Characteristics of Topside Equatorial Ionospheric Irregularities: An Investigation Using Swarm Echo Measurements

Ionospheric irregularities are a significant source of error in GNSS positioning and navigation as they cause scintillations in trans-ionospheric signals. The elliptical orbit and the high-rate GPS and in-situ irregularity measurements of the Swarm Echo satellite provide a unique opportunity to examine characteristics and distribution of scintillation-causing irregularities. To obtain the results presented in this research, the satellite was re-oriented in 293 sets of novel experiments to make the GAP-O antenna point in the zenith direction. The surface current measurements of the IRM instrument were employed as a measure for in-situ irregularities. In the present thesis, I carried out three studies to investigate distribution, morphology, and scintillation impacts of irregularities that form in the equatorial topside ionosphere during post-sunset hours. The results of the first study show that large-scale ionospheric structures (larger than 80 km) form predominantly below 500 km between 18 and 21 MLT, whereas small-scale irregularities (as small as 160 m) occur at all examined altitudes (330-1280 km) and post-sunset hours (18-24 MLT). The second study presents a novel approach to generate regional maps of small-scale scintillation-producing irregularities using single-satellite observations. I demonstrate that these maps, which are produced using different ionospheric GPS indices, including S4, TEC, and ROTI, can determine the horizontal geo-locations of small-scale irregularities, albeit with large uncertainties in the cross-track direction. The third study presents the spectral characteristics of simultaneous in-situ irregularities and GPS signal intensities. The PSDs of in-situ irregularities show one-component power law behavior, with the most common irregularity spectral index values between 1 and 2. The spectral index for stronger irregularities approaches 1.7. The GPS signal intensity PSDs also obey a power law, with spectral index values within the range of 1.8 and 2.2. Furthermore, the roll-off frequencies obtained from the intensity PSDs are between 0.4 and 2.5 Hz, Which is considerably higher than Fresnel frequencies estimated from ground-based GPS measurements at low latitudes, which fall between 0.2 and 0.45 Hz

Proceedings of the NASA Symposium on Global Wind Measurements

This Proceedings contains a collection of the papers which were presented at the Symposium and Workshop on Global Wind Measurements. The objectives and agenda for the Symposium and Workshop were decided during a planning meeting held in Washington, DC, on 5 February 1985. Invited papers were presented at the Symposium by meteorologists and leading experts in wind sensing technology from the United States and Europe on: (1) the meteorological uses and requirements for wind measurements; (2) the latest developments in wind sensing technology; and (3) the status of our understanding of the atmospheric aerosol distribution. A special session was also held on the latest development in wind sensing technology by the United States Air Force

The science case for the EISCAT_3D radar

The EISCAT (European Incoherent SCATer) Scientific Association has provided versatile incoherent scatter (IS) radar facilities on the mainland of northern Scandinavia (the EISCAT UHF and VHF radar systems) and on Svalbard (the electronically scanning radar ESR (EISCAT Svalbard Radar) for studies of the high-latitude ionised upper atmosphere (the ionosphere). The mainland radars were constructed about 30 years ago, based on technological solutions of that time. The science drivers of today, however, require a more flexible instrument, which allows measurements to be made from the troposphere to the topside ionosphere and gives the measured parameters in three dimensions, not just along a single radar beam. The possibility for continuous operation is also an essential feature. To facilitatefuture science work with a world-leading IS radar facility, planning of a new radar system started first with an EU-funded Design Study (2005–2009) and has continued with a follow-up EU FP7 EISCAT_3D Preparatory Phase project (2010–2014). The radar facility will be realised by using phased arrays, and a key aspect is the use of advanced software and data processing techniques. This type of software radar will act as a pathfinder for other facilities worldwide. The new radar facility will enable the EISCAT_3D science community to address new, significant science questions as well as to serve society, which is increasingly dependent on space-based technology and issues related to space weather. The location of the radar within the auroral oval and at the edge of the stratospheric polar vortex is also ideal for studies of the long-term variability in the atmosphere and global change. This paper is a summary of the EISCAT_3D science case, which was prepared as part of the EU-funded Preparatory Phase project for the new facility. Three science working groups, drawn from the EISCAT user community, participated in preparing this document. In addition to these working group members, who are listed as authors, thanks are due to many others in the EISCAT scientific community for useful contributions, discussions, and support

