Bob Camp wrote: > Unless you are making a GPS receiver from scratch (which you might be), there > is a certain “trust factor” that comes into using a GPS for timing. Since you > can’t play with the firmware, you trust that the guy who wrote it did a good > job.
As compared to internet facing software embedded systems seem to be unusually fragile, consider this paper on GPS receivers with adversarial signals: http://www.andrew.cmu.edu/user/tnighswa/GPS_CCS.pdf And the trust with using GPS goes beyond the quality of the construction of the receiver: You're trusting the the GPS constellation is working and correct (see the recent GLONASS failure) and you're trusting that there aren't random jammers going by, you're trusting that there isn't someone in physical proximity manipulating the signal intentionally (see the paper above), or even just random truckers going by with jammers (there have been past threads on time-nuts) about this. IIRC the stated US policy with respect to GPS signal integrity is that it may be intentionally degraded (and can be degraded in a geographically targeted manner) for e.g. political/military objectives, so you trust that you won't be the target or collateral damage of any such degradation or that it won't be severe enough to effect you. GPS driven timing works amazing well under most conditions most of the time and at a very low cost. The trade-off is that you're taking more fringe risk and greater trust. I sometimes worry that we're building too much public infrastructure which is depends on a single system (or on space based timing in general, since Kessler syndrome, while unlikely, is a risk that exists) now that loran is gone in the US. Of course, the attractiveness of GPS makes this self-fulfilling: Solid, long living, CS primary frequency sources would probably be much less expensive of GPS didn't cover so much of the commercial demand for them. There are newer receivers (e.g. ublox m8) that are concurrent mult-gnss which might help, or maybe not: who knows what the receiver will do if one system starts emitting crap? I am not especially confident that the software in these systems is well baked under exceptional conditions. If you're working on things with no availability requirements, no real-time requirements (e.g. able to go download after-the-fact GPS reliability and precise ephemeris from NGS), and aren't doing anything where your timing is likely to be intentionally attacked, say for test-lab purposes... then these issues may be less of a consideration. In the context of time-nuts though many people are interested in the art and science of precise time/frequency for pretty much its own sake... and the driving need for the lowest phase noise or best adev at some window might just be because it's possible. In that light, the extremes of autonomy, reliability, avoidance of systemic risk, and surviving attacks are also interesting parameters that I find to be interesting to explore, and they're ones which perhaps have inadequate commercial attention on them these days since it seems people are often (a little too) willing to trust and then point fingers when things fail. [Or at least this is an area I personally find interesting ... I wrote this back in 2011 not so long after I started reading time-nuts: https://people.xiph.org/~greg/decentralized-time.txt before I knew common-view time-transfer was already a thing, and when I knew a little less nothing than the nothing I know now about time/frequency standards.] In terms of the 5061A at least some of the old surplus units floating around out there are "non-working" for silly reasons, e.g. left sitting for a long time, and they'll actually lock up fine if left with the ion pump running for a few days, or the OCXO put back on frequency, or the gain adjusted.... though I wouldn't spend $1k just to find out. I picked up a 5061B for basically shipping costs a while back and it was up and running reliably after some minor repairs... though the beam current is low and it likely doesn't have much life left in the tube. It's hard to deny how interesting and finely built these devices are, objects of techno-lust in their own right, even in surplus-and-maybe-not-reliable and impossibly-expensive-to-refurbish condition. As an actual lab tool-- rather than a science project, sadly, I do have to agree that you're better off with a GPSDO than a surplus CS unless you happen to get really lucky in the surplus gear lottery. Of course, none of this is mutually exclusive. It's possible and reasonable to have both. _______________________________________________ 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.
