gpsprof.xml

gpsd, a popular GPS daemon.

<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE refentry PUBLIC 
   "-//OASIS//DTD DocBook XML V4.1.2//EN"
   "docbook/docbookx.dtd">
<refentry id='gpsprof.1'>
<refmeta>
<refentrytitle>gpsprof</refentrytitle>
<manvolnum>1</manvolnum>
<refmiscinfo class='date'>10 Feb 2005</refmiscinfo>
</refmeta>
<refnamediv id='name'>
<refname>gpsprof</refname>
<refpurpose>profile a GPS and gpsd, plotting latency information</refpurpose>
</refnamediv>
<refsynopsisdiv id='synopsis'>

<cmdsynopsis>
  <command>gpsprof</command>  
      <arg choice='opt'>-f <replaceable>plot_type</replaceable></arg>
      <arg choice='opt'>-m <replaceable>threshold</replaceable></arg>
      <arg choice='opt'>-n <replaceable>packetcount</replaceable></arg>
      <arg choice='opt'>-s <replaceable>speed</replaceable></arg>
      <arg choice='opt'>-t <replaceable>title</replaceable></arg>
      <arg choice='opt'>-h</arg>
</cmdsynopsis>
</refsynopsisdiv>

<refsect1 id='description'><title>DESCRIPTION</title>

<para><application>gpsprof</application> 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.</para>

<para><application>gpsprof</application> uses instrumentation built
into <application>gpsd</application>.</para>

<para>To display the graph, use
<citerefentry><refentrytitle>gnuplot</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
Thus, for example, to display the default spatial scatter plot, do
this:

<programlisting>
gpsprof | gnuplot -persist
</programlisting>
</para>

</refsect1>
<refsect1 id='options'><title>OPTIONS</title>

<para>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:</para>

<variablelist>
<varlistentry>
<term>space</term>
<listitem>
<para>Generate a scattergram of fixes and plot a probable-error
circle.  This data is only meaningful if the GPS is held stationary
while <application>gpsprof</application> is running. 
This is the default.</para>
<para></para>
</listitem>
</varlistentry>
<varlistentry>
<term>uninstrumented</term>
<listitem>
<para>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.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>raw</term>
<listitem>
<para>Plot raw data.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>split</term>
<listitem>
<para>Each sentence has its RS232 latency time colored differently.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>cycle</term>
<listitem>
<para>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.)</para>
</listitem>
</varlistentry>
</variablelist>

<para>The instrumented time plot conveys the following information:</para>

<variablelist>
<varlistentry>
<term>RS232 time</term>
<listitem>
<para>Minimum time required to send the sentence from the GPS to
<application>gpsd</application>. This is computed, not measured, 
and may be an underestimate.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Other line latency</term>
<listitem>
<para>The transmission latency between the GPS and 
<application>gpsd</application> 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 <application>gpsd</application> generates at 
sentence-reading time, before it is decoded.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Decode time</term>
<listitem>
<para>Elapsed time between sentence reception and the moment that
<application>gpsd</application> ships the resulting update to
the profiling client.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>TCP/IP latency</term>
<listitem>
<para>Elapsed time between the moment that
<application>gpsd</application> ships the update to
the profiling client and the moment it is decoded and timestamped.</para>
</listitem>
</varlistentry>
</variablelist>

<para>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.</para>

<para>The -n option sets the number of packets to sample.  The default
is 100.</para>

<para>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).</para>

<para>The -t option sets a text string to be included in the plot
title.</para>

<para>The -h option makes <application>gpsprof</application> print 
a usage message and exit.</para>

</refsect1>
<refsect1 id='bugs'><title>BUGS AND LIMITAIONS</title>

<para>Probably overestimates TCP/IP latency somewhat, as that includes the
Python interpreter's decode time. A C client would be faster.</para>

</refsect1>
<refsect1 id='see_also'><title>SEE ALSO</title>
<para>
<citerefentry><refentrytitle>gpsd</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>gps</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>libgps</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>libgpsd</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>gpsfake</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>gpsctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>gpscat</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>gnuplot</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
</para>
</refsect1>

<refsect1 id='maintainer'><title>AUTHOR</title> 

<para>Eric S. Raymond <email>[e-mail]</email>.  There is a
project page for <application>gpsd</application> <ulink
url="http://gpsd.berlios.de/">here</ulink>.</para>

</refsect1>

</refentry>