115200 bps</td> 246 </tr><tr> 247 <td align="center">7</td> 248 <td align="center">128</td> 249 <td align="center">0x80</td> 250 <td>Write the sub-second fraction of the receive time stamp to the 251 clockstat file for all recognised NMEA sentences. This can be used to 252 get a useful value for fudge time2.<br><strong>Caveat:</strong> This 253 will fill your clockstat file rather fast. Use it only temporarily to 254 get the numbers for the NMEA sentence of your choice.</td> 255 </tr> 256 </tr><tr> 257 <td align="center">8</td> 258 <td align="center">256</td> 259 <td align="center">0x100</td> 260 <td>process <tt>$PGRMF</tt></td> 261 </tr><tr> 262 <td align="center">9-15</td> 263 <td align="center"></td> 264 <td align="center">0xFE00</td> 265 <td>reserved - leave 0</td> 266 </tr><tr> 267 <td align="center">16</td> 268 <td align="center">65536</td> 269 <td align="center">0x10000</td> 270 <td>Append extra statistics to the clockstats line. 271 Details below.</td> 272 </tr> 273 </tbody></table> 274 275 276 <p> 277 The default (mode 0) is to process all supported sentences at a linespeed 278 of 4800 bps, which results in the first one received and recognised in 279 each cycle being used. If only specific sentences should be 280 recognised, then the mode byte must be chosen to enable only the selected 281 ones. Multiple sentences may be selected by adding their mode bit 282 values, but of those enabled still only the first received sentence in a 283 cycle will be used. Using more than one sentence per cycle is 284 impossible, because 285 </p><ul> 286 <li>there is only <a href="#fudgetime2">fudge time2</a> available to 287 compensate for transmission delays but every sentence would need a 288 different one and 289 </li><li>using more than one sentence per cycle overstuffs the internal data 290 filters. 291 </li></ul> 292 The driver uses 4800 bits per second by default, but faster bitrates can 293 be selected using bits 4 to 6 of the mode field. 294 <p></p> 295 296 <p> 297 <strong>Caveat:</strong> Using higher line speeds does not necessarily 298 increase the precision of the timing device. Higher line speeds are 299 not necessarily helpful for the NMEA driver, either. They can be 300 used to accomodate for an amount of data that does not fit into a 301 1-second cycle at 4800 bps, but high-speed high-volume NMEA data is likely 302 to cause trouble with the serial line driver since NMEA supports no 303 protocol handshake. Any device that is exclusively used for time 304 synchronisation purposes should be configured to transmit the relevant 305 data only, e.g. one <tt>$GPRMC</tt> or <tt>$GPZDA</tt> per second, at a 306 linespeed of 4800 bps or 9600 bps. 307 </p> 308 309 <h4>Monitor Data</h4> 310 311 <p>The last GPS sentence that is accepted or rejected is written to the 312 clockstats file and available with <code>ntpq -c clockvar</code>. 313 (Logging the rejected sentences lets you see/debug why they were rejected.) 314 Filtered sentences are not logged.</p> 315 316 <p> 317 If the 0x10000 mode bit is on and clockstats is enabled, several extra 318 counters will be appended to the NMEA sentence that gets logged. 319 For example: 320 <pre> 321 56299 76876.691 127.127.20.20 $GPGGA,212116.000,3726.0785,N,12212.2605,W,1,05,2.0,17.0,M,-25.7,M,,0000*5C 228 64 0 0 64 0 322 </pre> 323 </p> 324 325 <table border="1"> 326 <caption>Clockstats</caption> 327 <tbody><tr> 328 <th align="center">Column</th> 329 <th align="center">Sample</th> 330 <th align="left">Meaning</th> 331 </tr> 332 333 <tr> 334 <td align="center">1</td> 335 <td align="center">56299</td> 336 <td>MJD</td> 337 </tr><tr> 338 <td align="center">2</td> 339 <td align="center">76876.691</td> 340 <td>Time of day in seconds</td> 341 </tr><tr> 342 <td align="center">3</td> 343 <td align="center">127.127.20.20</td> 344 <td>IP Address from server config line</td> 345 </tr><tr> 346 <td align="center">4</td> 347 <td align="center">$GPGGA,...0*5C</td> 348 <td>NMEA Sentence</td> 349 </tr><tr> 350 <td align="center">5</td> 351 <td align="center">228</td> 352 <td>Number of sentences received</td> 353 </tr><tr> 354 <td align="center">6</td> 355 <td align="center">64</td> 356 <td>Number of sentences accepted and used for timekeeping</td> 357 </tr><tr> 358 <td align="center">7</td> 359 <td align="center">0</td> 360 <td>Number of sentences rejected because they were marked invalid (poor signal)</td> 361 </tr><tr> 362 <td align="center">8</td> 363 <td align="center">0</td> 364 <td>Number of sentences rejected because of bad checksum or invalid date/time</td> 365 </tr><tr> 366 <td align="center">9</td> 367 <td align="center">64</td> 368 <td>Number of sentences filtered by mode bits or same second</td> 369 </tr><tr> 370 <td align="center">10</td> 371 <td align="center">0</td> 372 <td>Number of PPS pulses used, overrides NMEA sentences</td> 373 </tr> 374 </tbody></table> 375 376 Sentences like $GPGSV that don't contain the time will get 377 counted in the total but otherwise ignored. 378 379 <p> 380 <a href="https://support.ntp.org/bin/view/Support/ConfiguringNMEARefclocks">Configuring 381 NMEA Refclocks</a> might give further useful hints for specific hardware 382 devices that exhibit strange or curious behaviour. 383 </p> 384 385 <p> 386 To make a specific setting, select the corresponding decimal values from 387 the mode byte table, add them all together and enter the resulting 388 decimal value into the clock configuration line. 389 </p> 390 391 <h4>Setting up the Garmin GPS-25XL</h4> 392 393 Switch off all output with by sending it the following string. 394 <pre>"$PGRMO,,2<cr><lf>"</pre> 395 <p>Now switch only $GPRMC on by sending it the following string.</p> 396 <pre>"$PGRMO,GPRMC,1<cr><lf>"</pre> 397 398 <p>On some systems the PPS signal isn't switched on by default. It can be 399 switched on by sending the following string.</p> 400 <pre>"$PGRMC,,,,,,,,,,,,2<cr><lf>"</pre> 401 402 <h4>Fudge Factors</h4> 403 404 <dl> 405 <dt><tt>time1 <i>time</i></tt></dt> 406 <dd>Specifies the PPS time offset calibration factor, in seconds and fraction, with default 0.0.</dd> 407 <dt><a name="fudgetime2"><tt>time2 <i>time</i></tt></a></dt> 408 <dd>Specifies the serial end of line time offset calibration factor, in seconds and fraction, with default 409 0.0.</dd> 410 <dt><tt>stratum <i>number</i></tt></dt> 411 <dd>Specifies the driver stratum, in decimal from 0 to 15, with default 0.</dd> 412 <dt><tt>refid <i>string</i></tt></dt> 413 <dd>Specifies the driver reference identifier, an ASCII string from one to four characters, with 414 default <tt>GPS</tt>.</dd> 415 <dt><tt>flag1 0 | 1</tt></dt> 416 <dd>Disable PPS signal processing if 0 (default); enable PPS signal processing if 1.</dd> 417 <dt><tt>flag2 0 | 1</tt></dt> 418 <dd>If PPS signal processing is enabled, capture the pulse on the rising edge if 0 (default); capture on the 419 falling edge if 1.</dd> 420 <dt><tt>flag3 0 | 1</tt></dt> 421 <dd>If PPS signal processing is enabled, use the <tt>ntpd</tt> clock discipline if 0 (default); use the kernel 422 discipline if 1.</dd> 423 <dt><tt>flag4 0 | 1</tt></dt> 424 <dd>Obscures location in timecode: 0 for disable (default), 1 for enable.</dd> 425 </dl> 426 427 <p>Additional Information</p> 428 <p><tt>flag1</tt>, <tt>flag2</tt>, and <tt>flag3</tt> are ignored under Windows.</p> 429 <p><a href="../refclock.html">Reference Clock Drivers</a></p> 430 <hr> 431 <script type="text/javascript" language="javascript" src="scripts/footer.txt"></script> 432 </body></html> 433
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