Come to think of it... it would be fun to see if there is a time-error correlation on location on sky. It would help to indicate multi-path errors and possibly be instructive to identify and mitigate multi-path sources. Could potentially also show the difference of antenna multi-path handling capabilities.
Magnus,
If you look at the recent plots for the cheap GPS receiver http://www.leapsecond.com/pages/MG1613S/ you can see some correlation between position error and nsv (number of sats) or hdop (which I just added). Or if nothing else you see dramatic periodicity in receiver operation over 24 hours, as one would expect with GPS. Most of the GPSDO we've been talking about over the years use a fixed time constant and the excitement and effort goes into characterizing what TC to select -- based on the GPS engine, the ADEV of the OCXO, the resolution of the phase comparator, the ambient temperature range, antenna quality, sky view quality, or other environmental factors, etc. But it seems to me this is sub-optimal. You can pick a nice TC for the best case to be sure. And when you loose GPS lock the GPSDO goes into holdover mode. The TC is at the heart of how much the 10 MHz output is influenced by the GPS receiver vs. the OCXO. In theory you slide the TC to 0 and all you get is a rough GPS 1PPS; you slide the TC to infinity and you get a fine free-running OCXO. In this view holdover mode is really nothing more than a temporary pushing of TC to a large (infinity) value. So how about running a GPSDO (e.g., TBolt) for many days and building up a profile of SVN, Az/El, S/N, NSV, HDOP, etc. and correlating any of that to timing error? You measure the timing error with accuracy by comparing with external (cesium) ref or get measure it approximately by monitoring the internal TIC value. Either way you get a profile of good times vs. bad times, good sats vs. better sats, good Az/El vs. poor Az/El. You can go one step further and based on packet 0x8F-A7, recompute the timing residuals and thus change the weight of certain SVN or Az/El combinations (multipath). Then, and here's the trick -- you dynamically change the TC according to your profile. The TC would be shorter during those minutes when you had a high quality GPS fix and the TC would be longer during those periods of lesser quality. But the TC is no longer a hardcoded constant, it's completely based on past history or current indicators. It will vary from rather short to very long depending on actual live conditions. Holdover would then not be a special case, it would simply be when the TC was made large. Furthermore, if the default TC was selected for an ideal room temperature but the learned/learning profile discovered that the temp is changing more rapidly than expected, it simply reduces the TC to accommodate this. Thus the GPSDO adapts itself to current conditions, whether they are SV related, or sky-view related, or environmental, or changes in the OCXO, or OCXO oven, etc. This is a general case of those FTS cesium standards fitted with accelerometers; as I understand it, when they sensed sudden acceleration (shock) they simply shortened the PLL TC so the OCXO would remain locked to the Cs signal. When all was settled, the TC went long again. Much better than the old STC/LTC (short/long time constant) switch on older 5061A. Anyway, it seems this would work on a TBolt. I'm not sure how many dB of stability you'd gain when all was said and done. /tvb _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
