Showing papers on "Total electron content published in 1999"

Analysis and validation of GPS/MET radio occultation data in the ionosphere

Abstract: Global Positioning System (GPS) radio occultation signals received by a low Earth orbit (LEO) satellite provide information about the global distribution of electron den- sity in the ionosphere. We examine two radio occultation inversion algorithms. The first algo- rithm utilizes the Abel integral transform, which assumes spherical symmetry of the electron density field. We test this algorithm with two approaches: through the computation of bend- ing angles and through the computation of total electron content (TEC) assuming straight line propagation. We demonstrate that for GPS frequencies and for observations in LEO, the as- sumption of straight-line propagation (neglecting bending) introduces small errors when monitoring the F2 layer. The second algorithm, which also assumes straight-line propagation, is a three-dimensional (3-D) inversion constrained with the horizontal structure of a priori electron density fields. As a priori fields we use tomographic solutions and the parameterized real-time ionospheric specification model (PRISM) when adjusted with ionosonde data or ground-based GPS vertical TEC maps. For both algorithms we calibrate the occultation data by utilizing observations from the part of the LEO that is closer to the GPS satellite. For in- versions we use dual-frequency observational data (the difference of L1 and L2 phase ob- servables) which cancel orbit errors (without applying precise orbit determination) and clock errors (without requiring synchronous ground data) and thus may allow inversions to be computed close to real time in the future. The Abel and 3-D constrained algorithms are vali- dated by statistically comparing 4 days of inversions with critical frequency (foF2) data from a network of 45 ionosonde stations and with vertical TEC data from the global network of GPS ground receivers. Globally, the Abel inversion approach agrees with the foF2 correlative data at the 13% rms level, with a negligible mean difference. All tested 3-D constrained in- version approaches possess a statistically significant mean difference when compared with the ionosonde data. The vertical TEC correlative comparisons for both the Abel and 3-D con- strained inversions are significantly biased (-30%) by the electrons above the 735-km LEO altitude.

Simultaneous Global Positioning System observations of equatorial scintillations and total electron content fluctuations

Abstract: One aspect of the Global Positioning System (GPS) is the potential to conduct geophysical research, and worldwide networks of GPS receivers have been established to exploit this potential. Several research groups have begun using this global GPS data to study ionospheric total electron content (TEC) variations, also referred to as GPS phase fluctuations, as surrogates for ionospheric scintillations. This paper investigates the relationship between GPS amplitude scintillations and TEC variations for the same line of sight using observations from Ancon, Peru. These observations were taken under equatorial spread F conditions for three nights in April 1997. As expected, only when the spectrum of TEC fluctuations includes significant power at the Fresnel scale do scintillations appear. We also find that when the TEC fluctuation spectrum includes the Fresnel scale, the S4 scintillation index is roughly proportional to measures of TEC fluctuation for the weak scintillations observed. The proportionality constant varies from night to night, however, casting doubt on the ability to predict GPS S4 successfully from TEC fluctuation data alone. We also present a simple theoretical phase screen model and show that if a relationship between TEC fluctuation measures and S4 exists, that relationship depends on the power spectrum of phase variations at the screen. Unfortunately, the available TEC data, at 30 s per sample (with some aliasing apparently permitted), offer limited spectral information. A preliminary comparison of 1 s/sample data with the same data decimated to a 30 s/sample interval suggests, however, that the level of successful S4 prediction, based on TEC fluctuation measures alone, is comparable at either sample rate.

Ionospheric total electron content response to solar eclipses

Abstract: On October 24, 1995, and March 9, 1997, two solar eclipse events occur. It is therefore of interest to investigate how the ionosphere responded to the eclipses. Five global positioning system (GPS) ground-based receivers are specifically designed to observe large-scale ionospheric variations over the geomagnetic equatorial, equatorial anomaly crest, and midlatitude regions. Two-dimensional images of ionospheric total electron content (TEC) during the two eclipse periods are constructed. The deviations in the TEC images on eclipse days from those on reference days show that during the eclipse days the ionosphere experienced some large-scale changes. Four features of the TEC deviations, pre-ascension (PA), major depression (MD), sunset ascension (SA), and secondary depression (SD) have been observed. A detailed study shows that in geomagnetic low latitudes, PAs are possibly related to the locations of the equatorial anomaly crest. The latitudinal location, amplitude, and occurrence time of MDs suggest that the fountain effect is essential. SAs and SDs occurring in geomagnetic equatorial and low latitudes and appearing respectively before/around and after local sunset indicate that the prereversal enhancement plays an important role.

