Recent results of water vapor radiometry in assessing vertical lithospheric movements by using space geodetic radiowave techniques

Abstract Vertical components of lithospheric deformations are key elements in assessing geodynamic processes such as mountain building, precursors of volcanic eruption or subduction of lithospheric plates. The recent success of modern geodetic space techniques such as the satellite positioning system GPS relates mainly to the “horizontal” components of lithospheric movements. One of the limiting factors for the height component is the refraction caused by the non-ionized atmosphere. This part is non-dispersive and the refractive effect cannot be determined by using two carrier waves. Modeling of the content of water vapor is particularly difficult because of its high variability both spatially as well as temporally. The objective of this paper is to present results from an experiment recently carried out in the frame of the European Sea Level Fluctuation program, SELF. Transportable water vapor radiometers were used to determine the refraction effect of the wet component of the troposphere between several tide gauge sites and neighboring SLR/VLBI stations in Italy and France. The corresponding differential path delays of the GPS radio signals are on the order of several cm. The example shown in this paper is a baseline measured between the French SLR station Grasse, France, and the tide gauge site Genoa, Italy. Apparent temporal changes of height differences of up to 8 cm have been observed over a time span of 12 h. The consequence for assessing vertical movements of GPS based stations is discussed.