Alan Dodson

Atmospheric models, GPS and InSAR measurements of the tropospheric water vapour field over Mount Etna

[1] Dynamic models of atmospheric movement over the Mount Etna volcano are used to calculate the path delays affecting radar caused by variable water vapour in the troposphere. We compare these model results with the equivalent differential radar interferogram generated by two ERS-2 SAR images taken 35 days apart and the water vapour delay retrievals from a network of fourteen GPS stations distributed over the volcano. The atmospheric model delay field agrees wellwith thelong-wavelength spatial differences measured by InSAR and those measured by GPS. INDEX TERMS: 6924 Radio Science: Interferometry; 1243 Geodesy and Gravity: Space geodetic surveys; 3367 Meteorology and Atmospheric Dynamics: Theoretical modeling; 6964 Radio Science: Radio wave propagation; 8499 Volcanology: General or miscellaneous. Citation: Wadge, G., et al., Atmospheric models, GPS and InSAR measurements of the tropospheric water vapour field over Mount Etna, Geophys. Res. Lett., 29(19), 1905, doi:10.1029/2002GL015159, 2002.

Wet tropospheric effects on precise relative GPS height determination

SummaryConsiderable interest has been generated recently in the use of the Global Positioning System (GPS) for precise height determination. A major error source in these measurements is the propagation delay due to atmospheric water vapour. In order to achieve the high precisions required for such applications as absolute sea-level monitoring improvement of wet delay modelling is necessary. Results from a GPS campaign show a significant correlation (0.91) between the variability of the wet delay measured using a water vapour radiometer (WVR) at the Onsala site and the absolute value of the residual error in the height determination of a 134 km baseline from Onsala to Jönköping. This correlation indicates that the atmosphericvariability as inferred from the WVR data includes information on the quality of the GPS height estimate. During periods of high atmospheric activity, e.g., during the passage of a weather front, the use of a six-parameter gradient model reduces the spread for the vertical coordinate from 40 mm to 20 mm (with standard deviations of 17 mm and 9 mm respectively) over the 134 km baseline (less than 1 × 10−7) using 8 hour data spans on 11 different days over a six month period.

Assessment of EGNOS Tropospheric Correction Model

Within the implementation of the European Geo-stationary Navigation Overlay System (EGNOS), a significant residual error in positioning is due to tropospheric delay effects. The EGNOS guidelines recommend that tropospheric delay is modelled using an empirical correction algorithm based on a receivers height and estimates of meteorological parameters developed from average and seasonal variation data. However, such a simple average and seasonal variation model is unlikely to emulate temporal weather changes exactly. The potential errors involved in the application of the recommended algorithm and the consequent effects on the positioning errors, under typical UK weather conditions, are detailed in this paper. This was achieved by comparing tropospheric delays produced by the EGNOS model, with tropospheric delays estimated from high precision carrier phase GPS, over a one-year period for five UK stations. The RMS EGNOS model zenith tropospheric delay errors ranged from 4.0 to 4.7 cm, with maximum errors ranging from 13.2 to 17.8 cm. The errors were also shown to be spatially correlated. The subsequent effect on position error is shown to be dependent on the satellite elevation cut-off angle adopted and on whether or not the observations are weighted according to the satellite elevation angle. (This paper was first presented at GNNS 2000, the third European Symposium on global navigation satellite systems held in Edinburgh, Scotland from 1st to 4th May 2000.)

Atmospheric water vapour correction to InSAR surface motion measurements on mountains: results from a dense GPS network on Mount Etna

Abstract In order for synthetic aperture radar interferometry (InSAR) to effectively measure the pattern of surface deformations of dynamic phenomena, such as volcanoes, it is necessary to mitigate the effects of water vapour on this signal. One way to achieve this is to use a numerical model of the atmospheric conditions, calibrated using an independent measure of atmospheric water vapour, in order to determine the water vapour-based delay in InSAR. We describe an experiment in which such GPS-derived integrated water vapour (IWV) estimates were made using a dense array of continuous GPS receivers. Fourteen GPS receivers were deployed at stations on Mount Etna and continuous observations were recorded for a 10-day period in August/September 2000 and repeated over a 10-day period in October 2000, coincident with ascending and descending SAR image acquisition by the ERS-2 satellite. The results show maximum variations in IWV of 10 kg / m 2 between the days corresponding to the ERS-2 passes. The variations were not the same at all the stations and equated to variations in the zenith wet delay (ZWD) ranging from a few millimetres to about 6 cm. The differences in ZWD between the two passes would degrade the ability to measure surface deformations significantly if they were not modelled.

