A Compact Digital Implementation of an Impedance Probe

Abstract:

USU has constructed, tested and delivered a compact digital impedance probe as part of the SPORT Brazil/US CubeSat mission. SPORT is expected to launch in the 4^th quarter of 2021. The Sweeping Impedance Probe measures the dielectric properties of the ionospheric plasma and will be used in conjunction with an on-board magnetometer to find the absolute electron density. The upper hybrid plasma resonance is identified in both magnitude and phase within an impedance vs frequency curve from which electron density is determined. This technique is important because it is unaffected by spacecraft surface charging and allows for the determination of absolute electron density traceable to a fundamental plasma resonance. A damping of the probe-plasma resonances is expected due to electron-neutral collision thus providing a path for measuring thermosphere density provided the instrumentation has sufficient sensitivity.

The SPORT impedance probe switches between two modes: one in which it sweeps over 512 logarithmically spaced frequency steps, recording the real and imaginary parts of the impedance at each step, and a second mode where the probe tracks the frequency of the upper hybrid resonance. The SPORT impedance probe is novel in that the driving frequency of the impedance probe originates from a numerically controlled oscillator and DAC and the current input to the probe is monitored with a high-speed ADC both processing the RF baseband signal. The impedance is then computed digitally providing improvements in both speed and accuracy over previous implantations of impedance probes. Within this paper we will present and overview of the SPORT Sweeping impedance probe and then focus on the use of the increased sensitivity to observe relative changes in thermosphere density by monitoring the damping of resonances as observed in the impedance of the cylindrical probe.