High-frequency direction finding in space

Abstract

Transionospheric propagation experiments at high frequency (HF) are planned in the Enhanced Polar Outflow Probe small-satellite mission. Crossed dipoles connected to a HF receiver in the spacecraft will be used to measure parameters of incident waves originating from coordinated ground transmitters. This article is concerned with the measurement of one important wave parameter: its direction of arrival (DOA). Two important issues about the receiving antenna signals are addressed here: the effective length of the dipoles, and the computational method of determining the DOA from the amplitudes and relative phase of signals. The electric field radiated by the monopole elements composing the dipoles has been calculated using the NEC4 code. With recourse to the reciprocity principle that the effective length of the active dipole equals that of the same dipole as a receiving antenna, the dependence of the effective length on DOA has been computed, for various antenna-spacecraft configurations. For 3 m monopoles combined as dipoles, and for frequencies between 5 and 18 MHz, effective lengths can be well approximated using a simple expression for a short dipole, with modest adjustment to include asymmetries of the transmitting members. The radiation-reception patterns of isolated monopoles are quite symmetric, implying that the experiment does not depend critically on dipole signals. The antenna models provide a basis for the numerical technique for inverting the amplitude and phase of voltages induced by an plane wave incident on crossed dipoles to the DOA of the wave. The procedure assumes a cw elliptically polarized wave, taking account of plasma effects. The mathematics leads to expressions for the induced dipole voltages as functions of the wave vector azimuthal and polar angles. The inverse determination of the DOA angles is underspecified for only two dipoles, and requires selecting the true solution from two or more possible azimuth-polar pairs throughout the 4π sr solid-angle space surrounding the spacecraft. The selection of the solution will be guided by values expected for the known ground-transmitter and satellite positions, and by other wave parameter measurements.