The Onsala Space Observatory (OSO) has equipment for space geodetic measurements using Very Long Baseline Interferometry (VLBI), the Global Positioning System (GPS), and the Global Navigation and Satellite System (GLONASS). Also available is a Water Vapour Radiometer (WVR) measuring the emission from the sky on and off the water vapor emission line centered at approximately 22 GHz. All these techniques acquire data that are affected by the propagation delay of radio waves in the atmosphere.

We concentrate on the influence of the neutral atmosphere and present a comparison of estimated atmospheric parameters derived from the analysis of simultaneous VLBI, GPS, and WVR observations. GPS and WVR observations at OSO are performed continuously during the year while geodetic VLBI is observed in 24 hour sessions, in our case 15 days covering all seasons of the year 1997. We estimate the equivalent zenith wet delay and the linear horizontal gradients in the east and north direction for these simultaneous observation epochs. We will study and compare the estimated gradients using different elevation cut off angles for the analysis of GPS and VLBI data. WVR data are typically obtained at elevation angles above 20 degrees. We will also assess possible differences in the quality of the atmospheric estimates from the different types of VLBI experiments. Of the total of 15 experiments we have 6 EUROPE, 5 CORE-B-100, and 4 RDV experiments.