The science case for the EISCAT_3D radar

The EISCAT (European Incoherent SCATer) Scientific Association has provided versatile incoherent scatter (IS) radar facilities on the mainland of northern Scandinavia (the EISCAT UHF and VHF radar systems) and on Svalbard (the electronically scanning radar ESR (EISCAT Svalbard Radar) for studies of the high-latitude ionised upper atmosphere (the ionosphere). The mainland radars were constructed about 30 years ago, based on technological solutions of that time. The science drivers of today, however, require a more flexible instrument, which allows measurements to be made from the troposphere to the topside ionosphere and gives the measured parameters in three dimensions, not just along a single radar beam. The possibility for continuous operation is also an essential feature. To facilitatefuture science work with a world-leading IS radar facility, planning of a new radar system started first with an EU-funded Design Study (2005–2009) and has continued with a follow-up EU FP7 EISCAT_3D Preparatory Phase project (2010–2014). The radar facility will be realised by using phased arrays, and a key aspect is the use of advanced software and data processing techniques. This type of software radar will act as a pathfinder for other facilities worldwide. The new radar facility will enable the EISCAT_3D science community to address new, significant science questions as well as to serve society, which is increasingly dependent on space-based technology and issues related to space weather. The location of the radar within the auroral oval and at the edge of the stratospheric polar vortex is also ideal for studies of the long-term variability in the atmosphere and global change. This paper is a summary of the EISCAT_3D science case, which was prepared as part of the EU-funded Preparatory Phase project for the new facility. Three science working groups, drawn from the EISCAT user community, participated in preparing this document. In addition to these working group members, who are listed as authors, thanks are due to many others in the EISCAT scientific community for useful contributions, discussions, and support

