Abstract
The National Geodetic Survey (NGS) in cooperation with a number of federal agencies, state and local groups and universities is establishing GPS networks in the United States, east of the Rocky Mountains, which can be used to monitor strain and vertical deformation. These GPS networks are tied to a framework of some 14 fixed and mobile VLBI sites. In cooperation with the Nuclear Regulatory Commission (NRC), NGS established a 45 station GPS regional network in Nov.–Dec. 1987 which is tied to the VLBI framework. This network is scheduled for reobservation in 1989 and funds permitting, at regular intervals thereafter. A number of additional, more dense networks have been or are in the process of being established. The Tennessee Department of Transportation has established a 60 station statewide network to act as a reference network for surveying in conjunction with road construction. This network is expected to have an accuracy of a few parts in 107. NGS in cooperation with the NRC and the University of Maine established in 1986 a high accuracy GPS network in southeast Maine. In 1987 NGS in support of the Federal Aviation Administration (FAA) has established approximately 100 stations throughout Ohio with an accuracy in the 1:106 to 1:107 range. Toward the end of 1988, NGS, working in conjunction with several state agencies and the University of Florida, will establish a statewide network of about 140 stations with an accuracy in the 1:106 to 1:107 range. NGS, in cooperation with the Department of Energy, has also established a high accuracy to 1:107 to 1:108 GPS traverse from Florida to Maine connecting stations at tide gauge sites. The State of Texas is establishing a number of permanent GPS stations in support of highway surveying. These stations will allow strain monitoring across Texas at the 1:108 level. Additional networks are in the planning stage.
It is clear that large numbers of high accuracy GPS networks are being established throughout the eastern United States. Many of these networks are being established for other than geophysical purposes. In many cases the state highway departments and others are interested only in 1:106 accuracy. As a practical matter this means that to assure 1:106 accuracy a few parts in 107 accuracy (1 to 3 cm over 100 kms) is often attained, but this is by no means certain. Also there are normally no plans for systematic resurveys, only replacement of destroyed monuments. A challenge to the geophysical community is to interact with the groups undertaking the high accuracy surveys to assure that, at points of geophysical interest, satisfactory accuracies are achieved during initial epoch measurements. This means that a satisfactory number of observations are obtained and high accuracy reduction methods are used in obtaining differential positions from the data. The geophysical community must also develop plans for resurvey of geophysically interesting network components on a systematic basis.
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