[time-nuts] tracking position & orientation

[jimlux]

On 11/22/19 10:17 AM, Eric Scace wrote:
>     Thank you, everyone, for your enthusiastic guidance and observations to my quirky question.
> 
>     Quite a few mentioned the difficulties in measuring rotation over a short baseline. In response to the question of “is there another measurement point 10 miles away”, the quick answer is yes: NIST is on the opposite side of Boulder City from me.
> 
>     The question of a sturdy — i.e., dimensionally stable — antenna mount brought to mind something I learned during my home inspection. My house is built at the foothills of the Front Range in North Boulder. Soils there include a kind of clay that swells significantly when exposed to water. As a result, house foundations are build on a system of screw pilings that go down to bedroom. The house’s cellar floor is a concrete slab poured on corrugated steel plates supported by cross-web girders that sit on these pilings. The cellar walls (to which the higher-precision pendulum clocks are mounted) are poured concrete that also rests on these pilings. It seems the house foundation is probably a better reference point for antennas than something sitting on the ground at the corner of my (tiny) lot.


> 
>     Of course, my house and the neighbors’ houses are obstructions to signals for an antenna attached directly to the foundation walls.
> 
Interesting problem.. You could put a steel (or Invar?) pipe down the 
side of the house to hit the foundation, but then as the house moves 
from side to side (wind, temperature, humidity) it would move the 
antenna. Some form of rigid spaceframe around the house would result in 
aesthetic problems. And drilling a hole all the way from the roof to the 
foundation is probably a non-starter (although, in my house, there is a 
sort of utility chase that does go from top of 2nd floor to 1st floor), 
but there's probably something in the way (HVAC vents, most likely).

I'm not that much of a GPS nut (yet) (because you know, living in 
southern California, there's not much in the way of geodetic measurement 
infrastructure I can leverage <grin> - Self reliance - at the end of 
civilization as we know it, at least you'd be able to measure 
continental drift without depending on others...)



> 
>     For millimeter-scale position determination, this sounds like a more difficult situation. The house is generally wood framing with some structural steel elements (not in useful locations). Position measurements would contain noise from the diurnal/seasonal changes of the house framing. Maybe that could be averaged out?

Maybe, but if the uncertainties are big enough over time, then no amount 
of averaging helps.