Physics and Chemistry of the Earth, Part A: Solid Earth and Geodesy, 2001
ABSTRACT We present first results on the site coordinates and the atmospheric propagation delay p... more ABSTRACT We present first results on the site coordinates and the atmospheric propagation delay parameters from the permanent Global Positioning System site established at Chajnantor, in the Chilean Andes, at 5000 m altitude. The site is designed to ensure a long-term monument stability. The primary reason behind the site establishment is the future construction of the Atacama Large Millimeter Array (ALMA), a radio astronomy facility, at the same location. There is an interest to observe the site coordinate stability as well as the total radio propagation path delay variations which is possible employing the GPS technique. The fact that the site is known to have mainly a dry delay component makes it also a unique site for GPS accuracy studies. We present first results on the estimated site coordinates and the zenith total and wet atmospheric delays. Structure function results of the delays are included as well as the impact of using different elevation cut off angles in the data processing. We also discuss the performance of the mapping functions used for the estimation of the zenith delay terms.
The objective of this study, named AWARDS (Advanced microWAve Radiometers in Deep space Stations)... more The objective of this study, named AWARDS (Advanced microWAve Radiometers in Deep space Stations), is the preliminary design of a transmission Media Calibration System (MCS) to be located at an ESA Deep Space Antenna (DSA) site. The crucial aspect is the capability to accurately retrieve the tropospheric path delay along the line-of-sight of the deep space probe in order to allow precise tropospheric calibration of deep space observables (range and range-rate) with particular reference to the BepiColombo spacecraft and its primary DSA at Cebreros (ES). The study focuses on two main aspects which lead to the preliminary design of the Mercury Orbiter Radioscience Experiment (MORE) MCS: the characterization of current microwave radiometers (MWRs) available at ESA/ESTEC and the atmospheric fluctuation effects on the MCS error budget, in terms of the Allan standard deviation (ASD). In the course of the study, further critical aspects have been identified (effects of Sun contamination, ef...
The developments in global satellite navigation using GPS, GLONASS, and Galileo will yield more o... more The developments in global satellite navigation using GPS, GLONASS, and Galileo will yield more observations at various elevation angles. The inclusion of data acquired at low elevation angles allows for geometrically stronger solutions. The vertical coordinate estimate of a GPS site is one of the parameters affected by the elevation-dependent error sources, especially the atmospheric corrections, whose proper description becomes necessary. In this work, we derive time-series of normalized propagation delays in the neutral atmosphere using ray tracing of radiosonde data, and compare these to the widely used new mapping functions (NMF) and improved mapping functions (IMF). Performance analysis of mapping functions is carried out in terms of bias and uncertainty introduced in the vertical coordinate. Simulation runs show that time-correlated mapping errors introduce vertical coordinate RMS errors as large as 4 mm for an elevation cut-off angle of 5°. When simulation results are compared with a geodetic GPS solution, the variations in the vertical coordinate due to mapping errors for an elevation cut-off of 5° are similar in magnitude to those caused by all error sources combined at 15° cut-off. This is significant for the calculation of the error budget in geodetic GPS applications. The results presented here are valid for a limited area in North Europe, but the technique is applicable to any region provided that radiosonde data are available.
Physics and Chemistry of the Earth, Part A: Solid Earth and Geodesy, 2001
ABSTRACT We have made Monte Carlo simulations in order to quantify the influence of several error... more ABSTRACT We have made Monte Carlo simulations in order to quantify the influence of several error sources on the atmospheric delay derived in our regular “Precise Point Positioning” analysis of GPS data. Satellite position and clock errors were found to be the greatest sources of error. The RMS size of the atmospheric delay error due to these sources is suggested to be 4–8 mm, and the correlation of the delay error over 1000 km is 0.5-0.7. In our real-time analysis of the atmospheric delay satellite orbits and clock offsets are modelled in the Kalman filter. The modelled errors suggest an uncertainty in the atmospheric delay of 10–13 mm, and a decrease in the correlation coefficients to about 0.65 over a distance of 1000 km. For both the Precise Point Positioning and the real-time analysis the influence from antenna signal scatter will give additional error contributions.
