[time-nuts] Papers on timing for lunar laser ranging

Tim Lister listertim at gmail.com
Sat Jul 8 14:42:44 EDT 2017

On Fri, Jul 7, 2017 at 4:14 PM, jimlux <jimlux at earthlink.net> wrote:
> On 7/7/17 3:14 PM, Hal Murray wrote:
>> kb8tq at n1k.org said:
>>> 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 ….
> Note that these papers are talking about optical ranging to the reflectors
> left on the moon by the Apollo missions, but the actual work was being done
> recently (e.g. it's a Microsemi 5071 Cesium clock)

Right, this is the "third generation" of laser ranging. APOLLO started
in 2007, so given
the usual delays in obtaining grant funding and purchasing, the tech
(pre-upgrade to
the 5071A) is going to be early 2000s tech.

Forgive the ignorance, but why is there a large disparity between ADC
and DAC capabilities ?
For example, Linear Technology sell a 24 bit ADC for ~$7 but an 18 bit
DAC is $30-50...

>> When was the first GPSDO shipped as a commercial product?
> An interesting question - at least 20 years ago - XL-DC manual, Rev E, from
> 1997
> http://glacier.lbl.gov/gtp/DOM/Support/xl-dc-manual.pdf

I also found it interesting that the paper says that the GPSDO uses a
2000 sec Kalman
filter. I've heard of Kalman filters being used for GPS navigation but
not in timing use, although
I gather things like Thunderbolts use a ~1000 sec loop constant - is
this the same form of
filtering or have different forms of filtering become more popular and
Kalman filtering is no longer
used ?

>> There is an interesting tradeoff in GPSDO design.  With a specific DAC,
>> you
>> can get finer steps if you reduce the tuning range.  Has anybody built one
>> with a reduced range and a knob on the side to adjust the center point of
>> that range?  You would have to adjust that knob occasionally as the
>> crystal
>> you are tuning drifted.
> My mid 2000s 10 MHz OCXOs from Wenzel have both EFC and a manual adjustment
> of some sort (I'm not sure what's under the little cap on the side.. a
> trimmer cap or something?)

Right. Apparently the DAC values have changed by 3500 over the ~11
years, which given the
1.2e-11 DAC steps would give an accumulated change of 4.2e-8, agreeing
with the typical and
quoted ~1e-11/day drift/aging for a good OCXO.


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