Antenna Systems for NUTS

NTNU is aiming to build and launch a small student satellite compliant with thedouble CubeSat standard, by 2014. The NTNU Test Satellite (NUTS) will carrytwo radio tranceivers and a beacon transmitter, all located in the VHF and UHFamateur bands. The goal of this thesis was to build the whole antenna systems forthe spacecraft.Turnstile antennas were chosen both for UHF and VHF, since they yield thehighest received signal strength on ground throughout the whole pass of the satel-lite. In order to reach this conclusion, a study of how the choice of spacecraftantennas dynamically influence the link margin was carried out. Furthermore, theantennas was made by measuring tape, and is to be wrapped around the satelliteuntil deployment. This is for the satellite to be within the size constraints of adouble CubeSat during launch. The measuring tape will be tightened to the satel-lites body by nylon thread, which will be burnt off by Nichrome-wire to release themeasuring tape when the satellite is in-orbit. The measuring tape is mounted inbetween two circuit boards constituting the internal part of the antenna module.The feeding network is encapsulated by these two circuit boards. The module ismade such that the antenna complies with the CubeSat standard, and such that itcan be integrated with other modules on-board the satellite, such as the camera.Feeding the antennas turned out more difficult than first anticipated. Whereasconventional feeding networks for turnstile antennas typically comprises baluns andarrangements of coaxial cable comparable to the wavelength, such solutions are notfeasible within the satellite due to size constraints. As a result a lumped feedingnetwork was designed, that could be mounted on one of the circuit boards.Because of the many reactive components in the feeding network, it becamenecessary to determine how deviations in component values affect the performanceof the circuit. Statistical simulations was performed, and the deviations may causean unbalance between the amplitudes and phases of the outputs. But this resultsin less than half a dB additional losses in the link budget.A refinement of electromagnetic antenna simulations from previous work wasalso carried out. With this, the performance of the antennas was verified, andtheir lengths were optimized to yield the best possible match. The VHF antenna isnear-ideal both in terms of pattern and matching. The pattern of the UHF antennais somewhat distorted, but it is still considered to be the best suited antenna ofthose considered. The matching of the UHF antenna could have been better, butis considered acceptable. Moreover, the mutual coupling between the VHF andUHF antennas were studied, with the conclusion that the antennas may very wellco-exist on-board the satellite.Finally, both the feeding network and the VHF antenna were successfully builtand tested. The feeding network had an insertion loss of less than 2 dB, whereas thepattern of the antenna was measured to be near-ideal. Unfortunately, a fault hasoccurred on the UHF feeding network, which also prohibits the pattern for the UHFantenna to be measured. Nonetheless, all the parts constituting it has been built,and all the necessary simulations are done. Previous and future troubleshooting ofthe feeding network is also discussed

m-NLP Inference Models Using Simulation and Regression Techniques

publishedVersio

The role of particle precipitation on plasma structuring at different altitudes by in-situ measurements

The plasma in the cusp ionosphere is subject to particle precipitation, which is important for the development of large-scale irregularities in the plasma density. These irregularities can be broken down into smaller scales which have been linked to strong scintillations in the Global Navigation Satellite System (GNSS) signals. We present power spectra for the plasma density irregularities in the cusp ionosphere for regions with and without auroral particle precipitation based on in-situ measurements from the Twin Rockets to Investigate Cusp Electrodynamics-2 (TRICE-2) mission, consisting of two sounding rockets flying simultaneously at different altitudes. The electron density measurements taken from the multi-needle Langmuir probe system (m-NLP) were analyzed for the whole flight duration for both rockets. Due to their high sampling rates, the probes allow for a study of plasma irregularities down to kinetic scales. A steepening of the slope in the power spectra may indicate two regimes, a frequency interval with a shallow slope, where fluid-like processes are dominating, and an interval with a steeper slope which can be addressed with kinetic theory. The steepening occurs at frequencies between 20 Hz and 100 Hz with a median similar to the oxygen gyrofrequency. Additionally, the occurrence of double slopes increases where precipitation starts and throughout the rest of the flight. In addition, strong electron density fluctuations were found in regions poleward of the cusp, thus in regions immediately after precipitation. Furthermore, by investigating the integrated power of the fluctuations within different frequency ranges, we show that at low frequencies (10–100 Hz), the power is pronounced more evenly while the rocket encounters particle precipitation, while at high frequencies (100–1000 Hz) fluctuations essentially coincide with the passing through a flow channel

