[time-nuts] Alternative to GPS?
brooke95482 at att.net
Wed Oct 6 12:47:54 EDT 2010
I'm in Ukiah, Calif. where one of the Lattitude observatories is
located. Finding the Lattitude was a much harder problem than finding
the Longitude and the five Lattitude observatories (all at 39 degrees 08
minutes North Latitude) were operational for almost a century. They
used a specialized Zenith Telescopes and later tried using Photographic
Zenith Tubes, but the accuracy was about the same. The Navy used the
PZT to set the time for quite awhile. Note that the accuracy of the
1899 ZT is orders of magnitude better than a modern Software Bisque
Paramount telescope mount.
I've been trying for decades to find a way to measure the Earth's period
with enough precision to see the daily variations (need better than 1 ms).
The "seeing" prevents getting the needed precision.
Can you say more about ways to improve the "seeing" limit?
> Neville Michie wrote:
>> How far could you get passing time around from amateur station to
>> with a two way handshake system that establishes the instantaneous delay
>> on the two way path and assumes a delay of half that value? A time
>> The stations would need their own short term clocks so they could keep
>> their own time between contacts, and somewhere you would need heros
>> with primary standards to synch the whole system.
>> You would not be able to find a good position, because you would not
>> know the
>> propagation mode.
>> cheers, Neville Michie
> You could look at the data from things like OTH-B radar (Over the
> Horizon-Backscatter) to get a feel for this.
> You can also estimate position by looking at direction of arrival from
> multiple stations (HF-DF).
> My gut feel is that you'd be doing real well to estimate your position
> within a few km.
> OTOH, if you already know your position (e.g. you want some sort of
> time/frequency distribution that persists after the cataclysm that
> wiped out WWV and GPS), and you don't move, then position accuracy
> isn't a big deal.
> Over a sufficiently long period of time (years), one could probably
> calibrate out a lot of the variability in propagation(but what would
> you calibrate against?).
> I still think that if you want "infrastructure-light" calibration of a
> local standard, astronomical means are going to be easiest.
> Harrison's H4 did about 10 ppm over a month and a half, just to put
> things in perspective. Astronomical methods were the competitor, and
> presumably are in the same accuracy range.
> Then, there's the meridian/zenith crossing telescope thing used for
> astronomical time hacks. (Can we presume that one has an almanac,
> even if WWV or GPS are unusable?) A standard surveying theodolite
> (like a Wild T2) can be read directly to 1 second of arc, and one
> could easily see a star cross the hairline in the 28x scope as several
> distinct events (touch hairline, centered, not touching). By hand, you
> could probably time that to less than a second, but probably not to
> 100 ms. However, I can easily conceive of a telescope and sensor that
> could do it (especially because you could track the centroid of the
> star image down to 0.1 pixel, and do some form of interpolation to get
> the "meridian crossing" to, perhaps, 10 milliseconds?
> Once a day gives you 0.1 ppm, roughly.
> Atmospheric seeing problems might be the limiting factor here, but
> they are random bubbles, and so you could probably do some sort of
> image processing (like astronomical blind deconvolution) to get a
> "reduced uncertainty" estimate of star position vs time.
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