We will draw attention to a new technique for retrieval of water vapor fields, which is based on ... more We will draw attention to a new technique for retrieval of water vapor fields, which is based on the use of a small network of GPS receivers, and is referred to as GPS tomography. A subset of the inverse theory utilizing the integrated values of the field distribution is called tomography. To demonstrate the technique we use a permanent local GPS network consisting of 8 stations and located in the G"{o}teborg area (Swe- den). Its baselines range from a few to 15 km. Tomographic results from this network are derived by applying a tomographic method on the measured slant delay values at each site. The method is implemented in the software package LOTTOS (LOcal Tropospheric Tomographic SOftware developed by Alejandro Flores) and is able to produce a tomographic solution of the water vapor induced refractivity in the tropo- sphere. In tropospheric tomography the slant delay values are obtained by combining the estimated zenith delay values with the horizontal gradients. The geometry using GPS is somewhat restricted compared to the classical tomographic approaches where one can choose the direction of the scanning rays. In the case of GPS tomography, we are limited by the satellite constellation and therefore in certain areas of the field that we want to reconstruct some "information gaps" will be present. In order to reduce these uncertainties, additional vertical and horizontal constraints should be included in the solution. Other ways of increasing the information could be: optimization of tomographic grid used, inclusion of more GPS stations and/or additional independent data from GLONASS, GALILEO (in the future), GPS occultation receivers, radioson- des or scanning pointed water vapor radiometers (WVR). We first present simulations of a tomographic solution based on our particular site distribution, sensitivity analyses of the network, as well as some grid optimization results. Impact of GLONASS data inclusion on the water vapor zenith and gradient estimates is studied by comparisons to a WVR. First comparisons of the tomographic results using real data with results from measurements from radiosondes launched in the vicinity the GPS network are also presented. The achievable time resolution of the obtained refractivity profiles can be from some minutes to hours depending on the data availability and the vertical spatial resolution used, which we set to vary within few hundreds of meters.
Global Navigation Satellite Systems, GNSS are used in many applications demanding millimetre-leve... more Global Navigation Satellite Systems, GNSS are used in many applications demanding millimetre-level accuracy in positioning. Such applications include monitoring of crustal movements. The GNSS technique also facilitates estimates of the tropospheric water vapour content, an important parameter in numerical weather predictions and climate research. The accuracy of estimated parameters is however degraded by many error sources. Some of those are related to the satellite system and the ground-based receivers. During 2006 the International GNSS Service, IGS implemented absolute antenna calibration for both satellite antennas and the different antenna types used at the ground-based stations. The use of such calibration values improves the accuracy as antenna type related errors are mitigated. However, unmodeled error sources still remain since a antenna couples electromagnetically with its surrounding environment. The absolute antenna calibrations, to be implemented by the IGS, will however not compensate for site- dependent effects. To further improve the accuracy, the site-dependent effects and their dependency on the direction of the observation need to be identified and removed in the GNSS data analysis. In this presentation the site-dependent error sources have been studied for the stations in the Swedish permanent GNSS network, SWEPOS, as well as some stations in the global IGS network. Strong similarities in terms of site-dependent effects were found. Differences in the site characteristics, caused by multipath and different antenna surroundings imply that an individual calibration of each station may be required needed. We have developed and evaluated two different methods for calibration of site dependent effects.
ABSTRACT A review of microwave radiometric (MWR) instruments, algorithms and ancillary data for d... more ABSTRACT A review of microwave radiometric (MWR) instruments, algorithms and ancillary data for deriving total attenuation and sky noise in propagation experiments is presented. The paper also includes reference to MWR networks, experimental databases and identification of the channel condition in propagation experiments and telecom systems.
