Hi Jim:
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.
http://www.prc68.com/I/UkiahObs.shtml
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).
http://www.prc68.com/I/StellarTime.shtml
The "seeing" prevents getting the needed precision.
Can you say more about ways to improve the "seeing" limit?
Have Fun,
Brooke Clarke
http://www.PRC68.com
jimlux wrote:
Neville Michie wrote:
Hi,
How far could you get passing time around from amateur station to
station
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
relay.
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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--
Have Fun,
Brooke Clarke
http://www.PRC68.com
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