[time-nuts] Thunderbolt zero baseline

bg at lysator.liu.se bg at lysator.liu.se
Tue Aug 24 19:46:37 UTC 2010


Dear tbolt-nuts,

A friend and I have played a little with zero baseline tests and
Thunderbolts.

Have some very early results, on measurements from two identical TAPR
Tbolts. These were connected to the same antenna. Its a good antenna, with
a rather open skyview. One receiver was hooked on a 4-port passive GPS
splitter that is connected to the antenna with some 15m of LMR400. The
other tbolt got signals from a downstream 8-port HP L1-splitter.

Both receivers had identical configuration. Non-default settings with
tboltmon was,

    message 0x5A enabled         (incl. pseudorange and doppler)
    doppler smoothed pseudorange (not relevant for todays discussion)
    synchronized measurements    (what does this do exactly?)

Both receivers have done site surveys. Surveyed positions not identical.
Position diff TBD.

Raw serial data was collected with two sessions of 'TsipNmeaDemo', each of
the Tbolts connected with USB-serial cables. Some 20 minutes of continous
data was collected from the receivers. Message 0x5a was decoded.

In the first plot shows (tbolt1.dopper - tbolt2.doppler) for all 8 tracked
satellites. Results are scaled from Hz at L1 to m/s. A few satellits have
a std dev around 1cm/s, most around 5mm/s. This equals 8 measurements a
second  with a noise of 1/30ns or better.

     http://www.lysator.liu.se/~bg/timenuts/sd_doppler.png

Second plot shows a simple integration (summation) of the above
velocities. The worst satellite moves around 90cm (ca 3ns) in 20 minutes.

     http://www.lysator.liu.se/~bg/timenuts/csum_sd_doppler.png

Any comments? questions?

Has anyone else played with 0x5a on their Thunderbolts?

--

    Björn





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