Showing papers on "Total electron content published in 1994"

Estimation of the transmitter and receiver differential biases and the ionospheric total electron content from Global Positioning System observations

Abstract: In the estimation of the ionospheric total electron content from the Global Positioning System (GPS) observables, various instrumental systematic effects such as the biases in the GPS satellites and receivers must be modeled. This paper describes a procedure, based on a Kalman filtering approach, for estimating these instrumental biases as well as the total electron content at each GPS station, using dual GPS data. The method is applied to six data sets, of 48 hours each, spanning one year, from the Deep Space Network with GPS stations in Australia, Spain, and the United States. The formal errors for the estimated satellite biases and for the total electron content at each station are about 0.07 ns and 0.2×1016 el/m2, respectively. The variation in time of the satellite biases (relative to the mean of all of them) estimated in different epochs during 1-year period, is below 1 ns.

Ionospheric Response to the Sustained High Geomagnetic Activity During the March '89 Great Storm

Abstract: A simulation was conducted to model the high-latitude ionospheric to the sustaied level of high geomagnetic activity for the great magnetic storm period of March 13-14, 1989. The geomagnetic and solar activity indices and the Defense Meterological Satellite Program (DMSP) F8 and F9 satellite data for particle precipitation and high-latitude convection were used as inputs to a time-dependent ionospheric model (TDIM). The results of the TDIM were compared to both DMSP plasma density data and ground-based total electron content (TEC) measurements for the great storm period as well as with earlier storm observations. The comparisons show that the overall structure of the high-latitude ionosphere was dominated by an increased convection speed within the polar cap that led to increased ion temperatures. In turn, this enhanced the NO(+) density, raised the atomic-to-molecular ion transition height to over 300 km, decreased N(sub m)F(sub 2), increased h(sub m)F(sub 2), and in places either increased n(sub e) at 800 km or slightly decreased it. The morphology of the ionosphere under these extreme conditions was considerably different than that modeled for less distributed intervals. These differences included the character of the dayside tongue of ionization that no longer extended deep into the polar cap. Instead, as a result of the ion heating and consequent reduction in N(sub m)F(sub 2), a large polar hole occupied much of the polar region. This polar hole extended beyond the auroral oval and merged with the night sector midatitude trough. The limitaions associated with the applicability of the TDIM to the geomagnetic conditions present on March 13 and 14 are discussed. The primary limitations of the TDIM derive from the limited temporal resolution of the model input parameters and the lack of suitably dynamic thermospheric specification for the great storm conditions. These limitations leads to midlatitude ionospheric storm phases that do no follow those observed.

Apparatus and method for ionospheric mapping

Abstract: This invention is a unique single-site method of determining the local total electron content (TEC) of the ionosphere and its space-time variation using a global positioning system (GPS) ionospheric receiver. The TEC of the ionosphere is specified in terms of a space-time map of the local TEC in the vicinity of the receiver. Differential group and phase path data between two L-band frequencies (L1 and L2) for a plurality of the GPS satellites in view of the receiver station are analyzed by a least squares technique to extract both the ten parameters of a full second order space-time polynomial expansion for the vertical TEC (VTEC) and the differential delay biases associated with the space vehicles (SVs). The method is applicable to day and nighttime data.

Phase and phase‐difference ionospheric radio tomography

Abstract: Variants of ray ionospheric radiotomography (RT) are considered: the phase RT based on measuring the absolute phase or the total electron content and the phase-difference RT based on measuring the phase-difference RT based on measuring the phase difference or Doppler frequency. The phase technique is shown to give poor results with typical errors in determining the absolute phase. The previously developed technique of the phase-difference RT is presented.©1994 John Wiley & Sons Inc

Model‐assisted ionospheric tomography: A new algorithm

Abstract: Ionospheric tomography uses total electron content (TEC) records collected by longitudinally aligned stations, which receive a beacon satellite orbiting overhead. The electron density distribution is reconstructed for the region bounded by the satellite orbit and the line of ground receivers. A new reconstruction algorithm is describes which satisfies the TEC records, makes use of an ionospheric model, and allows the incorporation of complementary measurements. The new algorithm also accepts relative (rather than absolute) TEC as input data. A comparison of the new algorithm and one used recently shows significant improvement over early techniques, particularly when a scaled ionogram is included in the data.

