[time-nuts] New tide gauge uses GPS signals to measure sea level change

Tom Holmes tholmes at woh.rr.com
Wed May 28 21:11:17 UTC 2014

Thanks Jim.

So if, just for fun since this is time-nuts after all, I wanted to make a
similar measurement in my back yard here in the relatively stable Ohio,
would I be able rig something up to monitor the position changes? Obviously
a lot of averaging  of GPS position data would be needed but I'm not sure my
Z3801 or any of my navigation receivers have the necessary resolution to see
even 10 mm.

Tom Holmes, N8ZM

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Jim Lux
Sent: Wednesday, May 28, 2014 4:54 PM
To: time-nuts at febo.com
Subject: Re: [time-nuts] New tide gauge uses GPS signals to measure sea
level change

On 5/28/14, 6:04 AM, Tom Holmes wrote:
> Which begs the question: just where the heck, exactly, is the center of
> Earth given that it is in the 'middle' of a molten and dynamic core. Are
> 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 

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 

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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