Scintillation on global navigation satellite signals and its mitigation

PhD ThesisThe scintillation effects on the Global Positioning system (GPS) or other GNSS (global navigation satellite system) receivers have been investigated by many researchers and several mitigation strategies have been proposed in this regard but the problem is not yet fully solved. This thesis covers the investigation of scintillation effects on GPS receivers and developing a mitigation approach which can play an important role in mitigating the effects of scintillation on these and other GNSS receivers. Firstly, a new GPS signal acquisition method known as the repetitive block acquisition (RBA) is presented which can be used to speed up the GPS signal acquisition in case fast acquisition is required. This acquisition method is implemented using coarse-acquisition (C/A) codes and tested by collecting real GPS data. The RBA method can also be used for other codes as well. It is rather difficult to show that how scintillation affects the acquisition process in a GPS receiver because mostly it results in tracking loop loss of lock due to cycle slip. However, during strong amplitude scintillation which is usually most important at low or near-equatorial latitudes, deep power fades resulting from amplitude scintillation result in the selection of long data records which leads to slow acquisition due to long acquisition times. It is shown in this thesis that, by using the RBA method, the acquisition time can be reduced to a fairly low level by reducing the number of computations involved in acquisition compared to other well-known methods such as the parallel FFT-based method and zero padding method (ZP). Secondly, the scintillation effects on the GPS tracking loop have also been investigated in this thesis and, based on this investigation, a new improved analogous phase scintillation index, σw φa, has been designed to more accurately represent the phase scintillation intensity at European high latitudes. This is then also validated using the real GPS data from Trondheim, Norway (63.41o N, 10.4o E). The σw φa uses dual frequency (L1 & L2) based vi time and spatial variations of total electron contents (TEC) at 1 Hz for estimating the phase scintillation values. For deriving the σw φa, the low frequency TEC fluctuations due to Doppler shift of the satellite/receiver motion and also due to the slowly varying background ionosphere need to be removed in order to consider only the high frequency TEC fluctuations which are responsible for scintillation due to the fast moving electron density irregularities which is done by using the wavelet transform. The σw φa is really an improved version of σφa where, rather than using time-invariant digital high pass filters (HPF), which according to several researchers are in-appropriate for filtering the non-stationary raw GPS signals affected by the ionospheric scintillation, a wavelet-based filtering technique is used. Although, the wavelet transform has been used previously in detrending raw amplitude and phase observations at 50 Hz for estimating the scintillation indices (amplitude and phase), due to the high sample data rate it may not be desirable to use this transform due to its very high computational cost. Since, σw φa uses TEC data at 1 Hz so this problem has been overcome. The performance of the new improved index (σw φa) is investigated and is also compared with the previously proposed σφa and σφ indices using one whole year of data from a GPS receiver at Trondheim, Norway (63.41o N, 10.4o E). The raw TEC observations and the σw φa index are then used in estimating the tracking phase jitter using two different methods. The phase jitter helps in defining the tracking thresholds for the tracking loops in a receiver which is useful in updating the tracking loop parameters during scintillation conditions as required in robust GPS/GNSS receiver designs because the phase jitter decides how wide the tracking (and thus the noise) bandwidth should be allowed in the tracking loop for the tracking to remain efficient. It is shown that if the phase jitter is estimated using the new proposed methods, generally a better estimate can be obtained compared to the previously proposed phase jitter estimation methods which employs σφa and σφ indices. These new phase jitter estimation methods can further be used in GPS/GNSS receivers for updating the tracking loop parameters during scintillation conditions and hence can serve as a good alternative for mitigating the effects of scintillation on GPS/GNSS receivers.Higher Education Commission (HEC) of Pakistan and the Sukkur Institute of Business Administration, Pakistan

Exploiting new GNSS signals to monitor, model and mitigate the ionospheric effects in GNSS

Signals broadcast by the Global Navigation Satellite Systems (GNSS) enable global, autonomous, geo-spatial positioning exploited in the areas such as geodesy, surveying, transportation and agriculture. The propagation of these signals is affected as they propagate through the Earth's upper atmosphere, the ionosphere, due to the ionic and electronic structure of the ionosphere. The ionosphere, a highly dynamic and spatially and temporally variable medium, can be the largest error source in Global Navigation Satellite System (Klobuchar 1991) in the absence of the Selective Availability. Propagation effects due to the ionosphere lead to errors in the range measurements, impact on receiver signal tracking performance and influence the GNSS positioning solution. The range error can vary from 1 to 100m depending on time of day, season, receiver location, conditions of the earth's magnetic field and solar activity (Hofmann-Wellenhof et al. 2001). This thesis focuses on modelling, monitoring and mitigating the ionospheric effects in GNSS within the scope of GNSS modernization, which introduces new signals, satellites and constellations. The ionosphere and its effects on GNSS signals, impact of the ionospheric effects at the receiver end, predicted error bounds of these effects under different solar, geomagnetic and ionospheric conditions, how these effects can be modelled and monitored with current and new (possible with GNSS modernization) correction approaches, degradation in the GNSS positioning solution and mitigation techniques to counter such degradation are investigated in this thesis. Field recorded and simulated data are considered for studying the refractive and diffractive effects of the ionosphere on GNSS signals, signal tracking performance and position solution. Data from mid-to-high latitudes is investigated for the refractive effects, which are due to dispersive nature of the ionosphere. With the use of multi-frequency, multi-constellation receivers, modelling of the refractive effects is discussed through elimination and estimation of these effects on the basis of dual and triple frequency approaches, concentrating on the benefit of the new GNSS signals. Data from the low latitudes is considered for studying the diffractive effects of the ionosphere, scintillation in particular, in GNSS positioning, and possible mitigation techniques to counter them. Scintillation can have a considerable impact on the performance of GNSS positioning by, for instance, increasing the probability of losing phase lock with a signal and reducing the accuracy of pseudoranges and phase measurements. In this sense, the impact of scintillation on signal tracking performance and position solution is discussed, where a novel approach is proposed for assessing the variance of the signal tracking error during scintillation. The proposed approach also contributes to the work related with scintillation mitigation, as discussed in this thesis. The timeliness of this PhD due to the recent and increasingly active period of the next Solar Cycle (predicted to reach a peak around 2013) and to the ongoing GNSS modernization give this research an opportunity to enhance the ionospheric knowledge, expertise and data archive at NGI, which is rewarding not only for this PhD but also for future research in this area

