diff options
author | Eric S. Raymond <esr@thyrsus.com> | 2005-02-16 05:23:02 +0000 |
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committer | Eric S. Raymond <esr@thyrsus.com> | 2005-02-16 05:23:02 +0000 |
commit | 6658ae0cc18dd1665037f57290828fe7200ef84c (patch) | |
tree | b2b6b45f11ca2800868f30bee1fac2912979160f /HACKING | |
parent | 30f252cedd7ca798eeaed20ba917ed499c0ede7a (diff) | |
download | gpsd-6658ae0cc18dd1665037f57290828fe7200ef84c.tar.gz |
This version breaks profiling (the problem is in gpsy.py somewhere)
but we need a checkpoint in order to merge Gary Miller's Magellan
changes.
Diffstat (limited to 'HACKING')
-rw-r--r-- | HACKING | 35 |
1 files changed, 23 insertions, 12 deletions
@@ -15,18 +15,29 @@ Next, profiling... If you build with --enable-profiling, an undocumented Z command in the daemon will cause it to emit a $ clause on every D request. The $ clause contains -two colon-separated fields: a millisecond-precision timestamp telling when -gpsd shipped the data, and the character length of the sentence containing the -timestamp data (a GPRMC, GPLL, or GPGGA sentence). - -The spread between D and Z timestamps measures the latency between the -GPS's fix measurement and when it became available to the client. For -it to be meaningful, the GPS has to ship timestamps with sub-second -precision. SiRF-II and Evermore chipsets ship times with millisecond -resolution. Your machine's time reference must also be accurate to -subsecond precision; I recommend using ntpd, which will normally give -you about 15 microseconds precision (two orders of magnitude better than -GPSes normally report). +four space-separated fields: + +(1) The character length of the sentence containing the timestamp data + (a GPRMC, GPLL, or GPGGA sentence) + +(2) A timestamp telling when gpsd received the data. + +(3) An offset from the received timestamp telling when gpsd decoded the data. + +(4) An offset from the received timestamp taken just before encoding the + response -- effectively, when gpsd was polled to transmit the data. + +(5) An offset from the received timestamp telling when gpsd transmitted + the data. + +The spread between D and the Z timestamps measures components of the +latency between the GPS's fix measurement and when it became available +to the client. For it to be meaningful, the GPS has to ship timestamps +with sub-second precision. SiRF-II and Evermore chipsets ship times +with millisecond resolution. Your machine's time reference must also +be accurate to subsecond precision; I recommend using ntpd, which will +normally give you about 15 microseconds precision (two orders of +magnitude better than GPSes normally report). Another helpful command is B. Without arguments, it triggers a response of the form <baudrate> <bits> N <stopbits>, e.g. "4800 8 N 1" describing |