Reference Coordinate System Requirements for Geodesy and Ocean Dynamics

Abstract

AbstractThe requirements for systems of reference in geodesy and ocean dynamics should be framed against the background of the distinct possibility of achieving in the foreseeable future measurements with a resolution of 1 part in 108. The current techniques promising this precision, with one exception, are based on observations to extraterrestrial sources/responders. The principal geodetic goals calling for such precision are, broadly stated, the determination of:1. Positions of stations in global tracking networks.2. Crustal motion.3. Polar motion.4. The radial component of instantaneous sea surface position from satellite altimetry for the estimation of phenomena related to ocean circulation.5. Earth model improvement.These goals cannot be achieved without the definition of adequate models for relating measurements from observing stations on Earth to extraterrestrial objects, possibly in an iterative manner in the first instance.While the specific choice of a system of geodetic reference is not critical for determinations of position in three dimensions, the maintenance of the continuity of geodetic concepts at the highest level of resolution in terms of four-dimensional considerations narrows the possible alternatives available.Criteria desirable in such a system of reference for observations of the highest precision made from the surface of a non-rigid Earth undergoing secular crustal motion are the following:1. These observations should be capable of direct relation to the system of reference in four dimensions.2. The crustal motion vector at a point should be defined in terms of the variations of the coordinatesof that pointwith time.3. Near-Earth orbital positions should have unambiguous relationships to the origin of ocean geoid determinations in four dimensions.4. Coordinates on the adopted system of reference should have relevance over an epoch of practical significance (e.g., one year).5. The scale for the space as obtained from the observations should not be influenced by factors other than the velocity of light and the adopted definition for the second.An important corollary is that only one point should be treated as “fixed” at the surface of the nonrigid Earth. Positional variations which have a periodic character, effective over an epoch of observation or less, are best treated as effects to be removed from observations by appropriate modeling. Reported secular changes in mean sea level require that the geoid be defined in relation to mean sea level in four dimensions for any determination of sea surface topography, thereby completing a unified system of reference which, would meet all the requirements of high-precision geodesy and ocean dynamics.