Showing papers on "Total electron content published in 1993"

Instrumental Biases in Ionospheric Measurements Derived from GPS Data

Abstract: Line-of-sight ionosphere measurements derived from differencing dual-frequency Global Positioning System (GPS) range data are corrupted by instrumental biases in both the receiver and GPS satellite transmitters due to hardware delays in the Ll and L2 signal paths. The line- of-sight differential delay can be modeled as the sum of a receiver bias, a satellite transmitter bias, and the line-of- sight ionospheric delay or TEC (total electron content). While the receiver bias can be calibrated directly for some types of receivers, the satellite biases must be estimated from the GPS data itself by using a model of the ionosphere. Ignoring the satellite (receiver) biases when computing TEC measurements from GPS will result in an error of 59 (k30) TECU (1 TEC unit = 1016 electrons/mete+. Using a global ionospheric shell model to fit GPS-based ionospheric delay data from a world-wide network of 30- 40 receivers, we can estimate, with a single fit, satellite biases for the entire GPS constellation and receiver biases for all the uncalibrated receivers. Current studies indicate that the estimated receiver biases agree with the hardware calibrations at the level of 1 nanosecond (ns) and the day- to-day scatter of the estimated satellite biases ranges from 0.3 to 0.5 ns. Preliminary results show our estimated satellite biases agree with other reported values only at the level of 0.7 ns (RMS difference over all satellites). Further investigation will be required to reconcile these differences. If the true accuracy is 0.5 ns, as derived from day-to-day scatter, then the total uncertainty in line-of- sight TEC measurements derived from GPS is 0.6 ns or 1.8 TECU (1 ns corresponds to 2.85 TECU).

Higher-Order Ionospheric Effects on the GPS Observables and Means of Modeling Them

Abstract: Based on relaistic modeling of the electron density of the ionosphere and using a dipole moment approximation for the earth magnetic field, we are able to estimate the effect of the ionosphere on the Global Positioning Systems (GPS) signal for a ground user. The lowest-order (1/f(exp 2)) effect, which is of the order of .1 - 30 meters of zenith group delay, is subtracted out by forming a linear combination of the dual frequencies of the GPS signal. One is left with second- (1/f(exp 3)) and third-order (1/f(exp 4)) effects which are estimated typically to be approximately 0 - 2 cm, and approximately 0 - 2 mm at zenith respectively, depending on the time of day, time of year, the solar cycle and the relative geometry of the magnetic field and the line of sight. Given the total electron content along a line of sight, we derive an approximation to the second-order term which is accurate to approximately 90% within the magnetic dipole moment model; this approximation can be used to reduce the second-order term to the millimeter level, thus potentially improving precise positioning in space and on the ground. The induced group delay, or phase advance, due to second- and third-order effects are examined for two ground receivers located at equatorial and mid-latitude regions tracking several GPS satellites.

Experimental ionospheric tomography with ionosonde input and EISCAT verification

Abstract: Results are presented of tomographic images of ionospheric electron density obtained from experimental measurements of total electron content at a linear chain of stations. Comparison with independent measurements made using a special scan or the EISCAT incoherent scatter radar show broad agreement except under extreme ionospheric conditions. The large-scale horizontal features of the electron density are replicated well in the tomographic image. The vertical profile is dependent on the ionospheric model used to create the background ionosphere formed as an initial condition in the reconstruction process. The inclusion of additional independent information in the form of the electron density and height of the layer peak obtained from ionosondes, has been incorporated into the imaging procedure. The resultant tomographic images show improved agreement with the EISCAT measurements in the vicinity of the layer maximum

Tomographic reconstruction of ionospheric electron density with European incoherent scatter radar verification

Abstract: A campaign was conducted in late September 1991 to obtain experimental measurements for use in ionospheric tomography. Four stations receiving signals from the Navy Navigation Satellite System satellites were set up in a meridional chain in Scandinavia covering some 10° of latitude. Measurements of total electron content were made for six satellite passes over a 2-day period coinciding with an extended run of the CP3f latitudinal scan common program of the European incoherent scatter radar. A new reconstruction technique involving the use of two-dimensional basis vectors has been used to convert the total electron content measurements to images of electron density in a height versus latitude plane. Comparisons of the tomographic images and the independent measurements of electron density from the radar show good general agreement. Broadly similar troughs and enhancements are observed by the two techniques, and a latitudinal gradient in the height of the peak density is reproduced in both data sets.