Exploiting new GNSS signals to monitor, model and mitigate the ionospheric effects in GNSS

Signals broadcast by the Global Navigation Satellite Systems (GNSS) enable global, autonomous, geo-spatial positioning exploited in the areas such as geodesy, surveying, transportation and agriculture. The propagation of these signals is affected as they propagate through the Earth's upper atmosphere, the ionosphere, due to the ionic and electronic structure of the ionosphere. The ionosphere, a highly dynamic and spatially and temporally variable medium, can be the largest error source in Global Navigation Satellite System (Klobuchar 1991) in the absence of the Selective Availability. Propagation effects due to the ionosphere lead to errors in the range measurements, impact on receiver signal tracking performance and influence the GNSS positioning solution. The range error can vary from 1 to 100m depending on time of day, season, receiver location, conditions of the earth's magnetic field and solar activity (Hofmann-Wellenhof et al. 2001). This thesis focuses on modelling, monitoring and mitigating the ionospheric effects in GNSS within the scope of GNSS modernization, which introduces new signals, satellites and constellations. The ionosphere and its effects on GNSS signals, impact of the ionospheric effects at the receiver end, predicted error bounds of these effects under different solar, geomagnetic and ionospheric conditions, how these effects can be modelled and monitored with current and new (possible with GNSS modernization) correction approaches, degradation in the GNSS positioning solution and mitigation techniques to counter such degradation are investigated in this thesis. Field recorded and simulated data are considered for studying the refractive and diffractive effects of the ionosphere on GNSS signals, signal tracking performance and position solution. Data from mid-to-high latitudes is investigated for the refractive effects, which are due to dispersive nature of the ionosphere. With the use of multi-frequency, multi-constellation receivers, modelling of the refractive effects is discussed through elimination and estimation of these effects on the basis of dual and triple frequency approaches, concentrating on the benefit of the new GNSS signals. Data from the low latitudes is considered for studying the diffractive effects of the ionosphere, scintillation in particular, in GNSS positioning, and possible mitigation techniques to counter them. Scintillation can have a considerable impact on the performance of GNSS positioning by, for instance, increasing the probability of losing phase lock with a signal and reducing the accuracy of pseudoranges and phase measurements. In this sense, the impact of scintillation on signal tracking performance and position solution is discussed, where a novel approach is proposed for assessing the variance of the signal tracking error during scintillation. The proposed approach also contributes to the work related with scintillation mitigation, as discussed in this thesis. The timeliness of this PhD due to the recent and increasingly active period of the next Solar Cycle (predicted to reach a peak around 2013) and to the ongoing GNSS modernization give this research an opportunity to enhance the ionospheric knowledge, expertise and data archive at NGI, which is rewarding not only for this PhD but also for future research in this area

