1GPSPROF(1) GPSD Documentation GPSPROF(1)
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6 gpsprof - profile a GPS and gpsd, plotting latency information
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9 gpsprof [-f plot_type] [-m threshold] [-n packetcount] [-t title]
10 [-T terminal] [-d dumpfile] [-l logfile] [-r] [-D debuglevel]
11 [-h] [[server[:port[:device]]]]
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14 gpsprof performs accuracy, latency, and time drift profiling on a GPS.
15 It emits to standard output a GNUPLOT program that draws one of several
16 illustrative graphs. It can also be told to emit the raw profile data.
17 Information from the default spatial plot it provides can be useful for
19 establishing an upper bound on latency, and thus on position accuracy
20 of a GPS in motion.
21 gpsprof uses instrumentation built into gpsd.
23 To display the graph, use gnuplot(1). Thus, for example, to display the
25 default spatial scatter plot, do this:
26 gpsprof | gnuplot -persist
28 To generate an image file:
30 gpsprof -T png | gnuplot >image.png
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35 The -f option sets the plot type. The X axis is samples (either
36 sentences with timestamps or PPS time drift messages). The Y axis is
37 normally latency in seconds, except for the spatial plot. Currently the
38 following plot types are defined:
39 space
41 Generate a scattergram of fixes and plot a probable-error circle.
42 This data is only meaningful if the GPS is held stationary while
43 gpsprof is running. This is the default.
44 time
46 Plot delta of system clock (NTP corrected time) against GPS time as
47 reported in PPS messages.
48 uninstrumented
50 Plot total latency without instrumentation. Useful mainly as a
51 check that the instrumentation is not producing significant
52 distortion. It only plots times for reports that contain fixes;
53 staircase-like artifacts in the plot are created when elapsed time
54 from reports without fixes is lumped in.
55 instrumented
57 Plot instrumented profile. Plots various components of the total
58 latency between the GPS's fix time fix and when the client receives
59 the fix.
60 For purposes of the description, below, start-of-reporting-cycle (SORC)
62 is when a device's reporting cycle begins. This time is detected by
63 watching to see when data availability follows a long enough amount of
64 quiet time that we can be sure we've seen the gap at the end of the
65 sensor's previous report-transmission cycle. Detecting this gap
66 requires a device running at 9600bps or faster.
67 Similarly, EORC is end-of-reporting-cycle; when the daemon has seen the
69 last sentence it needs in the reporting cycle and ready to ship a fix
70 to the client.
71 The components of the instrumented plot are as follows:
73 Fix latency
75 Delta between GPS time and SORC.
76 RS232 time
78 RS232 transmission time for data shipped during the cycle (computed
79 from character volume and baud rate).
80 Analysis time
82 EORC, minus SORC, minus RS232 time. The amount of real time the
83 daemon spent on computation rather than I/O.
84 Reception time
86 Shipping time from the daemon to when it was received by gpsprof.
87 Because of RS232 buffering effects, the profiler sometimes generates
89 reports of ridiculously high latencies right at the beginning of a
90 session. The -m option lets you set a latency threshold, in multiples
91 of the cycle time, above which reports are discarded.
92 The -n option sets the number of packets to sample. The default is 100.
94 The -t option sets a text string to be included in the plot title.
96 The -T option generates a terminal type setting into the gnuplot code.
98 Typical usage is "-T png" telling gnuplot to write a PNG file. Without
99 this option gnuplot will call its X11 display code.
100 The -d option dumps the plot data, without attached gnuplot code, to a
102 specified file for post-analysis.
103 The -l option dumps the raw JSON reports collected from the device to a
105 specified file.
106 The -r option replots from a JSON logfile (such as -l produces) on
108 standard input. Both -n and -l options are ignored when this one is
109 selected.
110 The -h option makes gpsprof print a usage message and exit.
112 The -D sets debug level.
114 Sending SIGUSR1 to a running instance causes it to write a completion
116 message to standard error and resume processing. The first number in
117 the startup message is the process ID to signal.
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120 gpsd(8), gps(1), libgps(3), libgpsmm(3), gpsfake(1), gpsctl(1),
121 gpscat(1), gnuplot(1).
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124 Eric S. Raymond <esr@thyrsus.com>.
125The GPSD Project 10 Feb 2005 GPSPROF(1)