gpsprof
1
10 Feb 2005
gpsprof
profile a GPS and gpsd, plotting latency information
gpsprof
-f plot_type
-n packetcount
-o outfile
-r
-s speed
-t title
-T terminal type
-h
DESCRIPTION
gpsprof measures the various
latencies between a GPS and its client. It draws an illustrative
graph. It can also be told to save 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,
especially in combination with the static-precision reports from
gpsprobe1.
gpsprof uses instrumentation built
into gpsd that will only be present if
it was configured to support profiling.
Graphs are generated using
gnuplot1.
OPTIONS
The -f option sets the plot type. The X axis is samples
(sentences with timestamps). The Y axis is latency in seconds.
Currently the following plot types are defined:
raw
Plot raw data.
split
Plot filtered data. Sentences with an apparent latency larger
than 1 second are discarded. Each sentence has its RS232 latency
time colored differently. This is the default.
uninstrumented
Plot total latency without instrumentation. Useful mainly as a
check that the instrumentation is not producing significant distortion.
Each plot conveys the following information:
RS232 time
Minimum time required to send the sentence from the GPS to
gpsd. This is computed, not measured,
and may be an underestimate.
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.
Decode time
Elapsed time between sentence reception and the moment that
gpsd ships the resulting update to
the profiling client.
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.
The -n option sets the number of packets to sample. The default
is 100.
The -o option specifies the name of a file in which to dump the
profiling data;
gnuplot1
will read out from there. If this option is not specified, the data
will be written to a tempfile and discarded after the plot id made
The -s option sets the baud rate. Note, this will only work if
the chipset accepts a speed-change command (SiRF-II supports this
feature).
The -t option sets a text string to be included in the plot
title. Specifying the GPS make and model is a good use for it.
The -T option sets a terminal type for
gnuplot1
. It will normally default to "x11" at produce a display immediately,
but (for example) specifying "-T png" will instead cause a PNG image
of the graphic to be shipped to standard output.
The -h option makes gpsprof print
a usage message and exit.
BUGS AND LIMITAIONS
Probably overestimates TCP/IP latency somewhat, as that includes the
Python interpreter's decode time. A C client would be faster.
SEE ALSO
gpsd1
libgps3
libgpsd3
gpsprobe1
gnuplot1
AUTHOR
Eric S. Raymond esr@thyrsus.com. There is a
project page for gpsd here.