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.\"Generated by db2man.xsl. Don't modify this, modify the source.
.de Sh \" Subsection
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\fB\\$1\fR
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..
.de Sp \" Vertical space (when we can't use .PP)
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.de Ip \" List item
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.ie \\n(.$>=3 .ne \\$3
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..
.TH "GPSPROF" 1 "" "" ""
.SH NAME
gpsprof \- profile a GPS and gpsd, plotting latency information
.SH "SYNOPSIS"
.ad l
.hy 0
.HP 8
\fBgpsprof\fR [\-f\ \fIplot_type\fR] [\-m\ \fIthreshold\fR] [\-n\ \fIpacketcount\fR] [\-s\ \fIspeed\fR] [\-t\ \fItitle\fR] [\-h]
.ad
.hy
.SH "DESCRIPTION"
.PP
gpsprof measures the various latencies between a GPS and its client\&. It emits to standard output a GNUPLOT program that draws an illustrative graph\&. It can also be told to emit the raw profile data\&. The information it provides can be useful for establishing an upper bound on latency, and thus on position accuracy of a GPS in motion\&.
.PP
gpsprof uses instrumentation built into gpsd\&.
.PP
To display the graph, use \fBgnuplot\fR(1)\&. Thus, for example, to display the default spatial scatter plot, do this:
.nf
gpsprof | gnuplot \-persist
.fi
.SH "OPTIONS"
.PP
The \-f option sets the plot type\&. The X axis is samples (sentences with timestamps)\&. The Y axis is normally latency in seconds\&. Currently the following plot types are defined:
.TP
space
Generate a scattergram of fixes and plot a probable\-error circle\&. This data is only meaningful if the GPS is held stationary while gpsprof is running\&. This is the default\&.
.TP
uninstrumented
Plot total latency without instrumentation\&. Useful mainly as a check that the instrumentation is not producing significant distortion\&. It only plots times for sentences that contain fixes; staircase\-like artifacts in the plot are created when elapsed time from sentences without fixes is lumped in\&.
.TP
raw
Plot raw data\&.
.TP
split
Each sentence has its RS232 latency time colored differently\&.
.TP
cycle
Report on the set of sentences or packets emitted by the GPS, their send intervals, and the basic cycle time\&. (This report is plain text rather than a gnuplot script\&.)
.PP
The instrumented time plot conveys the following information:
.TP
RS232 time
Minimum time required to send the sentence from the GPS to gpsd\&. This is computed, not measured, and may be an underestimate\&.
.TP
Other line latency
The transmission latency between the GPS and gpsd not accounted for by RS232 time\&. Total line latency (the sum of this bar and RS232 time) is measured; it begins with the GPS sentence's timestamp and ends with a timestamp that gpsd generates at sentence\-reading time, before it is decoded\&.
.TP
Decode time
Elapsed time between sentence reception and the moment that gpsd ships the resulting update to the profiling client\&.
.TP
TCP/IP latency
Elapsed time between the moment that gpsd ships the update to the profiling client and the moment it is decoded and timestamped\&.
.PP
Because of RS232 buffering effects, the profiler sometimes generates reports of ridiculously high latencies right at the beginning of a session\&. The \-m option lets you set a latency threshold, in multiples of the cycle time, above which reports are discarded\&.
.PP
The \-n option sets the number of packets to sample\&. The default is 100\&.
.PP
The \-s option sets the baud rate\&. Note, this will only work if the chipset accepts a speed\-change command (SiRFstarII and SiRFstarIII support this feature)\&.
.PP
The \-t option sets a text string to be included in the plot title\&.
.PP
The \-h option makes gpsprof print a usage message and exit\&.
.SH "BUGS AND LIMITAIONS"
.PP
Probably overestimates TCP/IP latency somewhat, as that includes the Python interpreter's decode time\&. A C client would be faster\&.
.SH "SEE ALSO"
.PP
\fBgpsd\fR(8), \fBgps\fR(1), \fBlibgps\fR(3), \fBlibgpsd\fR(3), \fBgpsfake\fR(1), \fBgpsctl\fR(1), \fBgpscat\fR(1), \fBgnuplot\fR(1)\&.
.SH "AUTHOR"
.PP
Eric S\&. Raymond <esr@thyrsus\&.com>\&. There is a project page for gpsd here: \fIhttp://gpsd.berlios.de/\fR\&.
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