IGCP 565 Project Workshop

IGCP 565 Workshop 1: Science of geodetic monitoring of the hydrological cycle

Abstract:

Improving spatial and temporal resolution through combined geodetic observations
Hans-Peter Plag
Nevada Bureau of Mines and Geology and Seismological Laboratory, University of Nevada, Reno, NV, USA

Over the last three decades, space-geodetic observations have increased in accuracy by about an order of magnitude every decade. Today, space-geodetic techniques allow the determination of positions with respect to a global reference frame with unprecedented accuracy. The techniques permit the measurement of changes in the geometry of the Earth's surface with an accuracy of millimeters over distances of several 1000 km. Moreover, geodetic imaging techniques increasingly gain importance, particularly when integrated with the traditional point-based approach of geodesy. Gravity satellite missions measure time-variable gravity with unprecedented accuracy, and the accuracy observations of time-variable Earth rotation has increased with a comparable speed. Based on these techniques, temporal changes in the Earth's shape, rotation and gravity field (the “three pillars” of geodesy) are provided with increasing accuracy and spatial and temporal resolution. Among others, these observations record the “fingerprints” of mass movements in ocean, atmosphere, ice sheets and terrestrial water storage. On time scales of months to decades, mass redistribution in the fluid envelop of the solid Earth in fact induces the most dominant persistent signals in geodetic observations.

Most importantly, the three pillars are linked through one unique Earth system. Changes in Earth's shape and rotation lead to gravity field changes, and changes in rotation and gravity field force changes in Earth's shape. Changes in these quantities induce mass redistribution particularly in ocean and atmosphere. Therefore, the traditional approach of geodetic analyses which aims at separation of the different contributions does not appear appropriate at a level where the interaction between atmosphere, ocean, terrestrial hydrosphere, and solid Earth are far above the accuracy level. Moreover, observations of the “three pillars” provide the basis for the realization of the reference system that is required in order to assign time-dependent coordinates to points and objects, and the accuracy of the reference frame is related to the accuracy to which time-dependent changes in the three pillars can be predicted. Predicting the geodetic signals cause by surfical mass relocation with an accuracy comparable to that of the observations requires a Earth system model taking into account these interactions. Analysing the geodetic observations jointly will allow for an exploration of the strengths of the individual techniques and a mitigation of their weaknesses. Combination of space-geodetic observations and in-situ observations helps to increase the spatial resolution.