The Unstructured Particle-In-Cell Method with Applications for Objects in Ionospheric Plasmas

Spaceborn instruments such as the Langmuir probe are essential to understand our own ionosphere. In this dissertation we explore the behaviour of such instruments through computer simulations, and find that some of the assumptions about them may not be well justified, causing degraded performance. We also develop new theories to counteract these effects, as well as new methods for simulating such objects in plasmas. One of the ways to measure the electron density in the ionosphere is by applying a positive voltage to a thin wire – a Langmuir probe – and then exposing it to the plasma in the ionosphere. The probe will then attract electrons, and using the so-called OML theory, this current of electrons can be used to calculate the electron density. However, the OML theory relies on several simplifying assumptions that are not always well satisfied, leading to a reduced accuracy. Ionospheric plasmas containing objects can be simulated on a computer using the Particle-In-Cell method, and an unstructured, tetrahedral mesh allows for arbitrary geometries, such as that of Langmuir probes attached to a satellite. Using such simulations, we quantify the electron current collected by probes of short length, or situated in a non-Maxwellian plasma. This is not covered by the usual OML theory, and this knowledge may therefore be used to improve ionospheric measurements. We also revealed by simulations that multineedle Langmuir probes mounted on a small satellite may charge the satellite sufficiently to render the measurements invalid, unless care is taken. Finally, we have also contributed with new numerical methods for simulating objects connected in arbitrary circuits in plasmas

Antenna Systems for NUTS

NTNU is aiming to build and launch a small student satellite compliant with thedouble CubeSat standard, by 2014. The NTNU Test Satellite (NUTS) will carrytwo radio tranceivers and a beacon transmitter, all located in the VHF and UHFamateur bands. The goal of this thesis was to build the whole antenna systems forthe spacecraft.Turnstile antennas were chosen both for UHF and VHF, since they yield thehighest received signal strength on ground throughout the whole pass of the satel-lite. In order to reach this conclusion, a study of how the choice of spacecraftantennas dynamically influence the link margin was carried out. Furthermore, theantennas was made by measuring tape, and is to be wrapped around the satelliteuntil deployment. This is for the satellite to be within the size constraints of adouble CubeSat during launch. The measuring tape will be tightened to the satel-lites body by nylon thread, which will be burnt off by Nichrome-wire to release themeasuring tape when the satellite is in-orbit. The measuring tape is mounted inbetween two circuit boards constituting the internal part of the antenna module.The feeding network is encapsulated by these two circuit boards. The module ismade such that the antenna complies with the CubeSat standard, and such that itcan be integrated with other modules on-board the satellite, such as the camera.Feeding the antennas turned out more difficult than first anticipated. Whereasconventional feeding networks for turnstile antennas typically comprises baluns andarrangements of coaxial cable comparable to the wavelength, such solutions are notfeasible within the satellite due to size constraints. As a result a lumped feedingnetwork was designed, that could be mounted on one of the circuit boards.Because of the many reactive components in the feeding network, it becamenecessary to determine how deviations in component values affect the performanceof the circuit. Statistical simulations was performed, and the deviations may causean unbalance between the amplitudes and phases of the outputs. But this resultsin less than half a dB additional losses in the link budget.A refinement of electromagnetic antenna simulations from previous work wasalso carried out. With this, the performance of the antennas was verified, andtheir lengths were optimized to yield the best possible match. The VHF antenna isnear-ideal both in terms of pattern and matching. The pattern of the UHF antennais somewhat distorted, but it is still considered to be the best suited antenna ofthose considered. The matching of the UHF antenna could have been better, butis considered acceptable. Moreover, the mutual coupling between the VHF andUHF antennas were studied, with the conclusion that the antennas may very wellco-exist on-board the satellite.Finally, both the feeding network and the VHF antenna were successfully builtand tested. The feeding network had an insertion loss of less than 2 dB, whereas thepattern of the antenna was measured to be near-ideal. Unfortunately, a fault hasoccurred on the UHF feeding network, which also prohibits the pattern for the UHFantenna to be measured. Nonetheless, all the parts constituting it has been built,and all the necessary simulations are done. Previous and future troubleshooting ofthe feeding network is also discussed