Tomographic imaging of the ionospheric mid-latitude trough

Abstract: The paper describes an experimental campaign carried out in the United Kingdom in December 1990 during which simultaneous observations were made, using transmissions from the Navy Navigational Satellite System (NNSS) satellites, at four stations covering a latitudinal range of some 8°. The resulting measurements of total electron content have been used in a reconstruction algorithm to image the electron density on a two-dimensional grid for each of the satellite passes monitored. Results are presented showing the development of the mid-latitude trough throughout one night. The potential usefulness of tomographic techniques in ionospheric sensing is discussed in the light of the results obtained

Diurnal double maxima patterns in the F region ionosphere: Substorm-related aspects

Abstract: Daytime double maxima (twin peaks or bite-outs) in the ionospheric total electron content (TEC) at middle and lower latitudes are found to be related to substorm signatures shown in both auroral electrojet and ring current variations. Case studies reveal that during substorm onset and recovery phases, the penetration of magnetospheric convection electric fields and their subsequent {open_quotes}overshielding{close_quotes} effects may be the major dynamical sources of these events. A theoretical low-latitude ionospheric model is used to simulate the dynamical effects of electric field disturbances on F region electron density and TEC. It is demonstrated that the diurnal double maxima in TEC can be created by a combined effect of E x B drift and altitude-dependent F region chemical loss. The required zonal electric fields are found to have greater penetration efficiency in the early evening sector and their latitudinal requirements appear to change with local time. The time scales for the modeled penetration and overshielding effects are 2-3 hours. Modeling results also show that considerable structuring in the local time variation of the ionospheric {open_quotes}equatorial anomaly{close_quotes} can occur due to the interplay of convection electric field penetration and overshielding effects. The possible cause of the midday bite-out ionospheric disturbances by themore » meridional winds associated with traveling atmospheric disturbances (TADs) is also addressed in modeling studies, but the specialized nature of the required TADs makes this a less well understood substorm-related mechanism. 64 refs., 14 figs., 3 tabs.« less

Observations of electron density irregularities in the plasmasphere using the VLA radio interferometer.

Abstract: Transverse irregularities in the line-of-sight total electron content have been studied with the Very Large Array radio-interferometer. Unresolved cosmic radio sources are used to back-illuminate the geoplasma. The baseline-differenced electrical phase time series are Fourier analyzed and then used to fit the constants (amplitude and trace wavevector) for a plane-wave model at each frequency. At frequencies > 0.003 Hz, i.e. above the spectral region dominated by atmospheric gravity waves, the VLA database reveals a distinct class of irregularities with apparent propagation azimuths within ±15° of magnetic east and with trace propagation speeds in the range 0.1-1.5 km s −1 . It is shown that these magnetic eastward directed (MED) irregularities cannot be explained as any perturbation generated by compressional waves at ionospheric heights. An alternative model is proposed, wherein they are geomagnetically-aligned irregularities essentially frozen in the plasmasphere, corotating with the Earth. The relative motion of the radio lines-of-sight past the corotating irregularities produces exactly the class of high-frequency disturbances seen at > 0.003 Hz. The frequency, and the trace speed, are artifacts of the relative motion of the line-of-sight. The trace speed can be used to infer where, along the line-of-sight, the irregularity occurs. This tool allows statistics of the location, shape, and incidence of the irregularities to be studied in some detail

