gpsprof - profile a GPS and gpsd, plotting latency information
gpsprof [-f plot_type] [-m threshold] [-n packetcount] [-s speed]
[-t title] [-h]
gpsprof measures the various latencies between a GPS and its client. It
emits to standard output a GNUPLOT program that draws an illustrative
graph. It can also be told to emit the raw profile data. The
information 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
The -f option sets the plot type. The X axis is samples (sentences with
timestamps). The Y axis is normally latency in seconds. Currently the
following plot types are defined:
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.
Plot total latency without instrumentation. Useful mainly as a
check that the instrumentation is not producing significant
distortion. It only plots times for sentences that contain fixes;
staircase-like artifacts in the plot are created when elapsed time
from sentences without fixes is lumped in.
Plot raw data.
Each sentence has its RS232 latency time colored differently.
Report on the set of sentences or packets emitted by the GPS, their
send intervals, and the basic cycle time. (This report is plain
text rather than a gnuplot script.)
The instrumented time plot conveys the following information:
Time required to send the sentence from the GPS to gpsd. This
measured from the time of the last zero-length read before the
packet to when the packet sniffer recognizes a complete sentence,
so there is a small aountt of computational overhead mixed in.
Elapsed time between sentence reception and the moment that gpsd
ships the resulting update to the profiling client.
Elapsed time between the moment that gpsd ships the update to the
profiling client and the moment it is decoded and timestamped.
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 -s option sets the baud rate. Note, this will only work if the
chipset accepts a speed-change command (SiRFstarII and SiRFstarIII
support this feature).
The -t option sets a text string to be included in the plot title.
The -h option makes gpsprof print a usage message and exit.
gpsd(8), gps(1), libgps(3), libgpsd(3), gpsfake(1), gpsctl(1),
Eric S. Raymond firstname.lastname@example.org. There is a project page for gpsd