Using continuous GPS and absolute gravity to separate vertical land movements and changes in sea-level at tide-gauges in the UK

Researchers investigating climate change have used historical tide-gauge measurements from all over the world to investigate the changes in sea-level that have occurred over the last century or so. However, such estimates are a combination of any true sea-level variations and any vertical movements of the land at the specific tide-gauge. For a tide- gauge record to be used to determine the climate related component of changes in sea-level, it is therefore necessary to correct for the vertical land movement component of the observed change in sea-level. In 1990, the Institute of Engineering Surveying and Space Geodesy and Proudman Oceanographic Laboratory started developing techniques based on the Global Positioning System (GPS) for measuring vertical land movements (VLM) at tide-gauges in the UK. This paper provides brief details of these early developments and shows how they led to the establishment of continuous GPS (CGPS) stations at a number of tide-gauges. The paper then goes on to discuss the use of absolute gravity (AG), as an independent technique for measuring VLM at tide-gauges. The most recent results, from CGPS time-series dating back to 1997 and AG time-series dating back to 1995/1996, are then used to demonstrate the complementarity of these two techniques and their potential for providing site-specific estimates of VLM at tide-gauges in the UK.

The use of GPS measurements for water vapor determination

Abstract A workshop on the use of Global Positioning System (GPS) measurements in weather and climate with emphasis on water vapor determination, was organized by the National Environmental Research Councils (NERC) Environmental Systems Science Centre (ESSC), at the University of Reading, Reading, United Kingdom, and took place there 29–31 August 2001. This paper gives a broad overview and general background of the use of GPS data for weather and climate. It outlines the objectives of the workshop and presents ongoing national, regional, and international activities both for ground-based and satellite-based systems. This includes work in the United States, China, and Japan, and different European efforts, including activities under European Community programs. Data assimilation of GPS data for weather prediction and climate is discussed as are ways in which to develop GPS-based systems to become an integrated part of the World Weather Watch. This includes ways of systematically using GPS data from the in...

Ground-based GPS water vapour estimation: potential for meteorological forecasting

Abstract In order to assess the impact of ground-based GPS water vapour estimates in meteorological forecasting applications, estimates must be available within near real time and must be produced with an accuracy comparable with that of existing meteorological measurement techniques. Before the potential benefit of GPS-integrated water vapour (IWV) estimates in numerical weather prediction (NWP) models could be determined, extensive tests were performed to examine the effect of various process error models, constraints and estimation techniques involved within GPS processing procedures in order to devise a recommended processing strategy. Ten months of GPS data from a selection of stations, which are co-located with radiosonde launch sites, within the UK permanent GPS network, have been processed to validate these findings. Near real-time estimation issues have subsequently been examined, primarily using a sliding-window processing approach to demonstrate the feasibility of producing accurate GPS IWV estimates within the required time frame. Additionally, preliminary simulation work has been performed to assimilate GPS IWV into the UK Meteorological Office (UKMO) numerical weather prediction models, in preparation for impact assessments on the quality of mesoscale weather forecasts.

Global Positioning System aided autonomous construction plant control and guidance

The advent of Real Time Kinematic (RTK) Global Positioning System (GPS) positioning means that this technology can now be used for dynamic control and guidance applications. The paper outlines the recent advances in GPS technology, which have enabled RTK GPS to become a reality. Tests have been carried out in construction plant control and monitoring applications with a demonstrated precision of the order of a few millimeters. The results are compared with those achieved using a laser level and a digital automatic level.

Performance of Precise Point Positioning with Ambiguity Resolution for 1- to 4-Hour Observation Periods

Abstract Recent progress in integer ambiguity resolution at a single station has made it possible to achieve high positioning accuracy in static precise point positioning (PPP) using a short period of observations. In this paper, 12 stations across Europe are used to conduct short-period (i.e. one, two, three and four hours) static PPP with ambiguity resolution from Day 245 to 251 in 2007. It is demonstrated that, when over three hours of observations are used, PPP can achieve a success rate of 100% for ambiguity resolution, a 3D positioning accuracy of about 1.0 cm and an occurrence of less than 1.0% for degraded solutions. Moreover, for the fixed solutions, increasing the observation period hardly improves the horizontal positioning accuracy while still improving the vertical one. Therefore, it is proposed that at least three hours of observations should be used in the ambiguity-fixed static PPP if a reliable millimetre positioning accuracy is required in the engineering applications.

A new test of Earth tide models in central Europe

New improved ocean tide models allow a corresponding improvement in ocean tide loading computations. During recent years various groups have made more accurate tidal gravity observations in central Europe, using gravimeters carefully calibrated on various calibration lines. This progress with tidal gravity measurements and ocean loading corrections now allows a more comprehensive test of body tide models than was possible previously. It is shown that the O1 and M2 gravimetric factors, corrected using the latest ocean tide models, are very consistent between the sites, despite the different instruments and gravity calibration lines that have been used. These observations allow constraints to be put on the anelasticity of the Earth at tidal frequencies. The corrected gravimetric factors are consistent with recent model results for both the elastic PREM model and with α ≤ 0.09 for an anelastic PREM model with the Q for dissipation in shear varying as frequency to the power α.