A statistical characterization of local mid‐latitude total electron content

Abstract: The integrated line-of-sight electron density within the ionosphere, known as the total electron content (TEC), is commonly used to quantify ionospheric propagation effects. In order to extrapolate single-point measurements of TEC to other locations and times, some characterization of the TEC spatiotemporal variation must be available. Using a four-channel receiver tracking coded signals from the NAVSTAR Global Positioning System satellites, estimates of both the mean variation and correlation coefficient have been made for the approximately 1200-km or 1-hour local time radius ionospheric region within view of a mid-latitude station. Results were obtained for morning and midday over a 4-week period near the autumnal equinox in 1989. The derived mean variation was found to be well characterized by linear functions of the local time and latitude separation between the ground site and the ionospheric penetration point of the signal. The correlation coefficient during midday was found to decrease linearly with latitude, longitude, and time separation, with values of about 0.91 for a 1000-km separation and 0.98 for a 1-hour separation. During morning hours the longitude and time coefficients were similar to the midday values, but the latitude coefficient was found to have a nonlinear dependence, with values as small as 0.70. The combined results suggest that the decorrelation is due primarily to longer term TEC fluctuations, such as day-to-day variation in the TEC spatial dependence, rather than to transient effects such as traveling ionospheric disturbances. The analysis provides a spatiotemporal characterization of TEC that can be used to extrapolate TEC values from single-point measurements.

Remote sensing of auroral E region plasma structures by radio, radar, and UV techniques at solar minimum

Abstract: The unique capability of the Polar BEAR satellite to simultaneously image auroral luminosities at multiple ultraviolet (UV) wavelengths and to remote sense large-scale (hundreds to tens of kilometers) and small-scale (kilometers to hundreds of meters) plasma density structures with its multifrequency beacon package is utilized to probe the auroral E region in the vicinity of the incoherent scatter radar (ISR) facility near Sondrestrom. In particular, we present coordinated observations on two nights obtained during the sunspot minimum (sunspot number <10) January–February 1987 period when good spatial and temporal conjunction was obtained between Polar BEAR overflights and Sondrestrom ISR measurements. With careful coordinated observations we were able to confirm that the energetic particle precipitation responsible for the UV emissions causes the electron density increases in the E region. These E region electron density enhancements were measured by the ISR at Sondrestrom. The integrations up to the topside of these ISR electron density profiles were consistent with the total electron content (TEC) measured by the Polar BEAR satellite. An electron transport model was utilized to determine quantitatively the electron density profiles which could be produced by the particle precipitation, which also produced multiple UV emissions measured by the imager; these profiles were found to be in good agreement with the observed ISR profiles in the E region. Surprisingly large magnitudes of phase and amplitude scintillations were measured at 137 and 413 MHz in the regions of TEC enhancements associated with the particle precipitation. Steep phase spectral slopes with spectral index of 4 were found in these regions. Strength-of-turbulence computations utilizing the ISR electron density profiles and observed characteristics of phase and amplitude scintillations are interpreted in terms of an irregularity amplitude varying between 10 and 20% at a several-kilometer outer scale size in the E region extending approximately 50 km in altitude. This outer scale size is also consistent with the measured phase to amplitude scintillation ratio. An estimate of the linear growth rate of the gradient-drift instability in the E region shows that these plasma density irregularities could have been generated by this process. The mutual consistency of these different sets of measurements provides confidence in the ability of the different techniques to remote sense large- and small-scale plasma density structures in the E region at least during sunspot minimum when the convection-dominated high-latitude F region is fairly weak.

GPS L1-L2 Bias Determination

Abstract: : The GPS satellite L1-L2 bias T sub gd is determined using data from a TI-4100 receiver operating at Millstone Hill in Westford, MA. Pseudorange L1-L2 differences are computed. This difference is a measure of the total electron content (TEC) path delay along the line of sight plus the L1-L2 satellite and receiver bias. The difference of two measurements at the same time and satellite position cancels the TEC path delay and possible receiver bias and is a measure of the difference in satellite bias of the two satellites; i.e., Sv(a)L1-L2 - Sv(b)L1-L2. such direct measurements are not often available. Therefore, measurements at the same time are converted to 'zenith' TEC values using a mapping function. These difference are used in a least squares solution to determine the difference in bias between GPS satellites. Individual bias differences are obtained with an accuracy of 0.1 ns (o.285 TEC units) or better. By choosing one satellite as the reference, the individual satellite biases can also be obtained. The stability of each satellite bias is also estimated by examining a time series of the differences.

