Date: Friday, 10 April 1998
Special University Ph.D. Oral Examination
Time: 3:15pm (refreshments at 3:00pm)
Location: Durand 450
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Using an Electrodynamic Tethered Satellite as an Ionospheric Electric
Double Probe to Measure Vertical Electric Field and Estimate Tethered Satellite Motion
Scott D. Williams
Department of Aeronautics and Astronautics
This research develops new techniques for estimating tethered satellite motion and for measuring vertical electric
field in the F-region ionosphere. The Tethered Satellite System (TSS) was used to investigate the electrodynamic
behavior of a long, insulated te ther with charge collecting surfaces attached to each end. Accurate measurement of
induced potential across the tether, while deployed as a long electric double probe, is necessary to characterize fundamental
electrodynamic properties of tethered satellite systems.
Measurements of induced tether potential were obtained by the Tether Current and Voltage Monitor (TCVM) during the two missions
of the TSS, on the Space Shuttle Atlantis in August 1992 and on Columbia in February 1996. An accurate model of the electromotive
force (EMF) generated by the motion of the TSS through the Earth's magnetic field was developed. A model of tethered satellite
dynamics, the induced tether potential measurements, and the conventional measurements made by the deployer and Orbiter's Ku-band
Rendezvous Radar, were used in a Kalman Filter to estimate the motion of a tethered satellite relative to the Orbiter. The results
with and without the tether potential measurement were compared with the Relative Best Estimate of Trajectory (RELBET) provided by
NASA for the TSS satellite. The induced tether potential measurements and the motional EMF model were also used to determine the
component of the average ambient electric field between each end of the tether. The results were compared with previous satellite
and ground-based measurements of the vertical electric field for similar ionospheric conditions.
Experimental results confirm that the induced tether potential was measured by the TCVM with sufficient accuracy to characterize
tether system electrodynamics. By using the induced tether potential as an measurement update to a Kalman Filter, a new method to
estimate tethered satellite motion was demonstrated. The electric fields derived from the induced tether potential measurements and
the motional EMF model were found to be consistent with previous satellite and ground-based measurements for similar ionospheric
conditions. The tether's ability to act as a long baseline electric double probe, with a variation of two orders of magnitude in length,
makes the data unique and demonstrates a new capability for measuring the vertical component of the ionospheric electric field.
Comparison of the measured electric field from TSS-1R at different tether deployment lengths for corresponding local times reveals
that the average vertical component of the ambient electric field existed on scales of at least ~20km.