On the accuracy of integrated water vapor observations and the potential for mitigating electromagnetic path delay error in InSAR

Abstract. A field campaign was carried out in the framework of the Mitigation of Electromagnetic Transmission errors induced by Atmospheric Water Vapour Effects (METAWAVE) project sponsored by the European Space Agency (ESA) to investigate the accuracy of currently available sources of atmospheric columnar integrated water vapor measurements. The METAWAVE campaign took place in Rome, Italy, for the 2-week period from 19 September to 4 October 2008. The collected dataset includes observations from ground-based microwave radiometers and Global Positioning System (GPS) receivers, from meteorological numerical model analysis and predictions, from balloon-borne in-situ radiosoundings, as well as from spaceborne infrared radiometers. These different sources of integrated water vapor (IWV) observations have been analyzed and compared to quantify the accuracy and investigate the potential for mitigating IWV-related electromagnetic path delay errors in Interferometric Synthetic Aperture Radar (InSAR) imaging. The results, which include a triple collocation analysis accounting for errors inherently present in every IWV measurements, are valid not only to InSAR but also to any other application involving water vapor sensing. The present analysis concludes that the requirements for mitigating the effects of turbulent water vapor component into InSAR are significantly higher than the accuracy of the instruments analyzed here. Nonetheless, information on the IWV vertical stratification from satellite observations, numerical models, and GPS receivers may provide valuable aid to suppress the long spatial wavelength (>20 km) component of the atmospheric delay, and thus significantly improve the performances of InSAR phase unwrapping techniques

Kinematic GNSS tropospheric estimation and mitigation over a range of altitudes

PhD ThesisThis thesis investigates the potential for estimating tropospheric delay from Global Navigation Satellite Systems (GNSS) stations on moving platforms experiencing a change in altitude. The ability to accurately estimate tropospheric delay in kinematic GNSS positioning has implications for improved height accuracy due to the mitigation of a major GNSS error source, and for the collection of atmospheric water vapour data for meteorology and climate studies. The potential for extending current kinematic GNSS positioning estimates of tropospheric delay from sea level based studies to airborne experiments, and the achievable height accuracy from a range of tropospheric mitigation strategies used in airborne GNSS positioning, are explored. An experiment was established at the Snowdon Mountain Railway (SMR), utilising the railway to collect a repeatable kinematic dataset, profiling 950 m of the lower atmosphere over a 50 day period. GNSS stations on stable platforms and meteorological sensors were installed at the extremities of the trajectory, allowing reference tropospheric delays and coordinates to be established. The retrieval of zenith wet delay (ZWD) from kinematic GNSS solutions using tropospheric estimation strategies is validated against an interpolated reference ZWD between GNSS stations on stable platforms, together with profiles from 100 m resolution runs of the UK Met Office Unified Model. Agreement between reference ZWD values and a combined GPS+GLONASS precise point positioning (PPP) solution is demonstrated with an accuracy of 11.6 mm (RMS), similar to a relative positioning solution and previous shipborne studies. The impact on the height accuracy from estimating tropospheric delay in kinematic GNSS positioning is examined by comparing absolute and relative GNSS positioning solutions to a reference trajectory generated from a relative GNSS positioning solution ii processed with reference to the GNSS stations on stable platforms situated at the extremities of the SMR. A height accuracy with a standard deviation of 72 mm was demonstrated for the GPS+GLONASS PPP solution, similar to a GPS-only relative solution, and providing an improvement over the GPS-only PPP solution.UK Natural Environment Research Council (NERC) studentship, and part of the work was funded by the Royal Institution of Chartered Surveyors (RICS) Education Trust

The estimation of precipitable water vapour from GPS measurements in South Africa