Electron density distribution derived from low latitude whistler studies

Abstract: The electron density distribution, total electron content in a flux tube and downward transport of flux of ionization at low latitudes are derived using the whistlers recorded during storm period at low-latitude stations Varanasi (geomag. lat 14°55'N), Nainital (geomag. lat. 19°1'N) and Gulmarg (geomag. lat. 24°10'N) in the Indian sector. The computed electron densities, electron tube contents (10 13 el/cm 2 tube) and downward fluxes (10 9 el/cm 2 s) are in good agreement with the results reported by other workers. It is shown that the downward transported fluw increases with the increase in magnetic activity

TOPEX ionospheric height correction precision estimated from prelaunch test results

Abstract: Free electrons in the ionosphere will lengthen the electromagnetic path between the TOPEX/Poseidon altimeters and the ocean surface. The path delay is proportional to the total electron content of the ionosphere along the line of sight between the altimeter and the surface. Since these ionosphere delays are also inversely proportional to frequency squared, the nearly simultaneous use of both Ku-band (13.6-GHz) and C-band (5.3-GHz) TOPEX altimeters permits a first-order correction for ionospheric delays. Using results from prelaunch ground testing of the TOPEX satellite altimeters, the authors present the residual height tracking noise after application of the ionosphere correction algorithm. Results are presented as function of ocean significant wave height and for both the 320-MHz and 100-MHz bandwidth of the C-band altimeter. >

Use of gps in the south of brazil under severe ionospheric conditions

Abstract: Severe ionospheric conditions, as e.g. scintillations or a very high electron content or large horizontal gradients of the electron content, can affect GPS observations in such a way that precise geodetic relative positioning becomes difficult or sometimes even impossible. A German-Brazilian joint project was established on the use of GPS in South Brazil, an area with severe ionospheric conditions. The GPS data are also used to monitor the electron content of the ionosphere.

A first-principle derivation of the high-latitude total electron content distribution

Abstract: Calculation of the high-latitude distribution of the vertical total electron content (TEC) is possible using a three-dimensional, time-dependent ionospheric model. Global and local comparisons may be made with observations of TEC. We compare the local diurnal variation of TEC calculated by the model with observations of TEC at Goose Bay, Labrador and Hamilton, Massachusetts. Data from the period of March 1–11, 1989, and monthly averaged data for solar maximum and solar minimum periods are examined. We extend the model to predict diurnal variations of TEC in the polar cap and compare these results with the observed TEC at Thule, Greenland, during an 8-day campaign from January 28 through February 4,1984. We propose a possible explanation for the large variability observed. We show that the “equivalent vertical content” TEC is very sensitive to horizontal F layer electron density gradients and that such “equivalent vertical” TECs may vary significantly from the true vertical TEC of the ionosphere. By incorporating these results, we calculate the vertical TEC distribution of the high-latitude ionosphere for a wide range of solar activity, seasons, and Kp variation represented by a recently completed Utah State University time-dependent ionospheric model data base. Finally, we discuss the possible uses of TEC as a diagnostic tool for testing ionospheric models.

Comparison of ionospheric total electron content measured using the difference Doppler and incoherent scatter methods

Abstract: Latitudinal profiles of the ionospheric total electron content in the auroral and sub-auroral region have been determined using the difference Doppler method. The phase variation of signals from Russian beacon satellites passing over a chain of receiving stations has been observed and, in order to solve the initial value of the phase difference, the two-station method has been applied. The chain consists of three receiving stations situated at approximately the same magnetic meridian. The observations were carried out simultaneously with an EISCAT CP-3 experiment scanning in the magnetic meridional plane. Comparisons with EISCAT results are made and an excellent agreement between the two measurements is often found. This shows that the two-station method is useful at high latitudes. The results also indicate that the Russian receiver chain gives valuable supporting material to EISCAT studies. In comparison with EISCAT, the receiver chain is able to monitor the F-region within a wider latitude range and it can also be used in studying the longitudinal dependence of high-latitude F-region phenomena. The possibility of installing a second chain of mobile receivers at the EISCAT geomagnetic longitude in future campaigns is discussed

