Measuring the Angle-of-Arrival and the Characteristic Polarizations of
  Radio Waves in Geospace

Bodo W. Reinisch and Gary S. Sales
University of Massachusetts 
Center for Atmospheric Research
Lowell, MA 01854

Satellites with three orthogonal antennas are able to measure the angle-of-arrival of
radio waves and their polarization ellipse. The technique is applicable to the ionosphere,
magnetosphere and interplanetary space. It will be applied for the first time by the Radio
Plasma Imager (RPI) instrument on the IMAGE mission scheduled for launch in January 2000
with an elliptical orbit varying in altitude from 1,000 km to 45,000 km. RPI
simultaneously samples all three antennas twice within an RF (or IF) cycle. The two
sampling times are spaced by one quarter of a wave period. This quadrature sampling method
is identical to the one used by the groundbased Digisondes where the samples are taken on
spaced receive antennas. Each three-antenna sample represents the instantaneous spatial E
field vector of the arriving wave. In a magnetoionic medium the waves are generally
elliptically polarized and the E vector has a component in the direction of the wave
normal. As long as the radio frequency is significantly larger than the plasma frequency
at the location of the satellite, this component is small and the plane of polarization
defines the wave normal.

The quadrature samples provide two orthogonal E vectors within the polarization plane and
the vector product defines the wave normal. Once the plane of polarization is determined,
the major and minor axes of the polarization ellipse and its tilt angle can be calculated
from the six voltage measurements at the three antennas. In the case of radio plasma
sounding used by RPI, the orientation of the transmitted wave is known and can be compared
with the polarization of the received signal to determine the Faraday effect.  The paper
also shows how to determine the amplitudes of the two characteristic waves, i. e. the
ordinary and extraordinary waves, from the measurements.