[time-nuts] Papers on timing for lunar laser ranging
kb8tq at n1k.org
Fri Jul 7 17:19:03 EDT 2017
> On Jul 7, 2017, at 4:51 PM, Tim Lister <listertim at gmail.com> wrote:
> Three recent papers from the APOLLO project showed up on my VoxCharta
> summary of arxiv.org which may be of interest to the time-nuts
> community. They are to do with improving the precision and absolute
> calibration of the lunar laser ranging project based at the 3.5m WIYN
> telescope at Apache Point.
> The links are:
> http://arxiv.org/abs/1707.00204v1 (Timing calibration of the APOLLO experiment)
> http://arxiv.org/abs/1706.09550v1 (An absolute calibration system for
> millimeter-accuracy APOLLO measurements)
> http://arxiv.org/abs/1706.09421v1 (APOLLO clock performance and normal
> point corrections)
> I was surprised by a few things: 1) that the size of GR effects on the
> Moon's position was at the 10 meter level (or ~0.01%
> measurable/testable level for the millimeter-level precision reached
> by APOLLO) and 2) the relatively coarse frequency steps the XL-DC
> GPSDO previously used was capable of. Does anyone know what sort of
> oscillator these used ? I thought most (good) OCXOs had/have a EFC at
> the 1e-13 level but maybe "back in the day" the availability of
> suitable DACs with a large dynamic range was more of an issue ?
Consider that in 1974, I could buy a nice new car for less than what a decent packaged
16 bit DAC cost. Go back into the 1960’s and you are up into the “several new cars”
range. Even for NASA projects cost did make it into the equation ….
A 1 ppm EFC and 16 bits gets you to 1.5x10^-11 if everything is perfect. In the typical
case you get to about 6x10^-11. Either way, that’s a long way from parts in 10^-13. It’s also
not the only driver for GPSDO’s doing parts in 10^-11 sort of performance out into the
hundreds of seconds range.
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