2012 12th Specialist Meeting on Microwave Radiometry and Remote Sensing of the Environment (MicroRad), 2012
ABSTRACT Turbulence is an important effect for different applications including different aspects... more ABSTRACT Turbulence is an important effect for different applications including different aspects of electromagnetic propagation in the atmosphere, especially for frequencies in the microwave part of the spectrum. Our parameter of interest is an integrated effect, namely the propagation delay of a signal propagating through the Earth's atmosphere to a receiver on the ground. A simulation method has been implemented in order to predict the statistics of atmospheric turbulence. For systems where calibration for the wet delay is done by means of radiometric measurements there is an uncalibrated remainder of the variability due to imperfections of the method: (1) the radiometer location, which may not be identical to the location of the receiving antenna that is subject to calibration, (2) a pointing offset on the sky, and (3) the radiometer beam that may sense another volume of the air than does the original signal. The size of the remainder depends on the assumptions made about the atmospheric variability when the instrument is calibrated. The analysis shows that the overall performance of the system to a significant level depends on the uncertainty in the true range of variation for the turbulence strength parameter Cn2.
Global Navigation Satellite Systems (GNSS) have revolutionised positioning, navigation and timing... more Global Navigation Satellite Systems (GNSS) have revolutionised positioning, navigation and timing, becoming a common part of our everyday life. Aside from the more standard applications, GNSS has proved to be an accurate sensor of atmospheric water vapour, the most abundant greenhouse gas, accounting for 60-70% of warming, and of obvious importance in weather forecasting. Atmospheric humidity is one of the most important parameters for forecasting extreme weather events and monitoring climate change, but is under-sampled in current operational meteorological and climate observing systems. A new EU COST Action (ES1206): Advanced GNSS Tropospheric Products for Monitoring Extreme Weather Events and Climate (GNSS4SWEC) will address new and improved capabilities from concurrent developments in both GNSS and meteorological communities to improve (short-range) weather forecasting and climate projections. Multi-GNSS will be utilised to develop new, more advanced products, stimulating the fu...
Physics and Chemistry of the Earth, Part A: Solid Earth and Geodesy, 2001
ABSTRACT We present first results on the site coordinates and the atmospheric propagation delay p... more ABSTRACT We present first results on the site coordinates and the atmospheric propagation delay parameters from the permanent Global Positioning System site established at Chajnantor, in the Chilean Andes, at 5000 m altitude. The site is designed to ensure a long-term monument stability. The primary reason behind the site establishment is the future construction of the Atacama Large Millimeter Array (ALMA), a radio astronomy facility, at the same location. There is an interest to observe the site coordinate stability as well as the total radio propagation path delay variations which is possible employing the GPS technique. The fact that the site is known to have mainly a dry delay component makes it also a unique site for GPS accuracy studies. We present first results on the estimated site coordinates and the zenith total and wet atmospheric delays. Structure function results of the delays are included as well as the impact of using different elevation cut off angles in the data processing. We also discuss the performance of the mapping functions used for the estimation of the zenith delay terms.
The objective of this study, named AWARDS (Advanced microWAve Radiometers in Deep space Stations)... more The objective of this study, named AWARDS (Advanced microWAve Radiometers in Deep space Stations), is the preliminary design of a transmission Media Calibration System (MCS) to be located at an ESA Deep Space Antenna (DSA) site. The crucial aspect is the capability to accurately retrieve the tropospheric path delay along the line-of-sight of the deep space probe in order to allow precise tropospheric calibration of deep space observables (range and range-rate) with particular reference to the BepiColombo spacecraft and its primary DSA at Cebreros (ES). The study focuses on two main aspects which lead to the preliminary design of the Mercury Orbiter Radioscience Experiment (MORE) MCS: the characterization of current microwave radiometers (MWRs) available at ESA/ESTEC and the atmospheric fluctuation effects on the MCS error budget, in terms of the Allan standard deviation (ASD). In the course of the study, further critical aspects have been identified (effects of Sun contamination, ef...
