GPS TEC and ionosonde TEC over Grahamstown, South Africa: first comparisons

The Grahamstown, South Africa (33.3°S, 26.5°E) ionospheric field station operates a UMass Lowell digital pulse ionospheric sounder (Digisonde) and an Ashtech geodetic grade dual frequency GPS receiver. The GPS receiver is owned by Chief Directorate Surveys and Mapping (CDSM) in Cape Town, forms part of the national TrigNet network and was installed in February 2005. The sampling rates of the GPS receiver and Digisonde were set to 1 s and 15 min, respectively. Data from four continuous months, March–June 2005 inclusive, were considered in this initial investigation. Data available from the Grahamstown GPS receiver was limited, and, therefore, only these 4 months have been considered. Total Electron Content (TEC) values were determined from GPS measurements obtained from satellites passing near vertical (within an 80° elevation) to the station. TEC values were obtained from ionograms recorded at times within 5 min of the near vertical GPS measurement. The GPS derived TEC values are referred to as GTEC and the ionogram derived TEC values as ITEC. Comparisons of GTEC and ITEC values are presented in this paper. The differential clock biases of the GPS satellites and receivers are taken into account. The plasmaspheric contribution to the TEC can be inferred from the results, and confirm findings obtained by other groups. This paper describes the groundwork for a procedure that will allow the validation of GPS derived ionospheric information with ionosonde data. This work will be of interest to the International Reference Ionosphere (IRI) community since GPS receivers are becoming recognised as another source for ionospheric information

Imaging the metaphysical in contemporary art practice : a comparative study of intertextuality, poststructuralism and metaphysical symbolism

It was then that I decided to investigate how contemporary forms of metaphysical imaging have evolved formally and stylistically. I began to question how such approaches might be informed by current philosophical thought, given that many contemporary theorists have adopted a sceptical view towards metaphysical discourse. This point of contention presented me with the initial challenge of finding an artist whose exploration of metaphysical content is supported by topical philosophical thought. I intended this inquiry to serve as a basis from which to develop my own approach to imaging metaphysical content and to situate it within the context of contemporary thought

Imaging the metaphysical in contemporary art practice : a comparative study of intertextuality, poststructuralism and metaphysical symbolism

It was then that I decided to investigate how contemporary forms of metaphysical imaging have evolved formally and stylistically. I began to question how such approaches might be informed by current philosophical thought, given that many contemporary theorists have adopted a sceptical view towards metaphysical discourse. This point of contention presented me with the initial challenge of finding an artist whose exploration of metaphysical content is supported by topical philosophical thought. I intended this inquiry to serve as a basis from which to develop my own approach to imaging metaphysical content and to situate it within the context of contemporary thought

Breaking the Rocket Equation: How Refuellable Spacecraft Change the Dynamics of Space Transportation

One of the chief tenets of space transportation has been the immutability of the Tsiolkovsky rocket equation, placing great emphasis on the specific impulse of a thruster to determine the propellant mass. For applications where all propellant must be carried from the start, this drives most vehicle designs to the highest possible ISP. The compromise is the typical tradeoff between ISP and thrust magnitude. Thus, transfers which require either impulsive maneuvers or a tight timeline will favor low-ISP platforms. If adequate infrastructure is provided to allow for a refuellable spacecraft, a smaller and lighter vehicle can be used. This vehicle will have a payload mass fraction more in line with an equivalent system with many multiples higher specific impulse. As effective specific impulse increases to values approaching the highest performance electric propulsion systems, the time to destination remains of the same order of magnitude as an impulsive orbital maneuver. Further work must be done to optimize for the positioning and design of fuel depot infrastructure, especially bearing in mind both interactions with high radiation in the Van Allen belts, and ideal orbital planes to seed with these refuel depots

Application of neural networks to South African GPS TEC modelling

The propagation of radio signals in the Earth’s atmosphere is dominantly affected by the ionosphere due to its dispersive nature. Global Positioning System (GPS) data provides relevant information that leads to the derivation of total electron content (TEC) which can be considered as the ionosphere’s measure of ionisation. This paper presents part of a feasibility study for the development of a Neural Network (NN) based model for the prediction of South African GPS derived TEC. The South African GPS receiver network is operated and maintained by the Chief Directorate Surveys and Mapping (CDSM) in Cape Town, South Africa. Vertical total electron content (VTEC) was calculated for four GPS receiver stations using the Adjusted Spherical Harmonic (ASHA) model. Factors that influence TEC were then identified and used to derive input parameters for the NN. The well established factors used are seasonal variation, diurnal variation, solar activity and magnetic activity. Comparison of diurnal predicted TEC values from both the NN model and the International Reference Ionosphere (IRI-2001) with GPS TEC revealed that the IRI provides more accurate predictions than the NN model during the spring equinoxes. However, on average the NN model predicts GPS TEC more accurately than the IRI model over the GPS locations considered within South Africa

Development of a regional GPS-based ionospheric TEC model for South Africa

Advances in South African space physics research and related disciplines require better spatial and time resolution ionospheric information than was previously possible with the existing ionosonde network. A GPS-based, variable degree adjusted spherical harmonic (ASHA) model was developed for near real-time regional ionospheric total electron content (TEC) mapping over South Africa. Slant TEC values along oblique GPS signal paths are quantified from a network of GPS receivers and converted to vertical TEC by means of the single layer mapping function. The ASHA model coefficients and GPS differential biases are estimated from vertical TEC at the ionospheric pierce points and used to interpolate TEC at any location within the region of interest. Diurnal TEC variations with one minute time resolution and time-varying 2D regional TEC maps are constructed. In order to validate the ASHA method, simulations with an IRI ionosphere were performed, while the ASHA results from actual data were compared with two independent GPS-based methodologies and measured ionosonde data

Superconducing Alloys with Weak and Strong Scattering: Anderson's Theorem and a Superconductor-Insulator Transition

We have studied the effects of strong impurity scattering on disordered superconductors beyond the low impurity concentration limit. By applying the full CPA to a superconductiong A-B binary alloy, we calculated the fluctuations of the local order parameters $\Delta_{A}, \Delta_{B}$ and charge densities, $n_{A}, n_{B}$ for weak and strong on site disorder. We find that for narrow band alloy s-wav e superconductors the conditions for Anderson's theorem are satisfied in general only for the case of particle-hole symmetry. In this case it is satisfied regardless whether we are in the weak or strong scattering regimes. Interestingly, we find that strong scattering leads to band splitting and in this regime for any band filling we have a critical concentration where a superconductor-insulator quantum phase transition occurs at T=0.Comment: 28 pages, 13 figure