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diff --git a/www/client-howto.txt b/www/client-howto.txt index ff12394f..f1ad061e 100644 --- a/www/client-howto.txt +++ b/www/client-howto.txt @@ -1,6 +1,6 @@ = GPSD Client HOWTO = Eric S. Raymond <esr@thyrsus.com> -v1.12, August 2011 +v1.13, February 2012 == Introduction == @@ -82,6 +82,18 @@ device-type-independent datagrams, but it can't impose timing and sequencing regularities on them that the underlying device doesn't already obey. +A note to authors of AIVDM clients: Unlike GPS data, you have the +problem that later sentences of (say) Type 1 don't obsolete the data +in earlier ones. This is a problem, because the library is designed +so that read calls pull any JSON reports waiting from the daemon and +interpret them all. + +To avoid losing data, you want to poll the daemon more often than once +per two seconds (that being the minimum transmission period for the +most frequently shipped sentences, Type 1/2/3). That way the read +buffer will never contain both a message and a later message of the +same type that steps on it. + == What GPSD does, and what it cannot do == GPSD solves some of the problems with GPS/AIS sensors. First: |