FIELD OBSERVATIONS AND MODEL SIMULATIONS OF LOW-LEVEL FLOWS OVER THE MID-ATLANTIC DURING AUGUST 1-5, 2006

For years, basic mountain, sea breeze, and low-level jet (LLJ) circulations have been studied, usually in locations with a high frequency of occurrence, sharp gradients, or significant geographic prominence. However, there is evidence that similar circulations exist in non-classic locations with more mild topography and atmospheric gradients. One such understudied area is the U.S. Mid-Atlantic region. The Water Vapor Variability - Satellite/Sondes (WAVES) 2006 field campaign provided a contiguous 5-day period of concentrated high resolution observations to examine fine-scale details of a weather pattern typical of the Mid-Atlantic summertime. These measurements presented an opportunity for an intensive modeling study to further investigate peculiar phenomena with verification against research-grade observations. The observations captured two significant events: an official LLJ and a cold front with a prefrontal trough. A pronounced diurnal cycle was revealed which can be categorized into three stages: (1) daytime growth of the planetary boundary layer (PBL), (2) flow intensification into a LLJ regime after dusk, and (3) interruption by downslope winds (DW) after midnight. The third stage is most interesting owing to the lack of literature documenting similar occurrences in the Mid-Atlantic, which can impact air quality forecasting. Prior to high resolution modeling of the case study, sensitivity studies were conducted examining four areas to which the model was believed most sensitive: (1) initial condition data, (2) cumulus schemes, (3) PBL parameterizations, and (4) initialization times. Results also revealed shortcomings in model precipitation and PBL profiles, model biases, urban anomalies, and tendencies for forecast convergence. High resolution regional modeling showed the evolution of these nocturnal events and were verified against WAVES observations. A hybrid solenoidal influenced afternoon and early evening circulation east of the mountains. Afternoon deepening of a lee trough by an oscillating warm air band influenced low-level wind fields. Wind flow was further influenced by the thermal wind that originated over sloping terrain. Airflow traversed the Appalachian barrier and moved down the east flank of the Appalachians with katabatic and hydraulic contributions. This DW swept the LLJ regime off to the southeast. The prefrontal LLJ outflow in the Midwest strengthened DW events as the cold front approached

Evaluation of precipitation forecasts by polarimetric radar

Over the last years, weather services have developed a new generation of high resolution mesoscale numerical weather prediction (NWP) models with the aim to explicitly predict convection. New methods are required to validate the representation of precipitation processes in these NWP models against observations. Polarimetric radar systems are especially suited for model validation as they provide information on the intensity and the microphysical characteristics of a precipitation event at a high temporal and spatial resolution. However, the observations can not be directly employed for model evaluation as polarimetric radar systems do not explicitly measure the parameters represented in microphysical parameterization schemes. In order to establish a relationship and allow for a direct comparison between the model parameters and the observations, the polarimetric radar forward operator SynPolRad (Synthetic Polarimetric Radar) has been developed. SynPolRad simulates synthetic polarimetric radar quantities out of model forecasts which permits an evaluation in terms of observed quantities. In a first step, the synthetic reflectivity, LDR, and ZDR are computed from predicted bulk water quantities and in a second step, the beam propagation in the model domain is simulated under consideration of refractivity and attenuation effects. In order to successfully employ SynPolRad for model evaluation purposes, the link between the forward operator and the mesoscale model has to conform as closely as possible to the model assumptions. However, in the case of a polarimetric radar forward operator not all the input parameters are defined by the model. Within this work, these free parameters are derived on theoretical terms accordingly to the model assumptions such that the polarimetric quantities match the thresholds of a hydrometeor classification scheme. Furthermore, special care is given to the representation of brightband signatures. The application of SynPolRad on two case studies proves the potential of the new method. A stratiform and a convective case study are chosen to assess the ability of mesoscale models to represent precipitation in different dynamical regimes. LMK (Lokal-Modell-Kürzestfrist) and MesoNH (Mesoscale Non-Hydrostatic Model) simulations considering different microphysical parameterization schemes are evaluated. The evaluation concentrates on the representation of life cycle, intensity, and the spatial distribution of synthetic reflectivity, LDR, and ZDR. Furthermore, hydrometeor types derived from the observed and synthetic polarimetric quantities employing a classification scheme are compared. Large discrepancies are found between the model simulations and the observations. However, the consideration of an additional ice hydrometeor category in the 3 component scheme significantly improves the performance of the LMK

Numerical modelling of mesoscale atmospheric dispersion

Fall 1992.Includes bibliographical references