On the information content in linear horizontal delay gradients estimated from space geodesy observations

We have studied linear horizontal gradients in theatmospheric propagation delay above ground-based stationsreceiving signals from the Global Positioning System (GPS).Gradients were estimated from 11 years of observations fromfive sites in Sweden. Comparing these gradients with thecorresponding ones from the European Centre for Medium-Range Weather Forecasts (ECMWF) analyses shows thatGPS gradients detect effects over different timescales causedby the hydrostatic and the wet components. The two stationsequipped with microwave-absorbing material below the antenna,in general, show higher correlation coefficients withthe ECMWF gradients compared to the other three stations.We also estimated gradients using 4 years of GPS data fromtwo co-located antenna installations at the Onsala Space Observatory.Correlation coefficients for the east and the northwet gradients, estimated with a temporal resolution of 15 minfrom GPS data, can reach up to 0.8 for specific months whencompared to simultaneously estimated wet gradients frommicrowave radiometry. The best agreement is obtained whenan elevation cut-off angle of 3\ub0 is applied in the GPS dataprocessing, in spite of the fact that the radiometer does notobserve below 20\ub0. We also note a strong seasonal dependencein the correlation coefficients, from 0.3 during monthswith smaller gradients to 0.8 during months with larger gradients,typically during the warmer and more humid part of theyear. Finally, a case study using a 15 d long continuous verylong-baseline interferometry (VLBI) campaign was carriedout. The comparison of the gradients estimated from VLBIand GPS data indicates that a homogeneous and frequentsampling of the sky is a critical parameter