[time-nuts] Did a member of time-nuts buy this?
kb8tq at n1k.org
Sun Dec 7 09:16:08 EST 2014
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.
> On Dec 6, 2014, at 11:45 AM, Gregory Maxwell <gmaxwell at gmail.com> 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:
> 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
> 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
> 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.
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