Showing papers on "Total electron content published in 1991"

Real-Time Ionospheric Monitoring System Using the GPS

Abstract: : In satellite tracking using ground-based radars, an estimate of the total electron content (TEC) along the path to the satellite is required to measure accurately the range of the satellite. This estimate is necessary because the radar wave travels at a slower speed as it propagates through the ionosphere. The range error Delta R that is introduced is dependent on the radar frequency f and on the TEC along the propagation path, and can be expressed by where Ne is the local electron density and R is the radar range. The TEC can vary significantly with the time of day, geomagnetic activity, and look direction. A real-time synoptic ionospheric monitoring system has been developed using data acquired from a T14100 Global Positioning System (GPS) receiver for use at the Millstone Hill satellite-tracking radar. The T14100 GPS receiver can track up to four GPS satellites at any one time. Each GPS satellite transmits signals at two different L-band frequencies: Ll (1575.42 MHz) and L2 (1227.6 MHz). ne TEC along the path to each satellite can be determined by combining both frequencies using the pseudorange and the integrated phase data. At Millstone, the TEC is measured every 3 s for each GPS satellite in view.

Model studies of the latitudinal extent of the equatorial anomaly during equinoctial conditions

Abstract: The latitudinal extent of the equatorial anomaly has been studied for equinoctial conditions using a theoretical model of the ionosphere which incorporates measured values of vertical E × B drift at the Earth's magnetic equator. Realistic values of neutral winds are also included. The equatorial anomaly region, typically between ±20° magnetic latitude, is that part of the world where the highest values of electron density and total electron content (TEC) normally occur and hence is very important to high-frequency propagation and to transionospheric propagation effects. During the daytime, upward E × B drift at the magnetic equator drives the ionization across field lines to higher latitudes, causing crests in ionization to occur at approximately ±15° dip latitude. The E × B drift mechanism is explained in detail by Hanson and Moffett (1966). The latitude range over which the anomaly makes a significant difference in values of ƒ0F2 and TEC is calculated as a percent departure from the case with no equatorial electric field. Results from the model studies with different values of realistic electric fields show that the effects of the anomaly can be more highly variable and widespread in latitude and local time than is generally assumed.

The effect of interhemispheric coupling on nighttime enhancements in ionospheric total electron content during winter at solar minimum

Abstract: In the present study values of TEC are determined for various satellite-to-ground-station raypaths in the northern (winter) hemisphere using electron concentration values generated by a fully time-dependent mathematical model of the plasmasphere. The values are then used in an investigation of the night-time occurrences of enhanced TEC at midlatitudes during winter at solar minimum. The modelled values of nighttime TEC show enhancements when there are large downward field-aligned flows of plasma in the topside ionosphere. Such downflows occur in the winter hemisphere after conjugate (summer) sunset and are associated with the postsunset decrease in plasma temperature. The magnitudes of the enhancements are found to be dependent upon the net gain of plasma resulting downflow through the 2500-km altitude level over that lost by chemical reactions with the neutral gases in the E- and F-regions. The results show that the magnitude and other characteristics of an enhancement, e.g. time of peak enhancement and duration, are modulated by the nighttime neutral air wind velocity and the satellite-to-ground-station raypath

Solar and magnetic activity effects on the latitudinal variations of nighttime TEC enhancement

Abstract: The effects of solar and magnetic activity on the latitudinal variations of nightime enhancement in total electron content (TEC) have been investigated by considering TEC data from then stations in the northern hemisphere. A discussion is also presented of the potential source mechanisms for the derived latitudinal variations in TEC enhancement

Evaluation of six ionospheric models as predictors of total electron content

Abstract: We have gathered total electron content (TEC) data from a range of mid-latitudes and low latitudes and longitudes for a wide range of solar activity. This data was used to evaluate the performance of six publicly available ionospheric models as predictors of total electron content. TEC is important for correcting modern DoD space systems, which propagate radio waves from the earth to satellites, for time delay effects of the ionosphere. The TEC data were obtained from polarimeter receivers located in North America, the Pacific, and the East coast of Asia. The ionospheric models evaluated are (1) the International Reference Ionosphere, (2) the Bent model, (3) the Ionospheric Conductivity and Electron Density model, (4) the Penn State model, (5) the Fully Analytic Ionospheric Model, and (6) a hybrid model consisting of the Union Radio Scientifique Internationale 88 (URSI-88) coefficients coupled with the Damen-Hartranft profile model. We will present extensive comparisons between monthly median TEC and model TEC obtained by integrating electron density profiles produced by the six models. These comparisons demonstrate that although most of the models do very well at representing ƒ0F2, none of them do very well with TEC, probably because of inaccurate representation of the topside profile. We suggest that one approach to obtaining better representations of TEC is the use of ƒ0F2 from the CCIR or URSI-88 coefficients coupled with a good climatological slab thickness model.