A status report on applying discrete inverse theory to ionospheric tomography

Abstract: Discrete inverse theory (DIT) provides an orderly framework in which to combine measurements of total electron content (TEC) with a priori information to image the ionosphere tomographically. We have developed a DIT-based tomographic processor for use with relative TEC data. The processor's a priori information comprises the global mean of over 17,000 profiles generated from an ionospheric model, for use as a “generic background”; empirical orthogonal functions (EOFs) spanning the same model profiles, for use as vertical basis functions; and a red power-law horizontal spectrum. Relative TEC data are used to evaluate coefficients multiplying the EOFs and harmonics, thus quantifying a perturbation electron-density field. The perturbation field, which need not be small, is added to the a priori background to produce the image. We present here several images produced by employing the processor with simulated TEC data based on in situ ionospheric measurements and incoherent-scatter radar observations.©1994 John Wiley & Sons Inc

Predicted and measured total electron content at both peaks of the equatorial anomaly

Abstract: The total electron content (TEC) of the ionosphere is a parameter of great importance for the systems which use transionospheric radiowaves and for the checking of the validity of ionospheric models. It is known that the highest TEC values in the world occur at the peaks of the equatorial anomaly (EA) regions at ∼15° either side from the magnetic equator. In this paper, TEC measurements obtained with Faraday technique at three stations placed near both peaks of the EA are used to check the validity of three ionospheric models to predict TEC in these regions. The TEC data of the southern peak of the EA were obtained at Tucuman (26.9°S, 294.6°E) in 1982, and those corresponding to the northern peak were Obtained at Palehua (21.4°N, 203.2°E) in 1978 and Delhi (28.6°N, 77.2°E) in 1968. Ionosonde data from Tucuman, Delhi, and Maui, (20.8°N, 203.5°E) are also used. Two widely used models are considered, namely: the international reference ionosphere and the semiempirical low-latitude ionospheric model. The third model considered is a Chapman layer with scale height equal to atomic oxygen scale height (CHOEA). In general, the models underestimate TEC during daylight hours at Tucuman suggesting that the real electron density profiles are larger in electron density magnitudes than those given by the models. For Palehua and Delhi the models' predictions are better than those for Tucuman, and a very good agreement among calculated and measured TEC values at Palehua for some months is observed. Taking into account the simplicity of the TEC calculus with CHOEA, this model would be an adequate alternative to predict TEC at stations placed near the northern crest of the EA, at least for the considered solar activity. This model allows also to calculate TEC at the southern crest of the EA with a similar degree of accuracy of those values obtained with the other models.

TEC and f0F2 comparison

Abstract: Following an investigation on the calibration of Faraday Rotation measurements by the slab thickness method, several interesting results have been obtained concerning the limits of application of TEC data in f0F2 modelling at middle latitudes.

A corrected method of the international reference ionosphere to be used in ghinese region

Abstract: A corrected method of the International Reference Ionosphere(CRI) is proposed when it is used in Chinese region, based on the long-term observations of the Chinese ionosonde stations.The method of predicting the ionospheric F layer in the Asia Oceania Region is adopted in calculation of f0F2 and M(3000)F2 instead of CCIR/URSI Coefficients,a more reasonable description of the Occurrence of F1 layer and a greater E peak height are used In the CRI.After various comparisons with measured data, it is shown that the CRI gives more accurate values than the IRI does when they are used in Chinese region.

