GPSPROF(1) 10 Feb 2005 GPSPROF(1)
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:
space
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.
uninstrumented
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.
raw
Plot raw data.
split
Each sentence has its RS232 latency time colored differently.
cycle
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 plots conveys the following information:
RS232 time
Minimum time required to send the sentence from the GPS to gpsd.
This is computed, not measured, and may be an underestimate.
Other line latency
The transmission latency between the GPS and gpsd not accounted for
by RS232 time. Total line latency (the sum of this bar and RS232
time) is measured; it begins with the GPS sentence's timestamp and
ends with a timestamp that gpsd generates at sentence-reading time,
before it is decoded.
Decode time
Elapsed time between sentence reception and the moment that gpsd
ships the resulting update to the profiling client.
TCP/IP latency
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 (SiRF-II supports 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.
Probably overestimates TCP/IP latency somewhat, as that includes the
Python interpreter's decode time. A C client would be faster.
gpsd(1), gps(1), libgps(3), libgpsd(3), gnuplot(1).
Eric S. Raymond <esr [AT] thyrsus.com>. There is a project page for gpsd
[1]here.
1. here
http://gpsd.berlios.de/
10 Feb 2005 10/08/2006 GPSPROF(1)