I some tests I was fortunate to be a part of the eLloran system was able to deliver very accurate time even in buildings. I won't quote numbers as it was 5 years ago. But suspect the details are online. The extra data channel allows for the transmission of various corrections. Regards Paul
On Thu, Aug 6, 2020 at 9:52 PM Forrest Christian (List Account) < [email protected]> wrote: > I probably need to clarify where I'm coming from in relation to my previous > message. > > I have a fair bit of background in dealing with using GPS clock sources for > synchronization at communication sites. Many of these sites are lucky to > have a rubidium oscillator in place for holdover, although some do (usually > the larger ones). And we're talking about thousands and thousands and > thousands of these sites. > > Where I was coming from is that at these sites, GPS can be a challenge - > it's in a narrow band, very low signal, and, at least from my worldview, it > seems like GPS interference is becoming more prevalent instead of less. > As a result, whether terrestrial or via satellite, it would be nice to > have a second alternative in a different band where one could obtain a > reasonably aligned 1PPS (+-1uSish). > > I'm hopeful that eLORAN will result in this result. I'm also curious > about the GPS L5 signal once it becomes operational since that is far > enough away frequency wise that one would hope that it wouldn't be affected > by the same interference source. > > On Thu, Aug 6, 2020 at 6:19 PM jimlux <[email protected]> wrote: > > > On 8/6/20 4:28 PM, Forrest Christian (List Account) wrote: > > > If you look at generally-available GNSS PNT solutions, you'll find a > few > > > failure modes: > > > > > > 1) Loss of a satellite (or two). This is why the constellations have > > more > > > satellites than is strictly necessary, so not a big deal. > > > > > > 2) Loss of control/failure in the control system/constellation wide > > > software failure, aka the recent Galileo failure. This is why you > have > > > multiple GNSS constellations. > > > > > > 3) Ground based interference (jamming, spoofing), etc. This is why > you > > > need a terrestrial backup, which doesn't really exist. > > > > > > For timing, I wouldn't be opposed to someone flying (or adding a > payload > > > to) a couple of geostationary satellites which live in a separate band > > from > > > GNSS. It would be interesting to be able to put up a small satellite > > dish > > > and get a highly reliable and hard to interfere with timing alternative > > to > > > GNSS. I know there are two way time transfer options out there, I'm > > more > > > thinking basically a fixed-location cesium clock in the sky. > > > > > > > Well, the GPS folks found that a Rb works better than a Cs, and both > > need ground monitoring and updating. > > > > One could rent a Single Channel Per Carrier transponder slot on a GEO > > satellite, feed a carrier derived from your ensemble of clocks to the > > uplink, and there you go. > > > > My google-fu is failing and I can't find even a rough cost for such a > > service. Maybe something like $50k/month? $600k/year. > > > > That 600k would probably buy you *one* space qualified Rb oscillator of > > "good enough" performance, maybe. USOs like used on GRAIL were > > $1M/each sort of items in qty 4. And then you need some TWTAs to amplify > > the signals for transmission - those are also pretty pricey. > > > > And then the launch cost.. I happen to know (because I just bought it at > > work) that you can push about 100-120kg to a GEO+1000km orbit for right > > around $10M. Rocketlabs might do 20-30kg to a similar orbit for half > that. > > > > > > All in all, I suspect that there are better uses of the $50M it would > > cost - You could buy a LOT of Cesium clocks for terrestrial use. > > > > If you're willing to do CSAC level performance, and you're willing to > > have it be in LEO, so you see it a couple times a day for 20 minutes, > > and you're willing to do some design and fab in your garage - under $5M. > > > > Look up CHOMPTT - CSAC and optical links. > > > > > > > https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=4407&context=smallsat > > > > > > > > > > _______________________________________________ > > time-nuts mailing list -- [email protected] > > To unsubscribe, go to > > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > > and follow the instructions there. > > > > > -- > - Forrest > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