The developments in global satellite navigation using GPS, GLONASS, and Galileo will yield more o... more The developments in global satellite navigation using GPS, GLONASS, and Galileo will yield more observations at various elevation angles. The inclusion of data acquired at low elevation angles allows for geometrically stronger solutions. The vertical coordinate estimate of a GPS site is one of the parameters affected by the elevation-dependent error sources, especially the atmospheric corrections, whose proper description becomes necessary. In this work, we derive time-series of normalized propagation delays in the neutral atmosphere using ray tracing of radiosonde data, and compare these to the widely used new mapping functions (NMF) and improved mapping functions (IMF). Performance analysis of mapping functions is carried out in terms of bias and uncertainty introduced in the vertical coordinate. Simulation runs show that time-correlated mapping errors introduce vertical coordinate RMS errors as large as 4 mm for an elevation cut-off angle of 5°. When simulation results are compared with a geodetic GPS solution, the variations in the vertical coordinate due to mapping errors for an elevation cut-off of 5° are similar in magnitude to those caused by all error sources combined at 15° cut-off. This is significant for the calculation of the error budget in geodetic GPS applications. The results presented here are valid for a limited area in North Europe, but the technique is applicable to any region provided that radiosonde data are available.
Physics and Chemistry of the Earth, Part A: Solid Earth and Geodesy, 2001
ABSTRACT We have made Monte Carlo simulations in order to quantify the influence of several error... more ABSTRACT We have made Monte Carlo simulations in order to quantify the influence of several error sources on the atmospheric delay derived in our regular “Precise Point Positioning” analysis of GPS data. Satellite position and clock errors were found to be the greatest sources of error. The RMS size of the atmospheric delay error due to these sources is suggested to be 4–8 mm, and the correlation of the delay error over 1000 km is 0.5-0.7. In our real-time analysis of the atmospheric delay satellite orbits and clock offsets are modelled in the Kalman filter. The modelled errors suggest an uncertainty in the atmospheric delay of 10–13 mm, and a decrease in the correlation coefficients to about 0.65 over a distance of 1000 km. For both the Precise Point Positioning and the real-time analysis the influence from antenna signal scatter will give additional error contributions.
We will draw attention to a new technique for retrieval of water vapor fields, which is based on ... more We will draw attention to a new technique for retrieval of water vapor fields, which is based on the use of a small network of GPS receivers, and is referred to as GPS tomography. A subset of the inverse theory utilizing the integrated values of the field distribution is called tomography. To demonstrate the technique we use a permanent local GPS network consisting of 8 stations and located in the G"{o}teborg area (Swe- den). Its baselines range from a few to 15 km. Tomographic results from this network are derived by applying a tomographic method on the measured slant delay values at each site. The method is implemented in the software package LOTTOS (LOcal Tropospheric Tomographic SOftware developed by Alejandro Flores) and is able to produce a tomographic solution of the water vapor induced refractivity in the tropo- sphere. In tropospheric tomography the slant delay values are obtained by combining the estimated zenith delay values with the horizontal gradients. The geometry using GPS is somewhat restricted compared to the classical tomographic approaches where one can choose the direction of the scanning rays. In the case of GPS tomography, we are limited by the satellite constellation and therefore in certain areas of the field that we want to reconstruct some "information gaps" will be present. In order to reduce these uncertainties, additional vertical and horizontal constraints should be included in the solution. Other ways of increasing the information could be: optimization of tomographic grid used, inclusion of more GPS stations and/or additional independent data from GLONASS, GALILEO (in the future), GPS occultation receivers, radioson- des or scanning pointed water vapor radiometers (WVR). We first present simulations of a tomographic solution based on our particular site distribution, sensitivity analyses of the network, as well as some grid optimization results. Impact of GLONASS data inclusion on the water vapor zenith and gradient estimates is studied by comparisons to a WVR. First comparisons of the tomographic results using real data with results from measurements from radiosondes launched in the vicinity the GPS network are also presented. The achievable time resolution of the obtained refractivity profiles can be from some minutes to hours depending on the data availability and the vertical spatial resolution used, which we set to vary within few hundreds of meters.