Includes bibliographical references (leaves 110-115).The propagation of the Global Positioning System (GPS) signal from the satellite to the receiver is affected by, among other factors, the atmosphere through which it passes and, whereas the affects of the ionosphere can be eliminated by the differencing of two transmitted frequencies, the affects of the troposphere remain one of the major sources of noise in traditional geodetic and positioning applications of GPS. This noise can, however, be turned into a signal for the meteorologist and, by applying suitable constraints and processing strategies, it is possible to estimate the amount of precipitable water vapour (PWV) in the atmosphere. The application of the GPS data for the estimation of PWV in the atmosphere is not a new concept and has been described in numerous publications and reports since the early 1990's (Bevis et al., 1992, Rocken et al., 1993). This project is, however, an attempt to test the technique using the South African network of permanent GPS base stations. This thesis sets out to answer four fundamental questions: i. In theory, can GPS observations be used to estimate the amount of precipitable water vapour (PWV) in the atmosphere? ii. What permanent GPS networks are being used in other countries around the world for similar applications and how successful are these applications? iii. Can data derived from the South African network of permanent GPS base stations, TrigNet, be used to estimate PWV with sufficient accuracy to be able to supplement the radiosonde upper air measurements of the South African Weather Service (SAWS)? iv. Is the estimation of PWV as derived from the GPS observations a true reflection of reality using the radiosonde ascent measurements and numerical weather model (NWM) data as a method of independent verification? The primary data sets used to estimate atmospheric PWV at hourly intervals for March 2004 were; i. GPS data derived from the South African network of permanent GPS base stations provided by the Chief Directorate of Surveys and Mapping (CDSM); and ii. Surface meteorological measurements supplied by the South African Weather Service (SAWS). The two independent data sets used to verify and test the technique were; i. Upper air measurements derived from radiosonde ascents provided by the SAWS. These measurements were used to compute Integrated Water Vapour (IWV) and then converted to PWV; and ii. PWV estimates derived from a Numerical Weather Model provided by the Department of Environmental and Geographical Sciences of UCT. By the comparing the estimates of PWV from the three techniques, viz. GPS, radiosonde and NWM, it was found that GPS will meet the accuracy requirements of the meteorologist and could be used to supplement radiosonde measurements for use in numerical weather models

Final results of the DFG funded project “Development of a tomographic water vapour sounding system based on GNSS data”

Since 2008 a group of scientists of the Leipzig Institute of Meteorology (LIM) and the German Research Centre for Geosciences Potsdam (GFZ) develops a method to derive water vapour profiles out of continuously available GNSS data (Global Navigation Satellite System). The aim of this project - supported by the Deutsche Forschungsgemeinschaft (DFG) - was to develop a processing system with related scientific algorithms, which uses data of regional GNSS ground networks to derive 3D water vapour distributions above these stations. This systems use the line of sight water vapour information from each ground station to every GNSS satellite in view (slants) as basis of a 3D tomographic reconstruction. At this time these reconstructions are based on GNSS data of about 330 German or near Germany located groundstations. This leads to a horizontal resolution of the reconstructed 3D water vapour field up to 40km and a vertical resolution of about 0.5km from the upper part of troposphere down to the atmospheric boundary layer (1km height).Seit 2008 befasst sich eine Arbeitsgruppe von Wissenschaftlern am LIM und dem GFZ in Potsdam im Rahmen eines DFG-geförderten Projektes mit der Ableitung von dreidimensionalen Wasserdampfverteilungen in der Atmosphäre aus Beobachtungsdaten regionaler GNSS-Bodennetze (GlobaleNavigationsSatellitenSysteme). Die Wasserdampfverteilungen können aus der atmosphärischen Information entlang der Sichtlinien zwischen den Bodenstationen und den sichtbaren GNSS-Satelliten (sogenannte Slants) berechnet werden. Diese zahlreichen Sichtlinien ermöglichen eine tomographische Verarbeitung der Daten. Der entwickelte tomographische Algorithmus nutzt derzeit bis zu 330 deutsche und nahe Deutschland gelegene GNSS-Stationen, was eine horizontale Auflösung der resultierenden 3D-Felder von 40km und einer vertikalen Auflösung von 0,5km bis hinab zur atmosphärischen Grenzschicht (bis 1 km über dem Boden) ermöglich