Nocturnal disturbances of total electron content and their correlation with VHF radio wave scintillations in the Pacific‐Asia region

Abstract: We have collected 136-MHz Faraday rotation and scintillation data of signals transmitted by the satellite ETS-2 and received at 13 ground stations in the Pacific-Asia region. Using this data base, a statistical analysis has been carried out by correlating the nocturnal TEC disturbances with the amplitude scintillations. Also presented are several typical events observed simultaneously over this network. An important result in the present paper is that the occurrence of amplitude scintillations is closely associated with the wavelike disturbances in TEC with periods below 2 hours. These periods agree with the TIDs produced by atmospheric gravity waves in the ionosphere. Furthermore, simultaneous recordings at several spaced stations indicate the disturbances are traveling. It is therefore deduced that wavelike disturbances in TEC observed are manifested as the ionospheric response to gravity waves in the neutral atmosphere.

Effects of Severe Ionospheric Conditions on GPS Data Processing

Abstract: One of the major error sources in GPS positioning is ionospheric refraction which causes signal propagation delays. Due to the dispersive character of these delays, dual frequency phase measurements can effectively be used to gain ionospheric corrections.

Ionospheric calibration for single frequency altimeter measurements

Abstract: This study is a preliminary analysis of the effectiveness (in terms of altimeter calibration accuracy) of various ionosphere models and the Global Positioning System (GPS) to calibrate single frequency altimeter height measurements for ionospheric path delay In particular, the research focused on ingesting GPS Total Electron Content (TEC) data into the physical Parameterized Real-Time Ionospheric Specification Model (PRISM), which estimates the composition of the ionosphere using independent empirical and physical models and has the capability of adjusting to additional ionospheric measurements Two types of GPS data were used to adjust the PRISM model: GPS receiver station data mapped from line-of-sight observations to the vertical at the point of interest and a grid map (generated at the Jet Propulsion Laboratory) of GPS derived TEC in a sun-fixed longitude frame The adjusted PRISM TEC values, as well as predictions by the International Reference Ionosphere (IRI-90), a climatological (monthly mean) model of the ionosphere, were compared to TOPEX dual-frequency TEC measurements (considered as truth) for a number of TOPEX sub-satellite tracks For a 136 GHz altimeter, a Total Electron Content (TEC) of 1 TECU 10(exp 16) electrons/sq m corresponds to approximately 0218 centimeters of range delay A maximum expected TEC (at solar maximum or during solar storms) of 10(exp 18) electrons/sq m will create 22 centimeters of range delay Compared with the TOPEX data, the PRISM predictions were generally accurate within the TECU when the sub-satellite track of interest passed within 300 to 400 km of the GPS TEC data or when the track passed through a night-time ionosphere If neither was the case, in particular if the track passed through a local noon ionosphere, the PRISM values differed by more than 10 TECU and by as much as 40 TECU The IRI-90 model, with no current ability to unseat GPS data, predicted TEC to a slightly higher error of 12 TECU The performance of PRISM is very promising for predicting TEC and will prove useful for calibrating single frequency altimeter height measurements for ionospheric path delay When adjusted to the GPS line-of-sight data the PRISM URSI empirical model predicted TEC over a day's period to within a global error of 860 TECU rms during a nighttime ionosphere and 974 TECU rms during the day When adjusted to the GPS derived TEC grid, the PRISM parametrized model predicted TEC to within an error of 847 TECU rms for a nighttime ionosphere and 1283 TECU rms during the day However, the grid cannot be considered globally due to the lack of sufficient numbers of GPS stations and large latitude gaps in GPS data It is the opinion of the authors that using the PRISM model and adjusting to the global sun-fixed TEC grid regenerated with a localized weighted interpolation offers the best possibility of meeting the 10 TECU global rms (or 2 cm at 136 GHz) ionosphere range correction accuracy requirement of TOPEX/Poseidon and should be the subject of further study However, it is clear that the anticipated requirement of 34 TECU global rms for TOPEX/Poseidon Follow-On (corresponding to the TOPEX/Poseidon performance) can not be met with any realizable combination of existing models and data assimilation schemes