Showing papers on "TEC 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

Transpiration efficiency in crops of semi-dwarf and standard-height sunflower

Abstract: This paper compares transpiration efficiency (TEc = shoot biomass corrected by seed oil-synthesis cost/ transpiration per unit vapour pressure deficit) of standard-height and semi-dwarf sunflower (Helianthus annuus L.) crops. Three irrigation regimes that differed in timing and amount of irrigation were applied. Transpiration was calculated as the difference between evapotranspiration (assessed by hydrological balance) and soil evaporation (measured using evaporimeters). Irrigation treatments and irrigation × cultivar interaction did not affect TEc. Across irrigation regimes, seasonal TEc was 4.70 and 3.64 g m-2 mm-1 kPa (P < 0.003) for the standard-height and semi-dwarf crops, respectively. Seasonal differences in TEc arose from differences during the post-anthesis period. The lower post-anthesis TEc of the semi-dwarf cultivar was related to a smaller radiation-use efficiency. Possible causes for the differences in post-anthesis radiation-use efficiency are discussed.

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.

Rat thymic epithelial cells in culture constitutively secrete IL-1 and IL-6

Abstract: To study the in vitro interactions between rat thymic non-lymphoid cells and thymocytes, we established a system for long-term cultivation of thymic epithelial cells (TEC). TEC were cultivated and successfully propagated for over 8 months in RPMI 1640 medium containing 15% FCS, dexamethasone, insulin, epidermal growth factor, and poly-L-lysin as an adhesive matrix. Their epithelial nature has been confirmed using monoclonal anti-cytokeratin (CK) antibodies. More than 95% of these cells were reactive with K 8.13 and CK 8 mAbs, which are pan-epithelial markers for rat TEC in situ. An epithelial cell clone (TE-R 2.5) established from a long-term TEC culture was 100% reactive with these anti-CK antibodies. Phenotypic analysis of TEC cultures was performed by a large panel of mAbs reactive with a subset of rat TEC or CK polypeptides as well as UIex europaeus agglutinin I using a streptavidin-biotin immunofluorescence assay. Although the results obtained demonstrated phenotypic heterogeneity among these cells, most cultures, including the TE-R 2.5 clone, were of subcapsular/medullary phenotype. Medium conditioned by TEC cultures exhibited IL-1 and IL-6 activities when tested on D10S and B9 sensitive cell lines, respectively. Cytokine activities were neutralized (IL-1) or significantly inhibited (IL-6) by specific polyclonal antibodies. In addition, both anti-IL-1 and anti-IL-6 antibodies reacted with TEC in culture and epithelial (CK-positive) cells on thymic cryostat sections, indicating that thymic epithelium provides an important intrathymic source for molecules contributing to T cell activation.

The Politics of Training in Britain: Contradictions in the TEC Initiative

Abstract: In the biggest shake-up of Britain’s training system since the 1964 Industrial Training Act, the Conservative government has created a new set of institutions, the Training and Enterprise Councils (TECs), to take over the delivery of training and enterprise development programmes, such as Youth Training (YT) and Employment Training (ET). With TECs, the government has handed over the ‘ownership’ of the training system to employers: almost £3 billion of public funds and in excess of 5,000 civil service jobs are being transferred to the private sector TECs in one of the Conservative’s most ambitious privatisation initiatives. Employment for the 1990s, the White Paper in which the TEC initiative was launched, stipulated that these locallybased institutions are to be controlled by private sector-dominated boards – twothirds of TEC directors must be senior figures from the business community. In contrast to this privileged role for the private sector, trade unions, voluntary groups and local authorities have been 23

Convergence behaviour of a jointly adaptive transversal and memory-based echo canceller

Abstract: A convergence analysis of a jointly adaptive transversal and memory-based or nonlinear echo canceller is presented. The coefficients of the transversal echo canceller (TEC) and the memory echo canceller (MEC) are updated simultaneously using the well-known least mean square (LMS) algorithm with separate adaptation constants and a common cancellation error. The amount of linear echo cancellation load-sharing between the TEC and MEC is shown to be governed by a fixed relationship that exists between the MEC coefficients and those TEC coefficients updated using the shared portion of the input symbol span. The average mean square error is derived for the case of binary input symbols and compared to that of an identical canceller adopting a sequential adaptation method where the TEC coefficients are adapted first and permitted to converge before MEC coefficient adaptation is initiated for jointly adaptive operation. Bounds for both adaptation constants are found for MSE convergence for the jointly adaptive case. The adaptation constants that yield the fastest jointly adaptive convergence are also derived. The sequential method is shown to have an overall convergence rate advantage that decreases as the ratio of the number of TEC coefficients to MEC coefficients becomes large. The theoretical results are supported by computer simulations.

