[time-nuts] Thunderbolt zero baseline

Bob Camp lists at rtty.us
Tue Aug 24 20:15:28 UTC 2010


Time data reported in the same manner supports the same "2 to 4 ns" over similar time periods.


On Aug 24, 2010, at 3:46 PM, bg at lysator.liu.se wrote:

> 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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