On 13-15 December 1993, following the 60th Anniversary Meeting of the PSMSL in London, a meeting was held at the Institute of Oceanographic Sciences Deacon Laboratory, Wormley, Surrey on the fixing of tide gauge bench marks.
The following text contains the Executive Summary of the report of that meeting (NOAA Technical Report NOSOE0006, edited by W.E.Carter, dated October 1994). The meeting was attended by the following scientists from whom more information may be obtained:
V.Ashkenazi, T.F.Baker, G.Blewitt, D.E.Cartwright, D.T.Pugh, P.L.Woodworth (UK), J.Bosworth, W.E.Carter, T.A.Clark, J.J.Degnan, S.K.Gill, G.A.Maul, G.T.Mitchum, R.Rapp, D.B.Zilkoski (USA), C.Boucher, C.Le Provost, G.Woppelmann (France), A.Cabezas (Chile), B.Engen (Norway), J.Kakkuri (Finland), A.Lambert, A.Mainvile (Canada), G.W.Lennon, J.Manning, W.Mitchell, P.Morgan, S.Turner (Australia), A.R.de Mesquita (Brazil), L.Pezzoli, S.Zerbini (Italy), B.Richter, H.Seeger (Germany), D.S.Rosen (Israel), J.Zielinski (Poland) and A.Tolkatchev (Intergovernmental Oceanographic Commission).
Advances in the Global Positioning System (GPS) now make it the method of choice for measuring vertical crustal motions at tide gauge stations to be used to monitor changes in absolute global sea level.
The minimum accuracy for vertical crustal velocities to be useful for sea level studies is estimated to be 1 to 2 mm per year over 5 year intervals and 0.3 to 0.5 mm per year over intervals of a few decades. The most cost effective operating mode to achieve these accuracies with GPS is to place permanent receivers directly at selected tide gauge stations and to continuously operate them throughout the life of the monitoring program.
Several sets of tide gauge stations were selected, including GLOSS sites with long records primarily for secular sea level change studies; some sites with short or no records at present but which could be of potential interest for secular change studies in otherwise data sparse regions; and stations, primarily at ocean islands, taken from the WOCE sea level network, for interannual studies.
The International GPS and Geophysical Service (IGS) of the International Union of Geodesy and Geophysics, already operates a global network of approximately 30 GPS stations, collects the data with rapid turn around, distributes the full data set to researchers, and computes Earth rotation time series and station coordinates with delays of less than 1 week. An expansion of the IGS network to include measurements at tide gauge stations appears to be the best opportunity to realize a global sea level monitoring network in the immediate future.
The central role now foreseen for GPS should not be interpreted as reason to discontinue or reduce ongoing efforts in complementary techniques, most particularly Very Long Baseline Interferometry (VLBI) and absolute gravimetry. The unique information from those techniques will provide constraints on the Earth models used in the analysis to extract changes in sea level.
Recommendation 1: The President of MSLT Commission should formally request that the IGS take on the additional duties of organising and managing the operation of the GPS global sea level monitoring network as a fully integrated component of the IGS-IERS International Terrestrial Reference Frame (ITRF). The products should be coordinates and velocities of the tide gauge stations bench (reference) marks in the ITRF system.
Recommendation 2: The Permanent Service for Mean Sea Level (PSMSL) archiving system should be designed to provide the vertical crustal velocities derived from selected IGS solutions, along with explanatory information including experts that can be contacted by users of the data.
For more information and copies of the report, contact:
Bill Carter firstname.lastname@example.org
Copies of the report may also be obtained via email@example.com.