Hi For a commercial system, yes things like jammers are indeed an issue. For a basement lab that’s not as big of a concern. In the context of “do I buy a $1,000 Cs or not” the risks on the Cs are much higher (by several orders of magnitude) than the signal integrity risks on GPS.
Bob > On Dec 6, 2014, at 11:45 AM, Gregory Maxwell <[email protected]> wrote: > > 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. _______________________________________________ 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.
