gpsprof - profile a GPS and gpsd, plotting latency information
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]]]]
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.
The -f option sets the plot type. Currently the following plot types are defined:
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.
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.
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.
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.
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.
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:
Delta between GPS time and SORC.
RS232 transmission time for data shipped during the cycle (computed from character volume and baud rate).
EORC, minus SORC, minus RS232 time. The amount of real time the daemon spent on computation rather than I/O.
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.
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.
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.
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:
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
Eric S. Raymond <esr [AT] thyrsus.com>.