On 2015-02-07, Rob <nom...@example.com> wrote: > walter.preunin...@gmail.com <walter.preunin...@gmail.com> wrote: >> Ok, so these questions might be off the wall. >> >> Is there any reason why I could not share the PPS output of say, my u-blox 7 >> GPS module on multiple computers? Would it be good or bad to peer these 2 >> systems with each other? > > We do that in our project to synchronize several systems to the same > GPSDO accurately. We use surplus "video distribution amplifiers" because > they neatly fit into this (BNC In/Out, correct levels). There also exist > dedicated devices for this.
Actually the biggest problem with this is that it takes time to service a gps interrupt. Thus the first pps to come in, or be serviced delays the servicing of the second interrupt. Thus the accuracy of the two GPS is very ( 10-20usec) different. (I call that "very" because a gps pps is capable of 1usec accuracy, so a factor of 10 worse is alot) > >> On the opposite end of the spectrum, would it be reasonable to have a PPS >> output from said module on a computer, with a second PPS (from another GPS >> module) on that same computer? > > I have that here at home, but mainly for evaluation of the results of PPS. > (comparing kernel-PPS to usermode-PPS etc) > > I'm not sure if that would bring much, aside of redundancy. > >> The Chronodot/DS323X chips have 1 Hz sqaure wave output and they have +/- >> 2ppm. It seems that the ATOM clock driver could be used, but would it be >> better to write a driver specifically for this chip? > > A special driver is not required for PPS, but it may be that you need to > write software to read the serial data when it is not supported by any of > the drivers. I wrote one it a Datum GPSDO, and I just made an external > daemon that writes the time info into SHM to be read by ntpd using the SHM > driver, so I don't need to try to get my software merged with ntpd. > >> And a side question: Is it the GPS module that calculates when the PPS goes >> active? Is this signal compensated for the time it takes the signal from the >> sats in the module, or on the SV? > > Yes, the module calculates the position and time fix from the signals of > at least 4 SVs, and then outputs the PPS related to the calculated time. > > This is automatically compensated for the delay between the SV and the > calculated position. _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