Ionospheric effects on synthetic aperture radar at 100 MHz to 2 GHz

Abstract: Recently, there has been increasing interest in the use of spaceborne synthetic aperture radar (SAR) for measuring forest biomass However, it is noted that conventional SAR using C-band or higher frequencies cannot penetrate into foliage, and therefore the biomass measurements require longer wavelengths, typically P-band (500 MHz) It is also known that the ionosphere is highly dispersive, causing group delay and broadening of pulses The variance of the refractive index fluctuations due to turbulence is approximately proportional toƒ−4 In addition, the Faraday rotation due to the geomagnetic field in the ionosphere becomes significant This paper presents an analysis with numerical examples of the following effects in the frequency range from 100 MHz to 2 GHz in order to show the frequency dependence and the effects of total electron content (TEC) of the ionosphere First, the ionospheric turbulence can reduce the coherent length below the equivalent aperture size, and the azimuthal resolution becomes greater than D/2 where D is the antenna aperture size Second, the ionospheric dispersion causes a shift of the imagery due to the group velocity Third, the dispersion also creates broadening of the pulse In addition, multiple scattering due to ionospheric turbulence gives rise to pulse broadening Fourth, we consider the Faraday rotation effect and show that the ellipticity change is negligible, but the orientation angle changes significantly at P-band Numerical examples are shown using typical ionospheric parameters, turbulence spectrum, and TEC values

The influence of the protonosphere on GPS observations: Model simulations

Abstract: The accuracy of the Global Positioning System (GPS) satellite navigation system can be degraded by propagation effects on ray paths through the ionized atmosphere. The bulk of the plasma resides in the F layer, and models of total electron content have been developed to compensate for the effects of this ionization. However, use of the GPS system involves long ray paths through the tenuous hydrogen-based plasma of the protonosphere, and little is known about the electron content in this region. In the present study, the Sheffield University plasmasphere ionosphere model has been used to determine the electron content on the protonospheric section of GPS ray paths. Results are presented for stations at midlatitudes in the European and American sectors at both extremes of the solar cycle. The results are discussed in terms of the geometry of the flux tubes and the known behavior of the plasma.

The midlatitude ionosphere during the total solar eclipse of March 9, 1997

Abstract: The total solar eclipse of March 9, 1997, was visible from some regions of China, Mongolia, and East Siberia during 0045–0130 UT. The eclipse coincided with a relatively long geomagnetically quiet period. During the total solar eclipse, the observations included oblique-incidence ionograms recording; also measurement the total electron content along specified directions to the visible Global Positioning System satellites and the Doppler sounding on various HF ray paths over the region under consideration were carried out. This paper presents results derived from studying the spatial pattern of midlatitude ionospheric response to this eclipse on the basis of a numerical simulations. Calculations have been executed using a model for ionosphere-plasmasphere coupling. Comparison of model results with data of all measurements showed a good qualitative and quantitative agreement. It is shown that by and large the behavior of the ionosphere during the eclipse manifests itself as a short-lasting (∼l-hour) rearrangement to nighttime conditions with the ion flow directed downward from the plasmasphere, as a rise of the F2 layer maximum by about 20 km, and as a twofold increase in electron density at the height of the maximum during the eclipse's totality phase. The electron temperature decreases by 200–400 K, while the ion temperature drops only slightly. It is found that changes in the spatial distribution of electron density along the HF ray paths during the eclipse give rise to variations of Doppler frequency shift with amplitudes of about 1 Hz and a duration of about 120 min. The findings reported in this paper do not validate the hypothesis that solar eclipses generate atmospheric gravity waves and associated traveling ionospheric disturbances.

