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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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CSR LogoLast modified June 07, 1999
Dr. Steve Nerem (
Dr. Mark Schenewerk (
Dr. Tonie vanDam (

Kwan-Dong Park (CSR graduate student)(

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