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 < gera...@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. > -- > ------------------------------------ > gera...@decampos.net > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.