The contribution of the protonosphere to GPS total electron content: Experimental measurements

Abstract: Global Positioning System (GPS) satellites have orbital altitudes of about 20,200 km, while satellites in the Navy Ionospheric Monitoring System (NIMS) constellation are in circular orbits at heights of about 1100 km. Independent measurements of the electron content in the ionized atmosphere can be made using the radio signals from both satellite constellations. Differences between the two estimates can be related to the electron content on the GPS ray paths above 1100 km, through the tenuous plasma of the protonosphere. Results are reported from some 21 months of simultaneous observations of both GPS and NIMS transmissions at a European midlatitude station at solar minimum. It is shown that the average differences between the electron contents measured by the two systems are in broad agreement with the predictions from an earlier modeling study of the effects of the protonosphere on GPS total electron content. The expected influence of ray path / flux tube geometry and the rapid depletion and slow refilling of the protonosphere in response to geomagnetic storm activity can be seen in the averaged measurements.

Long-term observations of VHF scintillation and total electron content near the crest of the equatorial anomaly in the Indian longitude zone

Abstract: Ionospheric VHF scintillation (SI ≥ 3 dB and saturated level) and total electron content (TEC) data obtained at Calcutta (subionospheric 21°N, 92.7°E geographic, 27°N dip) and ionosonde data at Kodaikanal ( 10.2°N, 77.5°E geographic, 3.5°N dip) for the period 1977-1990 have been analysed to show the importance of electrodynamic drift near the magnetic equator in controlling nighttime ionospheric F region ionization and irregularities in the equatorial region. Such long-term observations extending over a period of more than 13 years are possibly being reported for the first time from a location situated near the equatorial anomaly crest. Frequent and intense VHF scintillations near the equatorial anomaly crest during equinoctial and December solstice months around solar maximum years, have been identified with the equatorial F region irregularities. Simultaneous measurement of TEC at the same location shows that during solar maximum years the high F region ambient ionization is sustained for several hours in the postsunset period, often showing secondary enhancements during equinoctial months. Under solar minimum epoch, when scintillation is sparse, TEC in the above period shows a rapid decrease. At Kodaikanal, situated near the magnetic equator, during the equinoctial and December solstice months of solar maximum years, h/F values rise by more than 100 km in about an hour around sunset. These features are seldom observed during solar minimum epoch. A causative connection among h/F variation near the magnetic equator and the maintenance of high ambient ionization and occurrence of scintillation near the anomaly crest is established. Further, scintillation occurrence during the May-July months shows a remarkable hysteresis effect with solar activity level.

The effect of the protonosphere on the estimation of GPS total electron content: Validation using model simulations

Abstract: Simulated observations of total electron content (TEC) along ray paths from Global Positioning System (GPS) satellites have been used to validate the estimation of TEC using GPS measurements. The Sheffield University plasmasphere ionosphere model (SUPIM) has been used to create electron densities that were integrated along ray paths from actual configurations of the GPS constellation. The resultant slant electron contents were then used as inputs to validate the self-calibration of pseudo-range errors (SCORE) process for the determination of TEC from GPS observations. It is shown that if the plasma resides only in the ionosphere below 1100 km, then the SCORE procedure determines the TEC to a high degree of accuracy. When the contribution of the electrons in the protonosphere above 1100 km is included, the analysis results in TEC estimates that are high by some 2 TEC units (TECU) for conditions appropriate to European midlatitudes at solar minimum. However, if a restriction is placed in the analysis on use of observations equatorward of the station, then allowance can be made for the effect of the protonosphere. It is shown that with appropriate selection of the boundary for the observations, TEC can be estimated by SCORE to better than 1 TECU for the conditions of the simulation. Sample results are included from actual experimental observations using GPS to demonstrate the effect of compensation for the protonospheric plasma.

