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.
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