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

Journal article, 2019

We have studied linear horizontal gradients in the
atmospheric propagation delay above ground-based stations
receiving signals from the Global Positioning System (GPS).
Gradients were estimated from 11 years of observations from
five sites in Sweden. Comparing these gradients with the
corresponding ones from the European Centre for Medium-
Range Weather Forecasts (ECMWF) analyses shows that
GPS gradients detect effects over different timescales caused
by the hydrostatic and the wet components. The two stations
equipped with microwave-absorbing material below the antenna,
in general, show higher correlation coefficients with
the ECMWF gradients compared to the other three stations.
We also estimated gradients using 4 years of GPS data from
two co-located antenna installations at the Onsala Space Observatory.
Correlation coefficients for the east and the north
wet gradients, estimated with a temporal resolution of 15 min
from GPS data, can reach up to 0.8 for specific months when
compared to simultaneously estimated wet gradients from
microwave radiometry. The best agreement is obtained when
an elevation cut-off angle of 3° is applied in the GPS data
processing, in spite of the fact that the radiometer does not
observe below 20°. We also note a strong seasonal dependence
in the correlation coefficients, from 0.3 during months
with smaller gradients to 0.8 during months with larger gradients,
typically during the warmer and more humid part of the
year. Finally, a case study using a 15 d long continuous verylong-
baseline interferometry (VLBI) campaign was carried
out. The comparison of the gradients estimated from VLBI
and GPS data indicates that a homogeneous and frequent
sampling of the sky is a critical parameter.