[time-nuts] New tide gauge uses GPS signals to measure sea level change
Jim Lux
jimlux at earthlink.net
Wed May 28 16:53:51 EDT 2014
On 5/28/14, 6:04 AM, Tom Holmes wrote:
> Which begs the question: just where the heck, exactly, is the center of the
> Earth given that it is in the 'middle' of a molten and dynamic core. Are the
> satellite orbits so stable and/or measurable around the center of
> gravitational pull that the location can be determined from that? Where is
> the reference point? Is Archimedes fulcrum for moving the planet nearby?
>
> This would seem to play into the accuracy of the location of the GPS
> satellites at any given time-hack.
>
> T
There's a group of people around the world who worry about this kind of
thing. There's a difference between the barycenter (the "center of
mass") and the Earth centered coordinate system orgins. In fact, there's
somewhat of a dispute going on about whether we should update from
WGS84: that coordinate system was defined to be the same (within
measurement precision) as the previous origin, but now "within
measurement precision" has gotten smaller, and there's a difference.
But that raises a question of whether we need to redo all the state
plane coordinate transformations, since they're in terms of WGS84.
As to whether the satellite orbits are stable/measurable, the answer is
most certainly yes. It was fairly precise measurements of satellite
signals in the 60s that led to the changing of the geoid from an
ellipsoid to something more "pear" shaped.
Precise measurements of satellite orbits and Doppler rates is how we
infer the internal structure of the outer planets. A typical
measurement uncertainty is a few cm and a few mm/sec for a probe at
Jupiter.
One generally does some sort of fit to a gravitational model with some
number of spherical harmonic terms (ten or twenty, as I recall).
The ephemeris for GPS takes this into account, so if you also account
for things like the ionosphere, you can determine your position to cm
accuracy (absolute). Note well that the GPS ephemeris doesn't contain
all the spherical harmonics and effects.. what they do is reload the
ephemeris periodically with fewer terms so that you can use it for a few
days (as I recall, for satelites in "reasonable" orbits (not too low),
the uncertainty in satellite position after a day is on the order of
meters.
Some satellites are particularly stable: QuikScat was in a special
orbit, and as I recall, after a year, it was only a few hundred meters
from where the year old ephemeris predicted it would be. The orbit
height (803 km) had changed maybe 1 km.
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