| The requirements for defining and realizing a global vertical datum (GVD) with a precision on the dm- or even cm-level has become essential now, due to the improved measurement accuracy provided by the space geodetic techniques. This study identifies the different approaches in which such a global vertical datum could be evolved.;The first approach, which can be considered as an ideal approach, uses the best available geocentric station positions from present and future space geodetic networks, highly accurate geopotential model and surface gravity data around the space stations for defining and realizing a global vertical datum. The other simplified approach, which can be considered more of an operational development with a short time framework, is mainly dependent on the widely available GPS and DORIS tracking networks and accurate geoid height models for the GVD development.;After a review of the various heights and height systems that are in use today, detailed mathematical procedures for setting up the observation equations to define a global vertical datum in a least-squares sense, under different data scenarios, are discussed. A test computation to realize the first iteration global vertical datum with available data at 17 space geodetic stations in six regional vertical datums is also included. The gravimetric height anomaly/undulation computations at the space geodetic stations required in the test computation have been performed using both Modified Stokes' technique and least-squares collocation technique combining surface gravity data in a small cap around the stations with potential coefficients from OSU91A model. The test results show that in an idealistic approach the global vertical datum can be realized to an accuracy of ;For the simplified approach, height bias modeling procedures required to convert the orthometric height computed with respect to the geoid into the local vertical datum have been developed using least-squares collocation technique.;The report concludes with some specific recommendations for the acquisition and use of data for defining and realizing a global vertical datum. |