Global Navigation Satellite Systems, GNSS are used in many applications demanding millimetre-leve... more Global Navigation Satellite Systems, GNSS are used in many applications demanding millimetre-level accuracy in positioning. Such applications include monitoring of crustal movements. The GNSS technique also facilitates estimates of the tropospheric water vapour content, an important parameter in numerical weather predictions and climate research. The accuracy of estimated parameters is however degraded by many error sources. Some of those are related to the satellite system and the ground-based receivers. During 2006 the International GNSS Service, IGS implemented absolute antenna calibration for both satellite antennas and the different antenna types used at the ground-based stations. The use of such calibration values improves the accuracy as antenna type related errors are mitigated. However, unmodeled error sources still remain since a antenna couples electromagnetically with its surrounding environment. The absolute antenna calibrations, to be implemented by the IGS, will however not compensate for site- dependent effects. To further improve the accuracy, the site-dependent effects and their dependency on the direction of the observation need to be identified and removed in the GNSS data analysis. In this presentation the site-dependent error sources have been studied for the stations in the Swedish permanent GNSS network, SWEPOS, as well as some stations in the global IGS network. Strong similarities in terms of site-dependent effects were found. Differences in the site characteristics, caused by multipath and different antenna surroundings imply that an individual calibration of each station may be required needed. We have developed and evaluated two different methods for calibration of site dependent effects.
ABSTRACT A review of microwave radiometric (MWR) instruments, algorithms and ancillary data for d... more ABSTRACT A review of microwave radiometric (MWR) instruments, algorithms and ancillary data for deriving total attenuation and sky noise in propagation experiments is presented. The paper also includes reference to MWR networks, experimental databases and identification of the channel condition in propagation experiments and telecom systems.
2012 12th Specialist Meeting on Microwave Radiometry and Remote Sensing of the Environment (MicroRad), 2012
ABSTRACT Turbulence is an important effect for different applications including different aspects... more ABSTRACT Turbulence is an important effect for different applications including different aspects of electromagnetic propagation in the atmosphere, especially for frequencies in the microwave part of the spectrum. Our parameter of interest is an integrated effect, namely the propagation delay of a signal propagating through the Earth's atmosphere to a receiver on the ground. A simulation method has been implemented in order to predict the statistics of atmospheric turbulence. For systems where calibration for the wet delay is done by means of radiometric measurements there is an uncalibrated remainder of the variability due to imperfections of the method: (1) the radiometer location, which may not be identical to the location of the receiving antenna that is subject to calibration, (2) a pointing offset on the sky, and (3) the radiometer beam that may sense another volume of the air than does the original signal. The size of the remainder depends on the assumptions made about the atmospheric variability when the instrument is calibrated. The analysis shows that the overall performance of the system to a significant level depends on the uncertainty in the true range of variation for the turbulence strength parameter Cn2.
Global Navigation Satellite Systems (GNSS) have revolutionised positioning, navigation and timing... more Global Navigation Satellite Systems (GNSS) have revolutionised positioning, navigation and timing, becoming a common part of our everyday life. Aside from the more standard applications, GNSS has proved to be an accurate sensor of atmospheric water vapour, the most abundant greenhouse gas, accounting for 60-70% of warming, and of obvious importance in weather forecasting. Atmospheric humidity is one of the most important parameters for forecasting extreme weather events and monitoring climate change, but is under-sampled in current operational meteorological and climate observing systems. A new EU COST Action (ES1206): Advanced GNSS Tropospheric Products for Monitoring Extreme Weather Events and Climate (GNSS4SWEC) will address new and improved capabilities from concurrent developments in both GNSS and meteorological communities to improve (short-range) weather forecasting and climate projections. Multi-GNSS will be utilised to develop new, more advanced products, stimulating the fu...
Uploads
Papers by Gunnar Elgered