I added the ability for Lady Heather to measure a plot the difference of the
arrival times of each timing message (actually the time when Lady Heather
receives the last byte of the timing message from the operating system). The
end-of-message arrival time is time stamped to nominally nanosecond resolution
using either the Windows QueryPerformanceCounter() value or the Linux
nanosecond res clock function... (actual resolutions are system depenent). I
measured several different receivers in both NMEA mode and their native binary
mode.
A few generalizations: most receivers get the time messages out within a
window less than 50 milliseconds wide with a standard deviation of less than 10
milliseconds. Timing receivers tend to do a better job of it. The Trimble
Thunderbolt and Resolution-T receivers are particularly good (5 msec window, 1
msec standard deviation). The Z3801A is also rather good (10 msec window, 1.2
msec standard deviation).
There is usually not much difference the performance of the NMEA messages and
the native binary messages. The Navspark Venus based receivers have the
greatest differences.. their NMEA messages are about twice as consistent than
the binary messages. Occasionally you see a minor hicccup in the message time
when the operating system goes off and does something else (particularly like
when waking up from a screen saver mode).
The number of messages the receiver sends during each timing interval does not
seem to make much difference. I have a Ublox 8M tracking 20+ satellites and
sending over a dozen NMEA messages each second. It gets the timing message out
within a 30 msec window, 6.5 msec standard deviation.
Surprisingly, the communications channel type is not that important. I've
seen little difference between hardware serial ports and a USB / RS-232
adapter. Even running a TCP/IP link between Dallas and Seattle gave
surprisingly consistent messages (with a couple of spike to +/- 100 millisecond
or so differences. All of the tested receivers were running at 9600 baud,
the Navspark at 115,200, the Z3801A at 19,200.
Attached is a plot of the Trimble Thunderbolt running on a 3 GHz, single core
Windows XP box.
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