Structure of ionospheric irregularities from amplitude and phase scintillation observations

Abstract: The mutual coherence function Γ2, or the second moment of the complex amplitude of a radio wave which traverses through equatorial F region irregularities, is computed from amplitude and phase scintillation data. Theoretically, the equation satisfied by the coherence function has an analytic solution over the whole range of scintillation strength. This solution is directlly related to the structure function for the phase fluctuations produced by the irregularities. Hence the shape of the correlation function for variations in the total electron content along the signal path can be derived from the computed values of Γ2. With a suitable power-law model for the irregularities, an “intermediate break scale,” 10, as well as the rms density fluctuation are deduced from a comparison of computed values for short time lags with those expected from theory. During a postsunset scintillation event, 10 is found to increase with local time. In the context of the generalized Rayleigh-Taylor instability, which is the likely source of the irregularities, this increase may be attributed to a decline in the effective electric field prevailing in the region of the irregularities.

A study of transionospheric refraction of radio waves using the Clark Lake Radio Observatory

Abstract: The Clark Lake Radio Observatory, viewing four exceptionally strong, quasi-point celestial sources at 50 MHz, has been used to perform systematic, around-the-clock observations of transionospheric radio refraction during a 17-day campaign in February/March 1987. These data complement those gathered elsewhere, by concentrating on raypaths near and poleward of zenith. We observe average (rms fluctuating) transverse slant total electron content gradients which are weaker, by factors of 2 to >10, than those observed at other facilities sited further poleward and viewing sources located further equatorward. The frequently observed quasiperiodic oscillations are clustered in the frequency range f≤2 mHz. We do not see the expected suppression of refractive activity in this band during the night, despite the nocturnal reduction of ionospheric density

Ionospheric variability and the measurement of ocean mesoscale circulation with a spaceborne radar altimeter

Abstract: The propagation of radio waves broadcast to the Earth by a satellite is slowed by the total electron content (TEC) of the ionosphere. Spaceborne radar altimeters, like those aboard the Seasat and Geosat satellites, rely on the time delay between the transmission of a radar pulse and its reception, after reflection from the ocean surface, to estimate the range to the surface. The slowing of the electromagnetic waves by the ionosphere will increase the apparent distance between the satellite and the surface. Variations of the TEC of the ionosphere on horizontal spatial scales less than 300 km could potentially be confused for or mask the presence of sea surface height changes associated with mesoscale ocean circulation. Using dual-frequency broadcasts from Global Positioning System (GPS) satellites, the spatial variability of the ionosphere is estimated. It is shown that for altimetric monitoring of mesoscale ocean circulation, ionospheric variability is not a significant problem. For altimetric studies of ocean circulation on ocean basin scales, compensation for ionospheric variability maybe necessary.

Modeling the ionospheric electron content for the correction of altimetric measurements

Abstract: The TOPEX-POSEIDON oceanographic satellite (due to be launched in 1992) will proceed to high accuracy altimetric measurements of the sea surface. Since the altimeter signals will propagate through the ionosphere, they will be retarded with respect to their free-space propagation delay. As a result, the measured altitude will exhibit an apparent lengthening which must be considered. In order to correct this effect, the ionosphere total electron content (TEC) beneath the satellite has to be known. This paper addresses the problem of determining the TEC form Doppler measurements performed on telemetric signals propagating between the satellite and the ground stations of the DORIS positioning system. This is an inverse problem which, in general, does not admit a single-valued solution. Physical observations of the ionophere lead us to assume that the TEC along each half-revolution is regular such that we can select an appropriate solution. This solution is approximated by cubic splines. The computed results are compared to simulation results, based on the Bent ionospheric model and seem to be particularly promising.

Ionospheric model assessment using TRANSIT data

Abstract: : Total Electron Content (TEC) measurements derived from observation of polar-orbiting TRANSIT satellites have been incorporated into an assessment of the Bent ionospheric model, as implemented for the PAVPAW program. Both temporal and spatial assessments of the ionospheric model have been performed. By suitable adaptation of the programs developed for this assessment, similar assessments of other ionospheric models could be performed.

Modeling the Ionospheric Electron Content for the Correction of Altimetric Measurements

Abstract: The TOPEX-POSEIDON oceanographic satellite (due to be launched in 1992) will proceed to high accuracy altimetric measurements of the sea surface Since the altimeter signals will propagate through the ionosphere, they will be retarded with respect to their free-space propagation delay As a result, the measured altitude will exhibit an apparent lengthening which must be considered In order to correct this effect, the ionosphere total electron content (TEC) beneath the satellite has to be known This paper addresses the problem of determining the TEC form Doppler measurements performed on telemetric signals propagating between the satellite and the ground stations of the DORIS positioning system This is an inverse problem which, in general, does not admit a single-valued solution Physical observations of the ionophere lead us to assume that the TEC along each half-revolution is regular such that we can select an appropriate solution This solution is approximated by cubic splines The computed results are compared to simulation results, based on the Bent ionospheric model and seem to be particularly promising