10 Feb 2005
gpsprof
1
The GPSD Project
GPSD Documentation
gpsprof
profile a GPS and gpsd, plotting latency information
gpsprof
-f plot_type
-m threshold
-n packetcount
-t title
-T terminal
-d dumpfile
-D debuglevel
-h
DESCRIPTION
gpsprof performs accuracy and latency
profiling onm a GPS. 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.
gpsprof uses instrumentation built
into gpsd.
To display the graph, use
gnuplot1.
Thus, for example, to display the default spatial scatter plot, do
this:
gpsprof | gnuplot -persist
To generate an image file:
gpsprof -T png | gnuplot >image.png
OPTIONS
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:
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.
uninstrumented
Plot total latency without instrumentation. Useful mainly as a
check that the instrumentation is not producing significant
distortion. It only plots times for reports that contain fixes;
staircase-like artifacts in the plot are created when elapsed time
from reports without fixes is lumped in.
instrumented
Plot instrumented profile.
Plots various components of the total latency between the GPS's fix time
fix and when the client receives the fix.
For purposes of the description, below, start-of-reporting-cycle
(SORC) is when a device's reporting cycle begins. This time is
detected by watching to see when data availability follows a long
enough amount of quiet time that we can be sure we've seen the gap at
the end of the sensor's previous report-transmission cycle. Detecting
this gap requires a device running at 9600bps or faster.
Similarly, EORC is end-of-reporting-cycle; when the daemon has
seen the last sentence it needs in the reporting cycle and ready to ship
a fix to the client.
The components of the instrumented plot are as follows:
Fix latency
Delta between GPS time and SORC.
RS232 time
RS232 transmission time for data shipped during the cycle
(computed from character volume and baud rate).
Analysis time
EORC, minus SORC, minus RS232 time. The amount of real time the daemon
spent on computation rather than I/O.
Reception time
Shipping time from
the daemon to when it was received by gpsprof.
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.
The -n option sets the number of packets to sample. The default
is 100.
The -t option sets a text string to be included in the plot
title.
The -T option generates a terminal type setting into the gnuplot code.
Typical usage is "-T png" telling gnuplot to write a PNG file. Without
this option gnuplot will call its X11 display code.
The -d option dumps the raw plot data to a specified file for
pos-analysis.
The -h option makes gpsprof print
a usage message and exit.
The -D sets debug level.
SEE ALSO
gpsd8,
gps1,
libgps3,
libgpsd3,
gpsfake1,
gpsctl1,
gpscat1,
gnuplot1.
AUTHOR
Eric S. Raymond esr@thyrsus.com.