Information
1.
Research Objectives
The principle objective of this
study is to use Global Positioning System (GPS),
absolute gravity, and tide gauge measurements to
improve global data related to sea level change.
In addition, the information from this project
will improve the current models of glacial
isostatic adjustment (GIA).
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2. Tide Gauges and
GPS Sites in the Northeastern U.S.
For investigations of long term sea level
variability, only tide gauges with records longer
than 30-50 years are useful, since it takes the
averaging of much more tide gauge data to
determine long-term sea level change than it does
GPS observations to determine long-term crustal
motion. A number of tide gauges on the east coast
have been operational for 50 years or longer,
thus these gauges should initially be targeted
for GPS monitoring. This project will select six
tide gauges for GPS colocation based on a variety
of different factors including the length and
quality of the sea level record at the gauge,
site suitability (local geology), site security,
access to communications and power, the proximity
of the sites to one another (we want them well
distributed along the coast), etc. The plan is to
traverse the post-glacial rebound signal from the
peak of the peripheral bulge near Chesapeake Bay
up the east coast to Maine, where the rebound is
quite small. In addition, we will have the
opportunity to access nearby Coast Gaurd CORS
(Continuously Operationg Reference Station) and
IGS (International GPS Service) data for
comparision. Currently, we have installed two
additional sites near tide gauges in Maine, with
other sites along the northeast coast soon to
follow. The combination of this extended BAYONET
study with other GPS/tide gauge networks around
the world will provide new insight into global
sea level change that will nicely complement the
new results being derived from satellite
altimetry.
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3.
Installation of Continuous GPS Sites
The installation of a continuous GPS site
requires considering sky visibility,
monumentation, site security, and the
availability of power and communications. The
preference is to locate the GPS antenna/receiver
as close to the tide gauge as possible, since we
can exploit the security, power, and
communications infrastructure already established
for the tide gauge. Monumentation will be
acheived either in the bedrock or using deep
anchors. As always, the effects of multipath will
be carefully considered for each case.
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4. GPS Data
Analysis
Each GPS site in the network accumulates about
14,000 observations per day, as the receivers
collect data at 30-second intervals, providing L1
and L2 phase and pseudorange observations. Once
daily each site downloads its data to the
national Geographic Survey at NASA. There they
are converted to Receiver Independent Exchange
(RINEX) format and placed on anonymous FTP sites
for immediate, public use. The GPS data analysis
is done through NOAA's routine analysis (PAGE4),
as well as by using JPL's GIPSY package. The
PAGE4 data processing at NOAA is handled by Dr.
Mark Schenewerk. Kwan-Dong Park is responsible
for the GIPSY processing of the GPS data at UT
Center for Space Research (CSR).
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5.
Analysis and Correction of Tide Gauge Data
Tide gauge corrections based on numerical
models of post-glacial rebound are sensitive to
the visco-elastic structure adopted for Earth
models. Furthermore, some sites may exhibit
anomalous motion not described by numerical
models of post-glacial rebound. Two methods are
used to correct the tide gauge data. The first
method uses estimates of the geophysical
parameters to determine the GIA corrections. The
second method uses estimates of uplift obtained
directly from GPS data to correct nearby sites.
The corrections from the first method will have a
smaller statistical uncertainty than those from
the second method, because the vector rates from
many lines will be used to etimate a few
parameters, but the the corrections from the
second method are less affected by systematic
arrors in the geophysical models or by local
crustal deformation unrelated to GIA. The goal is
to develop a set of corrections that minimize the
variations in sea level rise observed from one
site to the next.
The correction of the global tide guge data set
can only be done using the first method, since we
must use the geophysical models to produce
corrections for tide gauges outside of our
research area. By correcting the global tide
gauge data, we can improve upon previous
estimates of global mean sea level rise obtained
from this data.
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Last modified June
07, 1999 
