[time-nuts] Timing Distribution in Mountainous Terrain

[Bob Camp]

Hi

A lunar setup would only give you data for part of the day. You would relax the flywheel requirement. Net result likely would still be a maser / cesium combo at each site. Not real clear how you would model clouds and weather into the availability equation. Some of the things that 100% take out GPS also kick up a LOT of dust. There is a point where the dust locks up all the turbines and there's nothing to track....

Bob



On Sep 10, 2010, at 2:42 PM, jimlux <jimlux at earthlink.net> wrote:

> Stanley Reynolds wrote:
>> On the crazy side another common view object is the lunar laser ranging retroreflector array. Has been improvements in cost of lasers and telescopes in the past 41 years and it doesn't appear to be headed for shutdown anytime soon.
>> 
> 
> Hmm.. the SNR isn't all that huge on the echo. The target is say, 1 square meter, at a distance of 300,000 km.
> 
> The beam divergence coming back is about the same as the outbound (that is, in order to cover 300km on earth, you need to have a spot on the moon about 300km in diameter).. So the laser power will be spread out by a factor of 71E9.. (about 110 dB). The power reflected back, if intercepted by a 1 square meter aperture will have the same "loss"  for a round trip loss of around 220dB.
> 
> Radiate 10 Watts, and you're seeing -210dBW coming back.. That's going to be tough to detect. Just how many photons/second is that? And what's the dark current/shot noise of your detector (a PM tube, presumably)
> 
> The first experiments with the Apollo 11 reflector were done with the 3.1m antenna at the Lick observatory.. that's a pretty big telescope. The work at the McDonald observatory used almost as big a telescope, but did provide ranging to 1 cm, which is 0.03 ns...
> 
> If we make the assumption that a degradation in position accuracy to 10m requires 1/10,000 the power.. that would imply you could use a telescope with 1/100th the area, or about 1/10th the diameter.. that's in the reasonable range..30-45 cm aperture is an off the shelf commodity item.
> 
> http://www.physics.ucsd.edu/~tmurphy/apollo/apollo.html talks about "few picoseconds"...
> 
> they use a pulsed laser with a few watts average power 115mJ/pulse at 20Hz with a 3.5 meter telescope making a 1.8 km spot on the moon.  For common view sync, you need to have the spot bigger, as mentioned above, which is nice, because it means you don't need as big a telescope to collimate the beam.
> (interestingly, they use a XL-DC GPS disciplined oscillator as their reference)
> 
> 
> In other parts of that site, they say that getting cm scale precision requires about 10 photons..
> 
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