Showing papers on "TEC published in 1988"

A comparison of mapped and measured total ionospheric electron content using global positioning system and beacon satellite observations

Abstract: Total ionospheric electron contents (TEC) were measured by global positioning system (GPS) dual-frequency receivers developed by the Jet Propulsion Laboratory. The measurements included P-code (precise ranging code) and carrier phase data for six GPS satellites during multiple five-hour observing sessions. A set of these GPS TEC measurements were mapped from the GPS lines of sight to the line of sight of a Faraday beacon satellite by statistically fitting the TEC data to a simple model of the ionosphere. The mapped GPS TEC values were compared with the Faraday rotation measurements. Because GPS transmitter offsets are different for each satellite and because some GPS receiver offsets were uncalibrated, the sums of the satellite and receiver offsets were estimated simultaneously with the TEC in a least squares procedure. The accuracy of this estimation procedure is evaluated indicating that the error of the GPS-determined line of sight TEC can be at or below 1 x 10 to the 16th el/sq cm. Consequently, the current level of accuracy is comparable to the Faraday rotation technique; however, GPS provides superior sky coverage.

Developmental studies on expression of monoclonal antibody-defined cytokeratins by thymic epithelial cells from normal and autoimmune mice.

Abstract: A major component of the thymic microenvironment is a network of thymic epithelial cells (TEC) which are able to express class II major histocompatibility complex products and to secrete thymic hormones. In the present investigation, we used a panel of anti-cytokeratin (CK) antibodies to establish distinct cytokeratin-defined TEC subsets. Four subpopulations were identified. One, in the cortex, is defined by anti-CK8 and anti-CK18 monoclonal antibodies (MAb). The other three subsets are medullary, two minor ones respectively reactive with anti-CK19 and KL1 monoclonal antibodies (the latter being specific for CK3 and 10), and a major one characterized by negative reaction with the above-mentioned MAb but strongly positive after labeling with a polyclonal (and polyspecific) anti-keratin immunoserum. Ontogenetic studies revealed that the CK8+/18+ TEC subset is the first to be detected in fetal life. Moreover, the numbers of CK3/10+ cells and CK19+ cells decrease in aging normal mice, a phenomenon that seems to occur early in autoimmune mice. We also observed that these two medullary TEC subsets are sensitive to high-dose in vivo treatment with hydrocortisone, which stimulates a dramatic increase in CK3/10+ cells and a certain decrease in CK19+ cells. Our results indicate that a number of mouse TEC subsets can be distinguished by cytokeratin expression. Such a strategy can be applied to analyze TEC sensitivity to drugs and might also be useful to further understanding of differential TEC function regarding intrathymic T-cell differentiation.

Traveling ionospheric disturbances (TIDs) at mid-latitudes - solar cycle phase dependence

Abstract: Faraday observations of total electron content (TEC) at Haifa, Israel (32.87 deg N, 35.09 deg E), during periods near the maximum (1980) and minimum (1984) phases of the current solar cycle have yielded information about the structure and variability of the ionosphere during both epochs in general, and about traveling ionospheric disturbances (TIDs) in particular. The TEC is characterized by the large differences in the seasonally dependent absolute values, by the generally occurring spatially confined postsunset secondary maxima during solar maximum which are absent during solar minimum, and by the regularly appearing modulations of the structure due to TIDs. There is a definite seasonal and solar phase dependence in the characteristics of the TIDs. This is expressed in terms of their frequency of occurrence, likely phase of the diurnal variation during which the occurrences take place, their absolute magnitudes, and the time rate of change of these magnitudes. 9 references.

A Dual Frequency GPS Receiver Measuring Ionospheric Effects Without Code Demodulation and Its Application to Time Comparisons

Abstract: : The GPS (Global Positioning System) satellites have become widely used for daily time comparisons between the major time and frequency laboratories. The precision of time comparison by GPS satellites is now between one and a few tens of nanoseconds depending upon the distance between the laboratories. In the case of long-distance time comparisons, the ionospheric effect is one of the largest sources of uncertainty. To compensate for the ionospheric effect, we have developed a novel GPS receiver which measures the total electron content (TEC) along the signal path to the GPS satellite. It uses the property of cross-correlation between the P-code (Precise-code) signals which are transmitted Erom GPS satellites by L1 (1575.42 MHz) and 4, (1227.6 MHz), without demodulating P-code signal. Preliminary results using the prototype receiver give, for about 3 minutes observation time, an uncertainty in the measurement of TEC of 2 x 1016/m2, equivalent to an uncertainty in the delay of L1 signal of 1 ns. We have begun to apply the results of this receiver to the time comparisons between USA and Europe.

A comparison of equatorial electron content in the Indian and American longitudes

Abstract: Total electron content (TEC) measurements taken at the magnetically equatorial stations located at Ootacamund, India and Huancayo, Peru by the group delay technique from radio beacon signals transmitted from the ATS 6 geostationary satellite show excellent agreement, though these stations are at widely different longitudes and are at nearly opposite geographic latitudes. Data from both stations were taken during the solar minimum period 1975–1976. The equivalent slab thickness, the ratio TEC/Nmax, also indicated similar F region profile shape in the two longitude sectors. The standard deviation of equatorial daytime TEC is significantly smaller than at other latitudes.

