Estimation of tropospheric wet delay from GNSS measurements (original) (raw)
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A history and test of planetary weather forecasting
2010
I would like to first thank my advisor, Lynn Margulis, for her recognition that my unconventional thesis lies within the boundaries of the geosciences. She understands very well how ideas in science come in and out of fashion and things that were dismissed or ignored in the past may very well be keys to future insights. Second, I wish to thank her for her continuous support of my larger education in matters pertaining to the science of Gaia and in regard to the art of teaching college-level students. Her unique teaching style, which combines rigorous drill on basics and patient nurturing, has been a template for my ongoing development as a teacher. I wish also to thank my committee for their support of my thesis project. Frank Keimig helped me frame an unusual research study in conventional forms wherever possible. Brian Ogilvie stressed the need for objectivity in my historical writing and pointed me towards authors who had already explored and organized some of the territory relevant to my thesis. Rob DeConto always had a good sense of what I was attempting and made many useful suggestions regarding the details of a proper scientific study. Ted Sargent, with whom I had many interesting discussions about controversial subject matter in general, helped with the overall conceptualization of the project. Several others outside of the academic world have either stimulated thought on the topic or helped by directing me to resources. I wish to thank Valerie Vaughan for many discussions on astrology and for her impressive publication, Earth Cycles, an annotated bibliography of scientific matters and recent publications that pertain to geocosmic influences on climate and weather. I also wish to thank Barry Orr for help in technical and software-related matters and for discussions that helped me focus my study. Finally, I wish to thank my mother Lucy Scofield for her moral support and education-related financial help which allowed me to pursue knowledge with reduced pressures.
Radio Science, 1998
The "wet delay," the excess radio path length due to atmospheric water vapor, has been derived from 71 days of microwave radiometer measurements at the Onsala Space Observatory, Onsala, Sweden. The temporal and spatial variability in the wet delay was analyzed. When we estimated daily "variance rates," the parameter characterizing a random walk process, values in the range 3.1 x 10 -0 to 1.1 x 10 -7 m2/s were found for the timescales 10-20 min. We estimated horizontal gradients in the wet delay and found that the temporal variability of the gradient components changed significantly from day to day. The variations in the gradients were also found to be significantly larger if data acquired only at relatively high elevation angles were used in the calculation. The effect of the wet delay variations on Global Positioning System (GPS) geodetic estimates was analyzed by performing Monte Carlo simulations. We used a Kalman filter with parameters for geodetic GPS data processing, first modeling the atmosphere as a horizontally homogeneous random walk process in time. In this case the estimated wet delay was found to be more sensitive to a detuning of the Kalman filter than the vertical component estimates. The RMS errors in the wet delay estimates increased from 2.2 to 3.6 mm when the atmospheric variance rate changed from 1.0 x 10 -s to 1.0 x 10 -7 m2/s and when the filter parameter was set to 1.0 x 10 -s m2/s. When simulated wet delay gradients were added to the data, it was seen that if gradients are not estimated by the Kalman filter on days with large gradient variability, the scatter introduced by the gradients can dominate the other modeled error sources. 1Now at tal plate boundary deformation and deformation associated with unloading of glacial material (postglacial rebound). A Swedish network of permanent GPS receivers (SWEPOS) was established in 1993 by the Onsala Space Observatory and the National Land Survey (NLS) of Sweden. The SWEPOS network consists of 21 continuously operating GPS stations with an average separation of about 200 km. (See Figure 1.) SWEPOS was designed for continuous measurements of the contemporary vertical and horizontal crustal deformations. After three years of operation the contemporary land-uplift after the last glacial period in Scandinavia has been detected [BIFROST Project, 1996]. In Figure 2 the vertical uplift of the GPS station Sundsvall is shown. It has also been demonstrated that the "wet delay," the excess radio path length due to the atmospheric 719 720 JARLEMARK ET AL.: ROLE OF WET DELAY VARIABILITY IN SPACE GEODESY
Wet path delay and delay gradients inferred from microwave radiometer, GPS and VLBI observations
2000
Very Long Baseline Interferometry (VLBI) is collocated with a permanent Global Positioning System (GPS) receiver and a Water Vapor Radiometer (WVR) at the Onsala Space Observatory in Sweden. Both space geodetic techniques are affected by the propagation delay of radio waves in the atmosphere, while the remote sensing technique is sensitive to the atmospheric emission close to the center of the 22 GHz water vapor emission line. We present a comparison of estimated equivalent zenith wet delay and linear horizontal delay gradients from an independent analysis of simultaneous VLBI, GPS, and WVR observations. Using different constraints for the variability of the delay and the horizontal gradient in the analysis of the VLBI and the GPS data did not have a large influence on the agreement with the WVR estimates. We found that the weighted rms differences between wet delay estimates from the geodetic techniques and the WVR estimates generally increased for an increased variability in the atmosphere.
Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy VIII, 2016
In a radio interferometer, the geometrical antenna positions are determined from measurements of the observed delay to each antenna from observations across the sky of many point sources whose positions are known to high accuracy. The determination of accurate antenna positions relies on accurate calibration of the dry and wet delay of the atmosphere above each antenna. For the Atacama Large Millimeter/Submillimeter Array (ALMA), with baseline lengths up to 16 kilometers, the geography of the site forces the height above mean sea level of the more distant antenna pads to be significantly lower than the central array. Thus, both the ground level meteorological values and the total water column can be quite different between antennas in the extended configurations. During 2015, a network of six additional weather stations was installed to monitor pressure, temperature, relative humidity and wind velocity, in order to test whether inclusion of these parameters could improve the repeatability of antenna position determinations in these configurations. We present an analysis of the data obtained during the ALMA Long Baseline Campaign of October through November 2015. The repeatability of antenna position measurements typically degrades as a function of antenna distance. Also, the scatter is more than three times worse in the vertical direction than in the local tangent plane, suggesting that a systematic effect is limiting the measurements. So far we have explored correcting the delay model for deviations from hydrostatic equilibrium in the measured air pressure and separating the partial pressure of water from the total pressure using water vapor radiometer (WVR) data. Correcting for these combined effects still does not provide a good match to the residual position errors in the vertical direction. One hypothesis is that the current model of water vapor may be too simple to fully remove the day-today variations in the wet delay. We describe possible new avenues of improvement, which include recalibrating the baseline measurement datasets using the contemporaneous measurements of the water vapor scale height and temperature lapse rate from the oxygen sounder, and applying more accurate measurements of the sky coupling of the WVRs.
2013
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Acta geodaetica et geophysica, 2018
The tropospheric wet delay is a significant systematic error of GNSS positioning, nevertheless it carries important information to meteorologists. It is closely related to the integrated water vapour that is the upper limit of precipitable water. The zenith wet delay can be converted to the integrated water vapour using a simple conversion factor. This conversion factor can be determined with the empirical formulae derived from radiosonde observations. In the past decades, numerous models were derived for this purpose, but all of these models rely on radiosonde observations stemming from a limited area of the globe. Although these models are valid for the area, where the underlying radiosonde observations were measured, there are several examples that these empirical formulae are used to validate GNSS based integrated water vapour estimations all over the globe. Our aim is to create a global model for the conversion of the zenith tropospheric delay to the integrated water vapour for realtime and nearrealtime applications using globally available Numerical Weather Models (NWM). Thus our model takes into consideration the fact that the model parameters strongly depend on the geographical location. 10 years of monthly mean ECMWF (European Center for Medium-Range Weather Forecast) dataset were used for the derivation of the model parameters in a grid with the resolution of 1° × 1°. The empirical coefficients of the developed models depend on two input parameters, namely the geographical location and the surface temperature measured at the station. Thus, the new models can be used for both realtime and near-realtime GNSS meteorological applications. The developed models were validated using 6 years of independent global ECMWF monthly mean analysis datasets (2011-2016). The results showed, that the application of the original models outside the area of the underlying radiosonde data sets can result in a relative systematic error of 7-8% in the estimation of the conversion factor as well as the estimated IWV values.
Utilization of GPS Tropospheric Delays for Climate Research
Journal of Physics: Conference Series
The tropospheric delay is one of the main error sources in Global Positioning Systems (GPS) and its impact plays a crucial role in near real-time weather forecasting. Accessibility and accurate estimation of this parameter are essential for weather and climate research. Advances in GPS application has allowed the measurements of zenith tropospheric delay (ZTD) in all weather conditions and on a global scale with fine temporal and spatial resolution. In addition to the rapid advancement of GPS technology and informatics and the development of research in the field of Earth and Planetary Sciences, the GPS data has been available free of charge. Now only required sophisticated processing techniques but user friendly. On the other hand, the ZTD parameter obtained from the models or measurements needs to be converted into precipitable water vapor (PWV) to make it more useful as a component of weather forecasting and analysis atmospheric hazards such as tropical storms, flash floods, landslide, pollution, and earthquake as well as for climate change studies. This paper addresses the determination of ZTD as a signal error or delay source during the propagation from the satellite to a receiver on the ground and is a key driving force behind the atmospheric events. Some results in terms of ZTD and PWV will be highlighted in this paper.
Tropospheric delay statistics measured by two site test interferometers at Goldstone, California
Radio Science, 2013
Site test interferometers (STIs) have been deployed at two locations within the NASA Deep Space Network tracking complex in Goldstone, California. An STI measures the difference of atmospheric delay fluctuations over a distance comparable to the separations of microwave antennas that could be combined as phased arrays for communication and navigation. The purpose of the Goldstone STIs is to assess the suitability of Goldstone as an uplink array site and to statistically characterize atmosphere-induced phase delay fluctuations for application to future arrays. Each instrument consists of two~1 m diameter antennas and associated electronics separated by~200 m. The antennas continuously observe signals emitted by geostationary satellites and produce measurements of the phase difference between the received signals. The two locations at Goldstone are separated by 12.5 km and differ in elevation by 119 m. We find that their delay fluctuations are statistically similar but do not appear as shifted versions of each other, suggesting that the length scale for evolution of the turbulence pattern is shorter than the separation between instruments. We also find that the fluctuations are slightly weaker at the higher altitude site.