Tomographic reconstruction of the ionosphere using generalized singular value decomposition

Abstract: The total electron content (TEC) of the ionosphere, being the line integral of the electron density encou ntered by a transionospheric wave along its path of propagation, does not convey any information about the structures within the ionosphere. In recent years, computer-aided ionospheric tomography (CIT) has been increasingly used for the investigation of ele ctron density distribution in the low and mid-latitude ionosphere. A number of CIT algorithms have been proposed, but they have their individual shortcomings. In this communication, we present a new algorithm based on generalized singular value decomposition technique (GSVD). The algorithm was tested with TEC data generated using the International Reference Ionosphere (IRI-1995) model for the Indian low latitudes and measured by the Global Positioning System satellites along a chain of European mid-latitude stations. The reconstructed images using the GSVD technique have been found to match well with the model and the measured data. THE ionospheric total electron content (TEC) has been measured for many years now using the Faraday rotation and differential Doppler techniques. These measurements provide information about the temporal and spatial vari ation of TEC. Multistation TEC data have been used to study the horizontal (latitudinal as well as longitud inal) variation of ionospheric structure. However, since TEC represents the integral of the electron density along the path, the information about the spatial variation of ele ctron density along the path caused by the irregular stru ctures cannot be recover ed using conventional techniques. Austen et al. 1 demonstrated the feasibility of u sing the computerized tomography technique to successfully reconstruct ionospheric structures from TEC data. Computerized tomographic (CT) technique can be used when the line integral of the parameter to be determined (e.g. ele ctron density) is the measured data through the region of interest. CT tec hnique has had a revolutionary impact in the field of medical diagnostic imaging and is now ro utinely used to produce high quali ty images of a section of the human body. The technique has also been applied to the study of ocean structures

Ionospheric responses to a total solar eclipse deduced by the GPS beacon observations

Abstract: The total electron content (TEC) data during the total eclipse of March 9, 1997 were collected, which were observed by means of nine GPS receivers located at the eastern Asia. The responses of total TEC to the eclipse were analyzed. The results show that: 1) the eclipse led to apparent decrement in TEC that lasted for six to eight hours; 2) the maximum decrement occurred after the middle of the eclipse with time-delays varying from twenty minutes to about three hours; 3) the maximum absolute deviations of TEC on the eclipse day do not show a simple and consistent relationship to the maximum solar obscuration.

Satellite technology glimpses ionospheric response to solar eclipse

Abstract: The millennium's last solar eclipse was observed on August 11, 1999, over most of Europe, along the northeast coast of North America, and in the Near East and Middle East The eclipse was also observed by the global navigation satellite systems (GNSS), the Global Positioning System (GPS), and the Russian GNSS (GLONASS), because the transmitted signals can be used to infer the total electron content of the ionosphere Disruption of photoionization and thermospheric heating leads to numerous complex phenomena in the ionosphere Because of the supersonic speed of the Moon's cool shadow in the atmosphere, atmospheric gravity waves may be generated; these propagate upward and trace as traveling ionosphere disturbances in the ionosphere [Chimonas and Hines, 1970] Analogous to the atmospheric pressure on the Earth's surface, total electron content (TEC) can be understood as the “pressure” of the electron gas of the ionosphere, which will decrease with reduced energy input

A Global Positioning System Receiver for Monitoring Ionospheric Total Electron Content.

Abstract: : A system has been built around a dual-frequency NovAtel MiLLenniumTM Global Positioning System (GPS) receiver in order to measure the Total Electron Content (TEC) of the ionosphere and to detect radio wave scintillation. The software has been designed and developed to provide interactive control of the receiver and the logging of its data to an external removable disk. In addition, it features a graphical user interface which includes displays both in real-time and for the previous 24 hours of satellite locations and measured TEC values. Adequate calibration of TEC values is widely recognised as a difficult problem and in this implementation, calibration of the real-time results has been achieved satisfactorily to first order. The real-time displays make the equipment particularly useful for campaigns, but the more useful application is for routine unattended logging. Two systems have been deployed, one each in Malaysia and Indonesia under the Regional Engagement program. The intent is to monitor both ionospheric behaviour and GPS performance in equatorial regions as we approach the next peak in solar cycle activity in about the year 2000.

The protonospheric contribution to GPS total electron content: Two‐station measurements

Abstract: Results are presented from an experiment to estimate the contribution of plasma on ray paths through the protonosphere to measurements of total electron content (TEC) using Global Positioning System (GPS) signals. Simulations using the Sheffield University plasmasphere ionosphere model show that observations of GPS satellites made at two stations separated by a few degrees of latitude could involve a common ionospheric volume but very different intersection geometries of the ray paths with protonospheric flux tubes. Experimental results demonstrate that, on average, higher equivalent vertical TECs are measured on ray paths to the south than those to the north of the European midlatitude stations considered here. The observations are discussed in terms of the known asymmetries of the protonospheric flux tubes, and caution is advised in the use of thin-shell ionospheric models for precise determination of TEC or correction for its effects on GPS systems.