Performance and reliability of redundant thermoelectric coolers

Abstract: The authors derived analytical solutions for thermoelectric coolers (TECs) with two-branch redundancy design, any number of open and short elements, and excess resistance. The model was used to predict the thermal performance of the TEC under high thermal load and compared with experimental data with good agreement. The inherent failure rate of both the series and two-branch redundant TECs were calculated. The latter was found to provide an approximately eight-times improvement in the expected failure rate. This improvement is necessary for a TEC to handle a high thermal load while meeting the reliability requirement of telecommunication systems. >

Performance of BOC Ohmeda Tec 3 and Tec 4 vaporisers following tipping.

Abstract: The introduction of systems to facilitate the changing of vaporisers on the back bar of the anaesthetic machine has increased the risk of a vaporiser being tipped or inverted prior to attachment. The performance of three Ohmeda Tec 3 and three Ohmeda Tec 4 vaporisers following tipping to 30 degrees, 90 degrees and inversion to 180 degrees was measured using a calibrated Datex Capnomac AGM-103 analyser. Both types of vaporiser when switched off tolerated tipping and inversion without alteration of the anaesthetic concentration subsequently delivered. The same was true for the Tec 3 after tipping to 30 degrees and 90 degrees with the dial set at zero or above. However, after inversion of the Tec 3 vaporiser with the dial at zero or higher, the anaesthetic concentration delivered was much greater than shown on the dial, initially exceeding 12% for all agents. Should a Tec 3 vaporiser be inverted with the dial at any setting other than OFF, it is necessary to adequately flush it with fresh gas and check its output prior to use.

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.

An empirical model of TEC over the continent of China

Abstract: An empirical model based on Fourier harmonic analysis for calculating ionospheric total electron content (TEC) over Xinxiang, China (35.3 degrees N, 113.9 degrees E), is presented for the period November 1981 to June 1985. The data used are from observation of the radio beacon satellite ETS-2 during the descending half of the solar cycle from 1981 to 1985. The harmonic components and annual coefficients of the TEC are examined. The representative diurnal variations of modeled and observed TEC for four months in the different seasons in Xinxiang are shown. >

A Novel Integrin Involved in Thymocyte-Thymic

Abstract: Summary Thymocytes differentiate in the thymic microenvironment into immunocompetent T cell through the interaction with a variety of accessory cells, including thymic epithelial cells (TEC) . TEC play an important role in the selection process presenting self antigens in association with major histocompatibilitycomplex(MHC) molecules to the maturingT cells . TheT cell receptor recognizes the selfantigen-MHC complex, but other surface molecules help stabilize this interaction. Thus, the CD2/LFA3 and LFA-1/intercellular adhesion molecule 1 pairs have been shown to participate in the binding between lymphoid cells and TEC. Here we describe an integrin of the very late activation antigen subfamily composed by the known Nt chain and by a novel ci chain. This adhesion molecule is expressed on the surface of medullary TEC and is involved in the adhesion between TEC and thymocytes, but not peripheral blood T lymphocytes.

Variations in ionospheric range error with GPS look direction

Abstract: ionospheric total electron content (TEC) causes range errors In single-frequency GPS measurements and in other trans-Ionospheric RF systems The GPS ionospheric correction model, broadcast in the downiink message, has been shown to provide an approximate 50% correction for these errors, on a statistical basis, in the middle-latitude regions such as CONUS There are several geographic regions outside the mid-latitudes where the geophysical behavior of the ionosphere is markedly different, and the GPS correction model will be less accurate In fact, the geographic area of the Ionosphere that can be observed from a single GPS site, tracking satellites to very low elevation angles, is large enough that a site nominally in the mid-latitudes will often view many GPS satellites through ionospheric effects from a less stable region, impacting range error correction accuracy Only limited quantities of data are available to examine the effects of look direction on ionospheric range errors This paper will examine some of this data, from sites that are outside or at the edge of the mid-latitudes Measures of TEC variability will be discussed in the context of observed mid-latitude variability and the GPS correction model

GPS Azimuth Determination Using Short Baseline Carrier Wave Interferometry

Abstract: : The U.S. Army Topographic Engineering Center (TEC) is involved in research regarding the determination of azimuth by measurement of the carrier phase of GPS satellite signals between two or more antennas. TEC awarded three contracts to develop a GPS Azimuth Determining System (GPS ADS). Presented is a discussion of the three approaches, and test data from limited Government tests.

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