Observation of total electron content, amplitude and phase scintillations in the auroral ionosphere

Abstract: The connection between the spatial gradient of the equivalent vertical total electron content (Ntot) of the ionosphere and scintillation patches are examined. It is shown that in the evening hours under quiet geomagnetic conditions, a steplike gradient of Ntot with latitude develops at the northern edge of the main ionospheric trough. Later in the evening, the above structure moves southward and the latitudinal gradient of Ntot becomes very pronounced and around it a scintillation patch is formed. During magnetic disturbances, the monotonic sharp gradient of Ntot is destroyed. A number of maxima of total electron content which become collocated with many scintillation patches are formed. Further, under disturbed conditions, the power law index of both the amplitude and phase scintillation spectra shows a marked variation with latitude which may be quasiperiodic.

Phase relationships between total electron content variations, Doppler velocity oscillations and geomagnetic pulsations

Abstract: The phase relationship between variations of ionospheric total electron content (TEC) and ground-level ULF geomagnetic pulsations has been examined for the advection and compression mechanisms. To determine the causal mechanism, several earlier studies have examined the phase difference between oscillations of Doppler velocity in ionospherically reflected radio waves and simultaneous ULF geomagnetic pulsations. In most instances it was found that the phase relation varied from event to event. With the application of Euler’s formula this study shows that in low geomagnetic and midgeomagnetic latitudes the phase differences between variations of TEC and ULF pulsations in the northward component of the geomagnetic field due to the advection and compression mechanisms are 0o and 180o, respectively. We also found that TEC variations tend to lead ionospheric Doppler velocity oscillations by 90o. Furthermore, it is shown that the phase relationship between ionospheric Doppler velocity oscillations and ULF pulsations of the northward component of the geomagnetic field, caused by the advection and compression mechanisms, are functions of the scale length, frequency of ULF waves, and geomagnetic dip.

Observations of TEC fluctuations from an explosion on the Earth`s surface

Abstract: The authors report observations of perturbations in the ionosphere total electron content (TEC) caused by acoustic waves propagating from a large chemical explosion in souther New Mexico at the earth`s surface. Fluctuations in TEC were measured by two arrays of receivers that monitor the phase of the 136 MHz beacons on two geostationary satellites. One array, located in northern New Mexico, observed fluctuations in the region where acoustic waves from the blast impinged directly on the ionosphere, while the second array, in Texas, was located to observe fluctuations caused by ducted acoustic waves. The TEC disturbance at the New Mexico array had an amplitude of about 2 {times} 10{sup 14} m{sup {minus}2} (more than 10 times the array noise level), while the amplitude at the Texas array, at a range of 900 km, was only a few times the instrumental noise level. Noise background analysis shows that the probability that a comparable or larger response at the New Mexico array might have been caused by a background noise event was less than 1%. The corresponding probability for the Texas array was 3%.

Investigation of the ionospheric short-term variability

Abstract: : Reliable HF communications require propagation assessment. Such assessment could be facilitated with the monitoring of ionospheric characteristics by continuously available passive means, i.e, measurements of the total electron content(TEC) using satellite-emitted signals without a need for burdening the electromagnetic spectrum. With the Global Positioning System (GPS) providing instantaneous time delay, or equivalently, TEC, values when needed, an assessment of HF propagation may be available on a near realtime basis. To assess this possibility a one year study of the correlation between TEC and foF2 using GPS time delay observation taken at Matera, Italy was undertaken during 1995-1996. This is a period of minimum solar activity with sunspot numbers varying between 6-18. The observed correlation coefficient varied between 0.55 in the winter of 1996 to about 0.75-0.8 during the summer of 1995. In addition to the seasonal variability of the correlation coefficient, a diurnal variability is also present with the coefficient normally maximizing during the day and minimizing in the predawn periods. The predawn minimum may be due to the contribution of plasmaspheric electron content. The correlation coefficient appears to increase with magnetic activity, indicating that TEC and foF2 behave similarly during magnetically active periods. Distribution of errors between measured foF2 values and predicted ones using a) standard predictions algorithms and b) TEC measurements converted to foF2 from a model of slab thickness, show that the errors are reduced when using TEC observations. The errors would be further reduced if the satellite subionospheric point would approach the location of the foF2 actual measurements.