Modelling atmospheric wet refractivity profile using ground and space-based global positioning system

Precise measurement of atmospheric water vapour has been very challenging due to some limitations of the conventional meteorological systems. Hence, there is a need for Global Positioning System (GPS) for meteorology or GPS meteorology. Therefore, the ground-based GPS meteorology and the space-based GPS Radio Occultation (GPS RO) techniques have been used. The major challenges of groundbased GPS meteorology approach include the lack of surface meteorological data collocating with the location of the ground-based GPS receivers as well as its inability to profile the atmosphere. Whereas the GPS RO technique has a problem of generating profile for the lower tropospheric region which holds the largest amount of water vapour. This research investigates an approach for estimating wet refractivity profile using GPS data. Three specific objectives were set for the study which was conducted in three phases. The first objective assessed GPS Integrated Water Vapour (GPS IWV) in which GPS IWV from interpolated meteorological data and the applicability of Global Pressure and Temperature (GPT2w) model for GPS meteorology was evaluated. The results revealed that the GPS IWV from Automatic Weather Station (AWS) presents good correlation with the radiosonde IWV, the standard deviation of the biases vary spatially from 3.162kg/m2 to 3.878 kg/m2. The actual influence of the errors of GPT2w meteorological parameters on GPT2w-based GPS IWV lies between 2kg/m2 and 3kg/m2, translating to an average relative accuracy of 1.2%. Meanwhile, the sensitivity of the GPS RO data to equatorial water vapour trend was evaluated to achieve second objective. It was found that the GPS RO IWV is highly comparable with the ground-based GPS IWV, having average bias of 1.8kg/m2. Finally, a methodology for GPS wet refractivity retrieval was developed towards achieving the third objective of this research. The Modified Single Exponential Function (MSEF) model for retrieving wet refractivity profile from ground-based GPS Zenith Wet Delay (ZWD) was realised. The output validation using profile from radiosonde and GPS RO observations showed high correlation in each case. In order to improve the performance of the MSEF model, an approach for integrating the ground-based and the space-based GPS data (GIWRef) was formulated. The GIWRef profile is highly correlated with the GPS RO profile, which showed an average improvement of 41% over the initial MSEF method with average correlation coefficient of 0.99. It can be concluded from the foregoing results of the study that the MSEF and GIWREF concepts developed in this work, presents a potential for augmenting weather forecasting and monitoring water vapour system

A GPS network for tropospheric tomography in the framework of the Mediterranean hydrometeorological observatory Cévennes-Vivarais (south-eastern France)

International audienceThe Mediterranean hydrometeorological observatory Cévennes-Vivarais (OHM-CV) coordinates hydrometeorological observations (radars, rain gauges, water level stations) on a regional scale in southeastern France. In the framework of OHM-CV, temporary GPS measurements have been carried out for 2 months in autumn 2002, when the heaviest rainfall are expected. These measurements increase the spatial density of the existing permanent GPS network, by adding three more receivers between the Mediterranean coast and the Cévennes-Vivarais range to monitor maritime source of water vapour flow feeding the precipitating systems over the Cévennes-Vivarais region. In addition, a local network of 18 receivers covered an area of 30 by 30 km within the field of view of the meteorological radar. These regional and local networks of permanent and temporary stations are used to monitor the precipitable water vapour (PWV) with high temporal resolution (15 min). Also, the dense local network provided data which have been inverted using tomographic techniques to obtain the 3-D field of tropospheric water vapour content. This study presents methodological tests for retrieving GPS tropospheric observations from dense networks, with the aim of assessing the uncertainties of GPS retrievals. Using optimal tropospheric GPS retrieval methods, high resolution measurements of PWV on a local scale (a few kilometres) are discussed for rain events. Finally, the results of 3-D fields of water vapour densities from GPS tomography are analysed with respect to precipitation fields derived from a meteorological radar, showing a good correlation between precipitation and water vapour depletion areas