Finite-length effects on cylindrical Langmuir probes

Kinetic simulations are used to compute current characteristics of finite-length cylindrical probes, with particular attention to end effects. Currents collected per unit lengths, as a function of distance to the ends, are calculated and fitted to empirical analytic functions. These fits, in turn, can be interpolated and used to predict probe characteristics; that is, collected current as a function of applied voltage, for a broad range of physical parameters of relevance to laboratory and space plasma

Inference of plasma parameters from fixed-bias multi-needle Langmuir probes (m-NLP)

New approaches are presented to infer plasma densities and satellite floating potentials from currents collected with fixed-bias multi-needle Langmuir probes (m-NLP). Using synthetic data obtained from kinetic simulations, comparisons are made with inference techniques developed in previous studies and, in each case, model skills are assessed by comparing their predictions with known values in the synthetic data set. The new approaches presented rely on a combination of an approximate analytic scaling law for the current collected as a function of bias voltage, and multivariate regression. Radial basis function regression (RBF) is also applied to Jacobsen et al's procedure (2010 Meas. Sci. Technol. 21 085902) to infer plasma density, and shown to improve its accuracy. The direct use of RBF to infer plasma density is found to provide the best accuracy, while a combination of analytic scaling laws with RBF is found to give the best predictions of a satellite floating potential. In addition, a proof-of-concept experimental study has been conducted using m-NLP data, collected from the Visions-2 sounding rocket mission, to infer electron densities through a direct application of RBF. It is shown that RBF is not only a viable option to infer electron densities, but has the potential to provide results that are more accurate than current methods, providing a path towards the further use of regression-based techniques to infer space plasma parameters

Theory and simulations of spherical and cylindrical Langmuir probes in non-Maxwellian plasmas

The collected current by spherical and cylindrical Langmuir probes immersed in an unmagnetized and collisionless non-Maxwellian plasma at rest are theoretically studied, and analytical expressions for the currents of attracted and repelled plasma particles are presented. We consider Kappa, Cairns and the generalized Kappa–Cairns distributions as possible models for the velocity field in the plasma. The current–voltage characteristics curves are displayed and discussed. Furthermore, comparisons with the collected currents in Maxwellian plasmas are given. The results of Particle-in-Cell (PIC) simulations of spherical and cylindrical probes in non-Maxwellian plasmas are also presented, and compared with the theoretical expressions. The results for the collected currents by the Langmuir probes obtained by PIC simulations are in good agreement with the corresponding analytical expressions

A Novel Method for Circuits of Perfect Electric Conductors in Unstructured Particle-in-Cell Plasma–Object Interaction Simulations

A novel numerical method has been developed that incorporates electrically conducting objects into particle-in-cell simulations of electrostatic plasma. The method allows multiple objects connected by voltage and current sources in an arbitrary circuit topology. Moreover, by means of an unstructured mesh, the objects can have arbitrary shapes. The electric potential of the objects is solved self-consistently by incorporating charge constraints into the finite element discretization of the Poisson equation. This method has been implemented in a new code, Particles-in-Unstructured-Cells (PUNC), suitable for rapid prototyping. The flexibility of this code has proven convenient to survey various methods, and an issue of reduced convergence rate of today's unstructured plasma-object interaction codes is highlighted. The results for a conducting sphere immersed in the Maxwellian plasma are in good agreement with previous studies