gpsprof(1)
NAME
gpsprof - profile a GPS and gpsd, plotting latency information
SYNOPSIS
gpsprof [-f plot_type] [-m threshold] [-n packetcount] [-t title]
[-T terminal] [-d dumpfile] [-l logfile] [-r] [-D debuglevel]
[-h] [[server[:port[:device]]]]
DESCRIPTION
gpsprof performs accuracy, latency, and time drift profiling on a GPS.
It emits to standard output a GNUPLOT program that draws one of several
illustrative graphs. It can also be told to emit the raw profile data.
Information from the default spatial plot 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 gnuplot(1). 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 (either
sentences with timestamps or PPS time drift messages). The Y axis is
normally latency in seconds, except for the spatial plot. 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.
time
Plot delta of system clock (NTP corrected time) against GPS time as
reported in PPS messages.
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 plot data, without attached gnuplot code, to a
specified file for post-analysis.
The -l option dumps the raw JSON reports collected from the device to a
specified file.
The -r option replots from a JSON logfile (such as -l produces) on
standard input. Both -n and -l options are ignored when this one is
selected.
The -h option makes gpsprof print a usage message and exit.
The -D sets debug level.
Sending SIGUSR1 to a running instance causes it to write a completion
message to standard error and resume processing. The first number in
the startup message is the process ID to signal.
SEE ALSO
gpsd(8), gps(1), libgps(3), libgpsd(3), gpsfake(1), gpsctl(1), gpscat(1), gnuplot(1).
AUTHOR
Eric S. Raymond <[email protected]>.
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