GPSPROF(1)

NAME
gpsprof - profile a GPS and gpsd, plotting latency information

SYNOPSIS

gpsprof [OPTIONS] [server[:port[:device]]]

gpsprof -h

gpsprof -V

DESCRIPTION

gpsprof performs accuracy, latency, skyview, 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 characterizing position accuracy of a GPS.

gpsprof uses instrumentation built into gpsd. It can read data from a local or remote running gpsd. Or it can read data from a saved logfile.

gpsprof is designed to be lightweight and use minimal host resources. No graphics subsystem needs to be installed on the host running gpsprof. Simply copy the resultant plot file to another host to be rendered with gnuplot(1).

gpsprof does not require root privileges, but it will run fine as root.

OPTIONS

The -f, --formatter option sets the plot type. Currently the following plot types are defined:

space
    Generate a scatterplot of fixes and plot probable error circles. This data is only meaningful if the GPS is held stationary while gpsprof is running. Various statistics about the fixes are listed at the bottom. This is the default plot type.

polar
    Generate a heat map of reported satellite Signal to Noise Ratio (SNR) using polar coordinates. A colored dot is plotted for each satellite seen by the GPS. The color of dot corresponds to the SNR of the satellite. The dots are plotted by azimuth and elevation. North, azimuth 0 degrees, is at the top of the plot. Directly overhead, elevation of 90 degrees, is plotted at the center. Useful for analyzing the quality of the skyview as seen by the GPS.

polarunused
    Similar to the polar plot, but only unused satellites are plotted. Useful for seeing which parts of the antenna skyview are obstructed, degraded, below the GPS elevation mask, or otherwise rejected.

polarused
    Similar to the polar plot, but only satellites used to compute fixes are plotted. Useful for seeing which parts of the antenna skyview are being used in fixes.

time
    Plot delta of system clock (NTP corrected time) against GPS time as reported in PPS messages. The X axis is sample time in seconds from the start of the plot. The Y axis is the system clock delta from GPS time.

instrumented
    Plot instrumented profile. Plots various components of the total latency between the GPS’s fix time 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.

uninstrumented
    Plot total latency without instrumentation. Useful mainly as a check that the instrumentation is not producing significant distortion. The X axis is sample time in seconds from the start of the plot. The Y axis is latency in seconds. 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.

-?, -h, --help
    Print a usage message and exit.

-d FILE, --dumpfile FILE
    Dump the plot data, without attached gnuplot(1) code, to a specified file for post-analysis.

-d LVL, --debug LVL
    Sets debug level.

-l FILE, --logfile FILE
    Dump the raw JSON reports collected from the device to the specified FILE.

-n SEC, --wait SEC
    Sets the number of seconds to sample. The default is 100. Most GPS are configured to emit one fix per second, so 100 samples would then span 100 seconds.

-r, --redo
    Replot from a JSON logfile (such as -l, logfile produces) on standard input. Both -n, --wait and -l, --logfile options are ignored when this one is selected.

-S STR, --subtitle STR
    Sets a text string to be included in the plot as a subtitle. This will be below the title.

-t STR, --title STR
    Sets a text string to be the plot title. This will replace the default title.

-T TERM, --terminal TERM
    Specify the terminal type setting in the gnuplot(1) code. Typical usage is "-T png", or "-T pngcairo" telling gnuplot(1) to write a PNG file. The default terminal is "x11".

    Different installations of gnuplot(1) will support different terminal types. Different terminal types may work better for you than other ones. "-T png" will generate PNG images. Use "-T jpeg" to generate JPEG images. "-T pngcairo" often works best, but is not supported by some distributions. The same terminal type may work very differently on different distributions.

    To see which terminal types your copy of gnuplot(1) supports:

    gnuplot -e "set terminal"

ARGUMENTS

By default, clients collect data from the local gpsd daemon running on localhost, using the default GPSD port 2947. The optional argument to any client may override this behavior: [server[:port[:device]]]

For further explanation, and examples, see the ARGUMENTS section in the gps(1) man page

SIGNALS

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.

EXAMPLES

To display the graph, use gnuplot(1) . Thus, for example, to display the default spatial scatter plot on your x11 display, do this:

    gpsprof | gnuplot -p

To generate an image file:

    gpsprof -T png | gnuplot > image.png

To generate a polar plot, and save the GPS data for further plots:

    gpsprof -f polar -T jpeg -l polar.json | gnuplot > polar.png

Then to make the matching polarused and polarunused plots and pngs from the just saved the GPS data:

    gpsprof -f polarused -T jpeg -r < polar.json > polarused.plot
    gnuplot < polarused.plot > polarused.png
    gpsprof -f polarunused -T jpeg -r < polar.json > polarunused.plot
    gnuplot < polarunused.plot  > polarunused.png

You can split the pieces up, so you do not need to run the entire chain at once. To allow tweaking settings without recollecting all the data. Like this:

    gpspipe -w -x 3600 ::/dev/ttyS0 > MY.raw
    gpsdecode  < MY.raw > MY.json
    gpsprof -r -T pngcairo -t "MY Title" < MY.json > MY.plt
    gnuplot MY.plt > MY.png
    display MY.png

The gpspipe saves one hour of raw data from the local gpsd device /dev/ttyS0 into MY.raw. It will take one hour to complete.

The gpsdecode converts the raw data in MY.raw into a gpsd JSON file called MY.json.

The gpsprof reads MY.json and creates a gnuplot program in MY.plt.

The gnuplot executes the program in MY.plt and creates the image file MY.png.

The display program paints MY.png on your desktop.

RETURN VALUES

0
    on success.

1
    on failure

SEE ALSO

gpsd(8), display(1), gnuplot(1), gpsctl(1), gps(1), libgps(3), libgpsmm(3), gpsprof(1), gpsfake(1).

RESOURCES

Project web site:

COPYING

This file is Copyright 2013 by the GPSD project
SPDX-License-Identifier: BSD-2-clause

AUTHOR

Eric S. Raymond