[time-nuts] Do I have a defective thunderbolt?

[David Martindale]

I wonder if you are suffering from multipath.  The GPS receiver assumes that
the measured delay in the signal from each satellite is along the most
direct path from satellite to antenna.  If your antenna "sees" a signal that
has been reflected from a nearby building, and the reflected signal is
stronger than the direct-path signal, the receiver may end up measuring the
delay of the reflected signal instead of the direct one.  That makes the
antenna appear to be further from the satellite than it really is, and
introduces an error into the position and time calculations.

If all of the satellites participating in the position/time calculation are
observed via their direct signals, then the solution ought to be about the
same (the correct solution) no matter what mix of satellites are used, and
there shouldn't be much change when a particular satellite starts or stops
being used.  But if a particular satellite is measured via its reflected
signal, then it will add an error to the solution, and that error
contribution will start and end abruptly when that satellite starts and
stops being used.

Can you set the mask angle high enough that only satellites for which the
antenna has a clear view get used?  The receiver knows where each satellite
is in the sky so it can select and reject by height above the horizon.  And
when the receiver is operating in fixed-position timing mode, it only needs
to receive a single satellite to provide timing information.

     Dave

On Wed, Nov 17, 2010 at 8:21 AM, Geraldo Lino de Campos <
geraldo at decampos.net> wrote:

> I have a thunderbolt acquired from TAPR. Almost every time the number of
> satellites change, there is an abrupt change in the DAC voltage – as high
> as
> 1 mv, sometimes. See
> www.decampos.net\LH\LH1<http://www.decampos.net/LH/LH1>.png
> - a graph with time constant 200 and Damping 4. Using much higher values
> improve the situation, as can be seen in
> www.decampos.net\LH\LH2.png<http://www.decampos.net/LH/LH2.png>, time
> constant 1000 and damping 100 (note the change in the DAC scale), but
> the jumps are still present.
>
> Is this normal, or I have a defective unit?
>
> The antenna is located in a window, with several buildings nearby, so the
> change in the number of satellites is frequent.
> --
> ------------------------------------
> geraldo at decampos.net
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