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author | Gary E. Miller <gem@rellim.com> | 2015-04-06 12:55:26 -0700 |
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committer | Gary E. Miller <gem@rellim.com> | 2015-04-06 12:55:49 -0700 |
commit | ba74b66fdaa5de3fa5b7ede93490bc20f6e2ee1b (patch) | |
tree | c70719599ac8a349bc7fc9de573e9c57809aaec3 /gpsd_json.xml | |
parent | 5967ffaffa6d1abf7d9b22f06bb8d7dbf71d7624 (diff) | |
download | gpsd-ba74b66fdaa5de3fa5b7ede93490bc20f6e2ee1b.tar.gz |
More doc PPS/TOFF tweaks.
Diffstat (limited to 'gpsd_json.xml')
-rw-r--r-- | gpsd_json.xml | 45 |
1 files changed, 33 insertions, 12 deletions
diff --git a/gpsd_json.xml b/gpsd_json.xml index a1a7bb39..7443949a 100644 --- a/gpsd_json.xml +++ b/gpsd_json.xml @@ -1138,6 +1138,11 @@ into this response.</para> between the host's clock time and the GPS time at top of second (actually, when the first data for the reporting cycle is received).</para> +<para>This message exactly mirrors the PPS message except for one +detail. The TOFF message reports the GPS time as derived from the GPS +serial data stream. The PPS message reports the GPS time as derived +from the GPS PPS pulse.</para> + <para>A TOFF object has the following elements:</para> <table frame="all" pgwide="0"><title>TOFF object</title> @@ -1191,29 +1196,32 @@ between the host's clock time and the GPS time at top of second <entry>precision</entry> <entry>Yes</entry> <entry>numeric</entry> - <entry>log(2) of source jitter in seconds</entry> + <entry>NTP style estimate of PPS precision</entry> </row> </tbody> </tgroup> </table> <para>This message is emitted once per second to watchers of a device -and is intended to report the offset between the in-band report of the +and is intended to report the time stamps of the in-band report of the GPS and seconds as reported by the system clock (which may be NTP-corrected) when the first valid timestamp of the reporting cycle -is seen.</para> +was seen.</para> -<para>The message contains second/microsecond pairs for two clocks; -the realtime clock without NTP correction (the result of -clock_gettime(CLOCK_REALTIME), but only to microsecond precision) -and the ordinary system clock (which may be NTP corrected).</para> +<para>The message contains two second/nanosecond pairs: real_sec and +real_nsec contain the time the GPS thinks it was at the start of the +current cycle; clock_sec and clock_nsec contain the time the system +clock thinks it was on receipt of the first timing message of the cycle. +real_nsec is always to nanosecond precision. clock_nsec is nanosecond +precision on most systems.</para> <para>Here's an example:</para> <programlisting> {"class":"TOFF","device":"/dev/ttyUSB0", "real_sec":1330212592, "real_nsec":343182, - "clock_sec":1330212592,"clock_nsec":343184} + "clock_sec":1330212592,"clock_nsec":343184, + "precision":-2}} </programlisting> </listitem> @@ -1226,6 +1234,19 @@ and the ordinary system clock (which may be NTP corrected).</para> <para>This message is emitted each time the daemon sees a valid PPS (Pulse Per Second) strobe from a device.</para> +<para>This message exactly mirrors the TOFF message except for one +detail. The TOFF message reports the GPS time as derived from the GPS +serial data stream. The PPS message reports the GPS time as derived +from the GPS PPS pulse.</para> + +<para>There are various sources of error in the reported clock times. +The speed of the serial connection between the GPS and the system adds +a delay to start of cycle detection. An even bigger error is added +by the variable computation time inside the GPS. Taken together the +time derived from the start of the GPS cycle can have offsets of +10 millisecond to 700 milliseconds and combined jjitter and wander of +100 to 300 millisecond.</para> + <para>A PPS object has the following elements:</para> <table frame="all" pgwide="0"><title>PPS object</title> @@ -1290,10 +1311,10 @@ emitting PPS, and reports the time of the start of the GPS second (when the 1PPS arrives) and seconds as reported by the system clock (which may be NTP-corrected) at that moment.</para> -<para>The message contains two second/nanosecond pairs: -real_sec and real_nsec contain the time the GPS thinks it was at the PPS edge; -clock_sec and clock_nsec contain the time the system clock thinks it was at -the PPS edge. real_nsec is always to nanosecond precision. clock_nsec +<para>The message contains two second/nanosecond pairs: real_sec and +real_nsec contain the time the GPS thinks it was at the PPS edge; +clock_sec and clock_nsec contain the time the system clock thinks it was +at the PPS edge. real_nsec is always to nanosecond precision. clock_nsec is nanosecond precision on most systems.</para> <para>There are various sources of error in the reported clock times. |