Equatorial F region irregularity morphology during an equinoctial month at solar minimum

Abstract: A large number of instruments was used in October 1996 to record activities in the equatorial ionosphere above South America. In a month at solar minimum, data were obtained at various levels of magnetic activity and various levels of ionospheric irregularity development. With this multi-instrumented study, it was possible to utilize optical data, radar, GPS transmissions, and ionosondes at various sites in the equatorial region. The concept of this paper is to review the plethora of events which occurred during this month with a view to describing the interrelationship of the wide variety of irregularity developments. Data were obtained on nights when no irregularities were observed at any location in the equatorial region across South America. There were nights when only localized irregularity structures with relatively narrow latitudinal and longitudinal effects were noted close to the magnetic equator. We noted the occasional presence in the 02–06 local time period of plume structures with data available from optical observations as well as from phase and amplitude scintillation. During a major magnetic storm on one night, October 22–23, a long lasting high altitude plume was detected by the Jicamarca radar. On this night, irregularities were noted all across South America and even beyond the western and eastern coasts. This plume produced ionospheric effects which could be traced to turbulence at over 2000 km above the magnetic equator. With additional data from high latitude stations and from Guam and Kwajelein, it was possible to link and compare irregularity development in the same time period over a large portion of the globe. The aim of this paper is to give a day-to-day picture of the occurrence and intensity of equatorial irregularity development over a month-long period rather than a short case study or the converse, long term statistics over several seasons. Using this database and the modeling of total electron content as a function of solar flux, we outline the possibilities and limitations for forecasting irregularity activity in this region for a period of low solar flux. Forecasting is limited and calls for experimental data for necessary and sufficient gradients and wind conditions for plumes to fully develop.

Ionospheric Responses to a Total Solar Eclipse Deduced by the GPS Beacon Observations

Abstract: The total electron content (TEC) data during the total eclipse of March 9, 1997 were collected, which were observed by means of nine GPS receivers located at the eastern Asia. The responses of total TEC to the eclipse were analyzed. The results show that: 1) the eclipse led to apparent decrement in TEC that lasted for six to eight hours; 2) the maximum decrement occurred after the middle of the eclipse with time delays varying from twenty minutes to about three hours; 3) the maximum absolute deviations of TEC on the eclipse day do not show a simple and consistent relationship to the maximum solar obscuration.

Algorithm for Model Calibration of GPS-Glonass Tec Observations

Abstract: Systematic difference between observed vertical TEC estimated from GPS-GLONASS observations and model average TEC provided by SMI-96 ionosphere-plasmasphere model has been found. To make relevant calibration of observed TECo, measured data have been averaged at three regional windows: [55–65°N, 23–37°E], [55–65°N, 83–97°E], and [45–55°N, 128–142°E], resulting in good correlation with SMI-96 TECm model values. The correlation coefficients and mean diurnal values of TECo and TECm are used for calibration of TECo until a reasonable consistency of vertical TECo with TECm is reached. The model-based algorithm preserves day-to-day TEC variability and provides monthly-mean data for model improvement.

Space correction of global models of electron number density in the ionosphere by receiving at one site signals from low-orbit satellites

Abstract: A technique for updating global models of the electron density N in real time is developed. It employs the ionospheric part of the Doppler shift due to the rate of change of the total electron content determined at one site from measurements of signals from the radio beacons aboard low-orbit satellites. This technique enables corrections of the Chiu model over a region of about 1,000 km in a North-South direction. To study a possibility of predicting N in an East-West direction using the corrected model, we employ the measurements of satellite signals received at three sites spaced up to 2,000 km. For updating global N models, it is determined that spacing between correction sites can be up to about 3,000 to 4,000 km.

GPS and Ionosphere

Abstract: This chapter contains sections titled: Introduction Calibrating GPS ground observations Modeling GPS observations of total electron content Application to calibrations for ocean altimetry Application to ionospheric science and space weather Summary and future directions Acknowledgments References ]]>