Date of Award:

5-2009

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Electrical and Computer Engineering

Advisor/Chair:

Edmund A. Spencer

Abstract

The Plasma Impedance Probe (PIP) is an RF-instrument that is used to measure plasma electron density and electron neutral collision Frequency. Radio Frequency probe techniques for the determination of plasma parameters are attractive especially because the RF response is not susceptible to spacecraft charging problems at frequencies above the electron plasma frequency where ion sheath effects are negligible. The work done in this thesis is presented as a compendium of two papers, the first paper dealing with PIP measurements on a sounding rocket mission and the second being a qualitative study of the effects of spacecraft aerodynamics on the impedance of antennas. Swept Impedance Probe measurements in a sporadic E-Layer observed during the Sudden Atomic Layer (SAL) sounding rocket mission are analyzed in the first paper to obtain absolute electron densities and electron neutral collision frequencies accurately. Three sets each of upleg and downleg impedance data are selected for the analysis. Initial estimates of the plasma parameters are obtained through a least mean square fit of the measured impedance data against the analytical impedance formula of Balmain. These initial parameters are used as a starting point to drive a finite difference computational model of an antenna immersed in a plasma called the Plasma Fluid Finite Difference Time Domain (PF-FDTD) model. The parameters are then tuned until a close fit is obtained between the measured impedance data and the numerical impedance data calculated by the PF-FDTD simulation. The electron neutral collision frequencies obtained from the more accurate PFFDTD simulation were up to 20% lower than the values predicted by Balmain's formula. The obtained collision frequencies are also lower than the quiet time values predicted by Schunk and Nagy when used in conjunction with neutral densities and electron temperature from the MSISE-90 model. Plasma impedance probe measurements on sounding rockets are affected by aerodynamics of the spacecraft body. The second paper analyzes the effects of density gradients and wake effects on the impedance of a dipole antenna. A Direct Simulation Monte Carlo (DSMC) code is used to simulate the aerodynamic conditions encountered during a typical sounding rocket flight. Conical electron density flow structure is used to approximate the density distribution around a spinning rocket flying through the lower ionosphere. The PF-FDTD code is modified to simulate the impedance of a dipole antenna under different flow conditions. A methodology is developed to find correction factors to rectify errors introduced due to spin modulation on sounding rockets, and the technique is applied for the SAL sounding rocket mission.

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