[time-nuts] New tide gauge uses GPS signals to measure sea level change
jimlux at earthlink.net
Tue May 27 20:16:12 EDT 2014
On 5/27/14, 10:24 AM, Brooke Clarke wrote:
> Classical tide gauges measure the height of the water relative to the
> gauge. But since the gauge is attached to a tectonic plate it's
> elevation is changing.
> This is one of the problems I have with the claim that sea level is
> rising at 2mm per year. i.e. how well can they back out the plate
> movement in historical tide gauge records?
Quite well. This is what geodesy specialists (geodesists?) do for a
living. There's guys and gals at JPL who measure the earth's rotational
axis to fractions of a cm and sub microsecond (with a lot of averaging
and accounting for all sorts of error and offset sources)
I would look for instance, at the Topex/Jason things where they
regularly measure sea surface height to mm accuracy (that's how they
infer ocean temperature and El Nino.. warm water floats on top, etc.)
One should also be aware that very precise altimetry has some
constraints on the publishable data. Over small time and geographic
scales one can see things like the wake of a submarine: the
oceanographers call that "noise" that has to be removed: other people
call that "signal".
In any case, I'd believe a general statement of "mm of seal level
change" over a sufficient averaging interval.
What's more important is why it rises (fresh water floats, so if you get
a warm summer that melts lots of ice, you can get a layer of freshwater
on top of the salt water.
Keeping this time nutsy, there is some research on using reflections of
gps signals to distinguish (e.g. you get a big reflection from the
air/water surface, but you get smaller reflections from the fresh/salt
It's similar to the things you can do with trying to look for the
complete correlation graph for a GPS signal, rather than just tracking
the biggest peak.
> The new system uses a pair of GPS antennas one pointed up and the other
> pointed down at the water.
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