- Status December 1995 -
by Gerhard Jentzsch
During the 12th International Symposium on Earth Tides in Beijing, August 1993, this working group was prolonged until the next meeting of the Earth Tide Commission (ETC) in Boulder, 1995. Therefore, the group met again in August/September 1994 in Bonn, at the Institute for Theoretic Geodesy, University of Bonn, and again Prof. M. Bonatz was our host. In all 37 scientists from 13 countries were present. The results of this meeting were published in volumes 121 & 122 of the Bulletin d'Information Marees Terrestres. We all acknowledge the support of the meeting by the 'Deutsche Forschungsgemeinschaft'. The working group should continue under a changed mandate: Following the resolution no. 7 (Beijing, August 1993), the group discussed the topics
Within this frame some achievements using superconducting gravimeters and the need for a comparison of current tidal analysis packages were discussed as well as the status of the data bank of the International Center for Earth Tides. In addition to the scientific goals of tidal measurements in remote areas the working group also discussed logistic and instrumental problems to develop a reliable recording equipment and to ensure the stability of the calibration esp. after hard transports. These ideas will be further developed. There is no conclusion on the topic related to space techniques; this will be discussed during the next meeting. The conclusions are reviewed briefly in the following:
(1) Data recording and exchange. Here, the handling of SG data within GGP led to the recommendation to store and to exchange high rate data at intervals of e.g. 1 min, and to use the so-called PRETERNA format (Wenzel 1995) for storage and exchange of high precision tidal data. With regard to the establishment of the 'Global Geodynamics Project' (GGP) we strongly encouraged operators of new and existing SG stations to consider the advantages of (a) an excellent site, located away from cultural, geological and electrical noise, and (b) a high-rate, precise data recording system with samples every 10 s or less at a precision of 7.5 digits for the gravity signal and 5.5 digits for the pressure, (c) a timing accuracy of at least 10 msec, to realize the benefits of new methods of data processing and interpretation. Further, we intended to perform a comparison of all tidal analysis packages currently in use. Unfortunately, due to the move of the responsible person (Prof. David Crossley) this task had to be postponed.
(2) The group discussed the increasing resolution and stability of tidal records with regard to the still existing problem of modelling small scale and short period air pressure effects in gravity. This resulted into the recommendation that effects of short period local air pressure variations should be studied applying a regional network of air pressure stations in addition to the local air pressure record.
(3) Tidal recording in remote areas touched the problem that most ocean tidal charts due to modeling problems in Arctic areas are only valid up to 65 deg or 70 deg N and that therefore tidal gravity measurements are necessary to provide boundary conditions for the evaluation of ocean tidal models by ocean tidal loading computations. This means that tidal gravity stations close or beyond the latitude of 65 deg N should be established. On the other hand, in the centers of the continents still wide areas exist lacking reliable tidal information. Since tidal parameters in those areas are needed for the improvement of tidal earth models because the effect of ocean tidal loading is small, the establishment of tidal gravity stations in central Asia was recommended to achieve state of the art tidal parameters.
(4) The last topic dealt with the International Center of Earth Tides (ICET). The group recognised the fruitful and encouraging work of the International Center for the collection and exchange of tidal data and for the documentation of the development of tidal research. This work should be extended to collect all available historic earth tide data, too. Further, realizing the increasing availability of high precision and high rate data the working group recommends that the Center should develop conditions under which such high rate data can be stored and made available using modern computer networks. Finally it was agreed that in accordance to the conclusion of the 1992 meeting of the working group the term 'quality factor' has definitely to be cancelled and replaced by the term 'internal consistency factor'. The working group will have another meeting in September 1996 again in Bonn. There, in addition to the conclusion of our tasks we want to prepare a final report to the next Earthtide Symposium to be arranged in Brussels, 1997, and to discuss resolutions to be proposed during this symposium.
December 12, 1995
Gerhard Jentzsch (chairman)
References: Wenzel, H.-G. (1995): Format and structure for the exchange of high precision tidal data. Bulletin d'Informations, Marees Terrestres, vol.121, 9097-9101, Bruxelles 1995.
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