BIFROST Project: Studies of Variations of Absolute Sea Level in Conjunction with the Postglacial Rebound of Fennoscandia

Determination of changes in absolute sea level like in the BIFROST project (Baseline Inference for Fennoscan-dian Rebound Observations, Sea level, and Tectonics) is shown as an example where a number of space geodetic techniques might gain from achieving a higher level of integration. According to the BIFROST project proposition it is feasable to determine changes of absolute sea level from changes of relative sea level by adding the movement of the crust as obtained with space geodetic techniques to the variations determined by tide gauges. Measurements with the Global Positioning System (GPS) offer the advantage of continuous, inexpensive observations in dense networks, but may suffer from relatively weak constraints on the translation and orientation of the site reference frame. Satellite Laser Ranging (SLR) and Very Long Baseline Interferometry (VLBI) augment GPS by providing high accuracy ties to the frames. Particularly the link of SLR to the gravity centre as the physical orbit centre appears important. The integration of GPS and VLBI promises to stabilize the orientation of the site reference frame.