The Progress of Modern Geodesy from the 22nd General Assembly of IUGG

The 22nd General Assembly of International Union of Geodesy and Geophysics (IUGG) was successfully held in University of Birmingham, from July 18 to 30, 1999 Twenty-eight Chinese geodesists participated in the IAG section meeting, and submitted forty papaers to the meeting. China nailonal report on geodesy was also submitted to the IAG. Much progress on geodesy has been achieved during past four years. From available progress reports and the submit-ted papers the following information can be extracted: ① High precision absolute and relative sur- face gravimetry allows to improve global/local gravity-based network and to monitor the. gravity changes due to the groundwater variations and related geodynamic phenomena such as tide.variation, earthquake, volcano and etc. ② Major progress in airborne gravimetry has been made,Under the normal operational condition, an accuracy of 1-3mGal over wavelength of 5~10km for the airborne gravimetry system has been achieved. But it is still very difficult to achieve a performance of 1~2m Gal over 1~2km using the current airborne scalar gravimetry system, to meet the requirement of geophysical exploration application. To increase the spatial resolution of air borne gravity data, new system such as the combination of airborne gravimetry: sytsem and a gravity gradiometry should be developed. The accuracy of GPS-derived accteleration Should be improved. Furthermore, the downward continuation of airborne gravity data needs more investigation, particularly for the mountain area. ③ With a successful completion of new gravity field missions (CHAMP, GRACE and probably GORCE), the static gravity field, represented by the geoid, will become available with a resolution of 80km half wavelength and accuracy of the order of 1-2cm on a global scale. Moreover, the temporal variations of the gravity field in the low to medium frequency range will be detected. The theory and methodology for the detenmination of the earth's gravity field will face a new challenge. These missions represent a quantum leap in gravity field research and will have an enormous impact on geoscientific research in general and on geodesy, oceanography and solid earth physics in particular. ④ Local and regional gcoid determination has become feasible in numerous well observed areas with accuracy of centimeter level and with a resolution of a few kilometers half wavelength, by using GPS/leveling derived geiod heights, surface gravity data, altimeter derived sea surface heights, very high resoluiton.DTM good earth's gravity field (such as EGM96) and etc (5) The standard deviation of the differences between the global 30, DTM JGP95E, ETOPO5 and TerrainBase is of the order of 100m, which indicates that the quality of the current DTM call not meet the, recent requirements Therefore, significant improvements are necessary in order to be created a reliable DTM with the same accuracy for all areas around the earth, proper for regional and glolbal geodetic, geophysieal and oceanographic applications, Space borne laser altimetry system and InSAP technology will be very promising tool for the improvements of DTMs. ⑥Some mew theories and methods have been used for the approximation of the earth's gravity field, such as wavelet transform, multiple-input output method, etc. Overdetermined geodetic boundary value problem (OGBVP), non-linear geodgetic boundary value problem and stochastic boundary value problem should be widely investigated in future. ⑦ VLBI and SLR are also primary techniques for monitoring the earth rotating parameters, the movements of global/local or regional plates and the earth crust deformation. ⑧ It has been proved that most important meteorological results from GPS/MET using ground-based and spaceborne-based GPS techniques was that the agreement with numerical weather prediction (NWP) models is noticeably better in data dense than in data poor ragions. This shows that GPS/MET results assimilated into NWP models can improve the weather forecast models. In addition, GPS/MET data will be helpful to study global/local climate changes.⑨ GPS-altimetry can be used for sea surface or ice sheet topography coverage, but some corresponding theoretical basis and data processing methods should be widely investigated.⑩High precision static and real-time or near real-time kinematic GPS technology can be used for the health monitoring purpose of large structures such as large dam, tall buildings and long-span or cable-stayed bridges.