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author | Eric S. Raymond <esr@thyrsus.com> | 2005-03-03 17:59:20 +0000 |
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committer | Eric S. Raymond <esr@thyrsus.com> | 2005-03-03 17:59:20 +0000 |
commit | abdc481e1bcba171a79701613848b60b10694c67 (patch) | |
tree | 4b07cbe393a44525f77906ffed2fd4eb57c6a392 /HACKING | |
parent | 0780f6ce5c7307b97336ea47eb8e83bee4dfb5cd (diff) | |
download | gpsd-abdc481e1bcba171a79701613848b60b10694c67.tar.gz |
Big documentation update to go with hotplugging.
Diffstat (limited to 'HACKING')
-rw-r--r-- | HACKING | 35 |
1 files changed, 14 insertions, 21 deletions
@@ -20,31 +20,24 @@ 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. +(2) An offset from the D time telling when gpsd received the data. -(3) An offset from the received timestamp telling when gpsd decoded the data. +(3) An offset from the D time telling when gpsd decoded the data. -(4) An offset from the received timestamp taken just before encoding the +(4) An offset from the D time 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 +(5) An offset from the D time 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 -the communications parameters of the link to the GPS. If profiling is -enabled, B may be followed by an equals sign and a baud rate; the link -will then change to that rate. This feature is not enabled by default -because it is not secured and can be used to effectively disable the GPS. +The Z figures measure 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 report). The distribution lincludes a Python script, gpsprof, that uses the Z command to collect profiling information from a running GPS instance. @@ -105,7 +98,7 @@ Next, autoconfiguration... One of the design goals for gpsd is to be as near zero-configuration as possible. Under most circumstances, it doesn't require either -the GPS type or the serial-line parameters rto connect to it to be +the GPS type or the serial-line parameters to connect to it to be specified. Presently, here's how the autoconfig works. 1. Ay each baud rate gpsd grabs packets until it sees either a @@ -163,7 +156,7 @@ some funky initialization string. Simply ship the string in the initializer for the NMEA driver. Because vendor control strings live in vendor-specific namespaces (PSRF for SiRF, PGRM for Garmin, etc.) your initializing control string will almost certainly be ignored by -anythinbg not specifically watching for it. +anything not specifically watching for it. Another good thing to send from the NMEA initializer is probe strings. These are strings which shjould elicit an identifying response from |