NAME

       gpsprof - profile a GPS and gpsd, plotting latency information

SYNOPSIS

       gpsprof [-D debuglevel] [-d dumpfile] [-f plot_type] [-h] [-l logfile] [-m threshold]
               [-n samplecount] [-r] [-S subtitle] [-T terminal] [-t title]
               [[server[:port[:device]]]]

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.

OPTIONS

       The -f 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 fixs 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. This plot only works if gpsd was built with
           the timing (latency timing support) configure option enabled.

       instrumented
           Plot instrumented profile. Plots various components of the total latency between the
           GPS's fix time and when the client receives the fix. This plot only works if gpsd was
           built with the timing (latency timing support) configuration option enabled.

           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 axs 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.

       The -d option dumps the plot data, without attached gnuplot code, to a specified file for
       post-analysis.

       The -D sets debug level.

       The -h option makes gpsprof print a usage message and exit.

       The -l option dumps the raw JSON reports collected from the device to a specified file.

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

       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 -S option sets a text string to be included in the plot as a subtitle. This will be
       below the title.

       The -t option sets a text string to be the plot title. This will replace the default
       title.

       The -T option generates a terminal type setting into the gnuplot code. Typical usage is
       "-T png", or "-T pngcairo" telling gnuplot to write a PNG file. Without this option
       gnuplot will call its X11 display code.

       Different installations of gnuplot 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.

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, do this:

           gpsprof | gnuplot -persist

       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

SEE ALSO

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

AUTHOR

       Eric S. Raymond <esr@thyrsus.com>.