Ionospheric effect of the magnetospheric substorms at middle latitudes

Abstract: A study is made of theF2-layer effect of magnetospheric substorms over the Mediterranean area using data from several ionospheric stations for selected events in the current sunspot cycle 21. The night-time enhancements in the critical frequency of theF2-layer (f 0 F2) and the total electron content (TEC) have been found with both premidnight and postmidnightf 0 F2 peaks and a subsequent decrease in the minimum virtual height of theF region (h′F). It is found that the enhancements occur through the nights under steady geomagnetic conditions and that the time at which it is seen at Rome and Grocka ionospheric stations is progressively earlier as geomagnetic activity increases. It has been further shown that this type of thef 0 F2 night-time increases is not always accompanied by an increase in TEC, although the reverse holds true during the nights of increased substorm activity. The fact that the considerable variability inf 0 F2, TEC andh′F at the onset of the substorm expansion are preceded by the ionospheric dynamics associated with these observations can be very useful in the identification of precursor indicative of short-term variations of ionospheric propagation conditions.

Improvements In Reliability Of Thermoelectric Coolers Through Redundant Element Design

Abstract: Thermoelectric coolers (TECs) are solid-state devices with no moving parts, and therefore are inherently very reliable. The inherent reliability of the cooler, however, is determined by the electrical arrangement of the p- and n- type thermoelements (elements). Typically, TECs are manufactured with the elements in series electrically, resulting in the highest voltage and lowest current requirements for a given number of elements, but also the lowest possible inherent reliability. For maximum TEC reliability, all of the elements should be connected electrically in parallel, but this results in the need for an expensive high current power supply. By using a redundant element design, the inherent reliability of the TEC can be dramatically improved with very minimal impact on TEC cost and performance, and more acceptable increases in current requirements. A brief description of the exponential reliability model is presented followed by an analysis of various electrical arrangements of the TEC elements in order to optimize the TEC design as it relates to 1) reliability, 2) electrical requirements, and 3) impact on system design and cost. Comparisons are made between the configurations and the results are tabulated and graphed.

Ionospheric scintillations and in-situ measurements at an auroral location in the European sector

Abstract: : The orbiting HiLat satellite offered a unique opportunity for studying the ionospheric scintillation parameters in relation to the in-situ measurements of ionization density, drift velocity, field-aligned current, and particle precipitation during the sunspot minimum period. This paper discusses the results of such a morphological study based on observations at the auroral oval station of Tromso, Norway. The dynamics of the spatial and temporal extent of this region are illustrated in the invariant latitude/magnetic local time grid. The geometrical enhancement of scintillations observed during the alignment of the propagation path with the local magnetic L-shell is shown to be the most consistent and conspicuous feature of scintillations in the nighttime auroral oval. The steepening of phase spectral slope in this region is indicative of the presence of L-shell aligned sheet-like irregularities at long scale lengths. The seasonal variational of total electron content (TEC) determined from the differential Doppler measurements of HiLat transmissions is discussed in relation to the in-situ density measurements at 830 km. The results are also utilized to illustrate the dependence of ionospheric structure parameters on short-term variability of solar activity during the sunspot minimum period. Special effort is made to illustrate that the joint study of scintillation/TEC and in-situ parameters provides an insight into the nature of magnetospheric coupling with the high latitude ionosphere. Keywords: Amplitude and phase scintillations; Total electron content; In-situ density; Auroral oval; Sunspot cycle variation; Irregularity anisotropy; Reprints.

Through-the-earth communication: Experiment results from Billie Mine and Mississippi Chemical Mine

Abstract: As part of the Lawrence Livermore National Laboratory (LLNL) effort to evaluate ''Through-the-Earth Communication'' (TEC) as an option for military communication systems, experiments were conducted involving transmission, reception, and performance monitoring of digital electromagnetic communication signals propagating through the earth. The two experiments reported on here not only demonstrated that TEC is useful for transmissions at digital rates above a few bits per second, but also provided data on performance parameters with which to evaluate TEC in various military applications. The most important aspect of these experiments is that the bit error rate (BER) is measured rather than just estimated from purely analytic developments. By measuring this important parameter, not only has more credibility been lent to the ''proof of concept'' goals of the experiment, but also a means for judging the effects of assumptions in BER theoretical models has been provided. 4 refs., 35 figs., 3 tabs.

Multiple‐scattering theoretic approach to the thermal expansion of inhomogeneous materials

Abstract: Expression for the effective thermal expansion coefficient (TEC) of statistically homogeneous and isotropic, inhomogeneous material has been derived within the framework of the multiple‐scattering theory. Then, from the general expression, effective TEC is obtained in the single‐grain scattering approximation, for this type of materials, consisting of piecewise homogeneous phases (grains). For simplicity, only spherical grains are considered. Previous exact results for two‐phase composites and polycrystals with crystallites having a preferred axis (e.g., tetragonal, trigonal, and hexagonal) are shown to follow from this approach in the single‐grain scattering approximation. Apart from this, it is shown that the effective TEC for general multiphase composites and polycrystals can be obtained in a self‐consistent way. Finally, the self‐consistent solutions thus obtained have been employed to calculate the effective TEC of polycrystals belonging to different symmetry classes, and the results are found to com...