Ionospheric Corrections to Precise Time Transfer Using GPS

Abstract: The free electrons in the earth's ionosphere can retard the time of reception of GPS signals received at a ground station, compared to their time in free space, by many tens of nanoseconds, thus limiting the accuracy of time transfer by GPS. The amount of the ionospheric time delay is proportional to the total number of electrons encountered by the wave on its path from each GPS satellite to a receiver. This integrated number of electrons is called Total Electron Content, or TEC. Dual frequency GPS receivers designed by Allen Osborne Associates, Inc. (AOA) directly measure both the ionospheric differential group delay and the differential carrier phase advance for the two GPS frequencies and derive from this the TEC between the receiver and each GPS satellite in track. The group delay information is mainly used to provide an absolute calibration to the relative differential carrier phase, which is an extremely precise measure of relative TEC. The AOA Mini-Rogue ICS-4Z and the AOA TurboRogue ICS-4000Z receivers normally operate using the GPS P code, when available, and switch to cross-correlation signal processing when the GPS satellites are in the Anti-Spoofing (A-S) mode and the P code is encrypted. An AOA ICS-Z receiver has been operated continuously for over a year at Hanscom AFB, MA to determine the statistics of the variability of the TEC parameter using signals from up to four different directions simultaneously. The 4-channel ICS-4Z and the 8-channel ICS-4000Z, have proven capabilities to make precise, well calibrated, measurements of the ionosphere in several directions simultaneously. In addition to providing ionospheric corrections for precise time transfer via satellite, this dual frequency design allows full code and automatic codeless operation of both the differential group delay and differential carrier phase for numerous ionospheric experiments being conducted. Statistical results of the data collected from the ICS-4Z during the initial year of ionospheric time delay in the northeastern U.S., and initial results with the ICS-4000Z, will be presented.

Nightglow Measurements of 630.0 nm Line from Kolhapur, India and Their Association with TEC Enhancements

Abstract: Nightglow measurements of 630.0 nm emission line by high resolution tilting photometer have been compared with simultaneous Faraday rotation measurements of the ionosphere's total electron content (TEC) from ETS-II satellite (-130°E) at a low latitude station, KOLHAPUR (Geog. lat. 16.8°N, long. 74.2°E, dip lat. 10.6°N) in India. The preliminary observational data are presented and discussed for seven nights during March, 1989 and April, 1990. The comparative study shows that in general the airglow enhancements are associated with the night time enhancement in TEC. The airglow enhancement due to downward motion of the F layer during pre-dawn hours does not correlate with TEC fluctuations. Further, the time rate of change of content has been used to obtain rough estimate of the nightglow intensity level at the station. On comparisons 630 nm emission intensities computed from TEC values and those predicted by MSIS-86 (Hedin, 1987) and FAIM (Anderson et al., 1989) models show an accepted level of agreement. Results focus the coupling between airglow producing irregularities and TEC at low latitude ionosphere.

Variability of total electron content in the high-altitude ionosphere following solar maximum

Abstract: : A data archive of Total Electron Content CIEC) data was developed through operation and maintenance of GPS satellite receiver equipment at Shemya, AK; Hanscom AFB, MA; Thule AB, Greenland; Tucuman, Argentina; and Agua Verde, Chile Scintillation data can be extracted ftom much of this database The Shemya, AK, data provide a valuable source for characterizing TEC morphology and variability with look direction at this site The Shemya data were also used in an initial assessment of the seasonal and directional specification accuracy of the Bent ionospheric model, specific to this site Analysis of an existing NWRA database has produced ionospheric trough boundary signatures in the sub auroral European sector These signatures will support studies aimed at real time detection of the trough boundary Innovative software techniques were developed to improve data quality in the areas of multipath rnitigation (the Multipath Template Technique) and the automated calibration of an installed receiver system for the combination of all system components' contributions to pseudorange error (SCORE:: Self-Calibration Of pseudoRange Errors)