In message <[EMAIL PROTECTED]>, Tom Clark writes: >Hi All, >The otherday I asked about the Jupoiter T receivers and Brooke mentioned the re >sults of his experience with it, I was curious what he meant when he spoke of t >he "clock zero-beat times (TvB calls them "hanging bridges")" .Brooke, can you >or Tom direct me to somewhere where I might get a better understanding of this? >Thanks; Rich
The mathematical explanation for the hanging bridge is that the PPS signal is superimposed with a function which is the modulus of the error between the receiver local clock and the 'real' GPS time. The modulus period is the frequency used to generate the PPS signal, it is the period of some frequency derived from the local receiver clock and typically in the 20...150 nsec range. As long as the receiver local clock is not close to perfect frequency, the resulting superimposed function more or less has a box-shaped histogram and it will cancel out by averaging over a few tens of samples. However, when the local receiver clock approaches perfect frequency, or even worse: holds the perfect frequency for long periods of time (if you're really lucky with the temperature), the function will take a random value inside the modulus range and stay there until the receiver clock drifts off frequency again. In this case there is no guarantee that any averaging period will be long enough to cancel out this error. The hanging bridge is the most common figure we see, but there are others that are even worse. The long and the short is, that unless you apply the "sawtooth correction", which is the receivers prediction of this noise function, your average, no matter the time period, can be up to the modulus interval wrong. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 [EMAIL PROTECTED] | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
