Report of GPS-99 Session 13
GPS for gravity field and geoid determination

The session highlighted the developments in geoid determination methods for height determination by GPS techniques. The current status of geoid determination includes both extensive gravimetric computations utilizing updated earth reference models ? notably EGM96 ? as well as the tayloring of geoids to GPS/levelling results. Satellite, airborne and shipborne gravity field determination techniques are developing rapidly to aid in the determination of improved geoid models, and presentations on these topics were included as well.

The developments of large-scale, state-of-the art high-resolution continental geoid models were presented by an invited paper of Dru Smith, who presented the new US geoid model ?Geoid99?, which represents a new high-resolution (1?) geoid of the US, taylored to more than 6000 GPS points on levelling benchmarks, and by a last-minute addition of a presentation of Heiner Denker, Germany, on the current European quasigeoid model EGG97. Both of these geoid models indicates that results of height determination with GPS is possible at an accuracy level of a few cm, and that the geoid accuracy in many cases are controlled by the accuracy of the available GPS/levelling data. H. Denker presented results of GPS surveys from Germany (covering the entire former East Germany), where an r.m.s. fit between GPS and levelling were below 2 cm, indicating that the old dream of a 1 cm geoid over extended regions are close to becoming a reality.

A number of papers on regional geoids were presented, illustrating partly some of the problems encountered in more rough areas with highly varying gravity fields. The progress of geoid model determination in Nepal was presented by T. Baral, local geoid determination experiments in Brasil and Korea presented by M. Stewart and U. Han, and results of a new, comprehensive geoid determination of Japan presented by Y. Kuroishi. The latter model was based on more than 0.5 million gravity values, and the introduction of a new levelling system with proper orthometric heights gave geoid shifts of more than 20 cm, highlighting the need for the use of a good and comprehensive implementation of geoid computations, taking into account the necessary theoretical refinements.

The progress in gravity field data collection for improved geoid determination were illustrated by talks on improvement of marine gravimetry through use of differential GPS (presented by M. Satomura), and by the prospects of using satellite missions to improve the long-wavelength gravity field using new satellite missions (the geoid utilization of the Taiwan ROCSAT-3/COSMIC mission presented by B. Chao, USA). Airborne gravimetry will be an effective method in the future for filling the ?mid wavelength? band in geoid determination: Two papers addressed the issue, one by the European AGMASCO group (presented by R. Forsberg), showing how airborne gravity can yield accuracies of 2 mgal, transforming into geoid accuracies of 5-10 cm regionally, and one paper on recent test flights of current Japanese helicopter-based developments of new accelerometer hardware, with a potentially very high resolution and accuracy, presented by J. Segawa.

The session took place on Tuesday afternoon, and attracted an audience of 30-35 participants.

Rene Forsberg

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