New Techniques in Spaceborne Sounding - RPI and TOPADS

B.W. Reinisch, D.M. Haines, G.S. Sales
University of Massachusetts, Center for Atmospheric Research
600 Suffolk St., Lowell, MA, 01854 USA
Tel: 978 934-4900, Fax: 978-459-7915, e-mail: Bodo_Reinisch@uml.edu

R.F. Benson, S.F. Fung, D.L. Gallagher, J.L. Green, W.W.L. Taylor
NASA, GSFC and MSFC

J.L. Bougeret,  R. Manning,
Observatoire de Paris, Meudon, France


In January 2000, NASA's IMAGE satellite will be launched into a high inclination elliptical 
orbit with an altitude of 7 RE at apogee. The IMAGE payload includes the Radio Plasma 
Imager (RPI). RPI will use active Doppler radar techniques for the remote sensing of the 
magnetospheric plasma structures using frequencies from 3 kHz to 3 MHz MHz detecting 
electron densities from N = 0.1 to 105 cm-3. For the first time, a spaceborne sounder will 
measure the angle-of-arrival, Doppler shift and wave polarization of the received echoes 
and not only the amplitudes as function of frequency and range. The measuring techniques 
are adopted from the groundbased Digisonde systems and adapted to the space environment.
Two orthogonal 500 m dipoles are used for transmission with a near omni-directional
radiation pattern. A third shorter dipole of 20 m length is used to receive the 
echoes on three orthogonal antennas. By quadrature sampling the three receiver outputs, 
the angle-of-arrival of the diverse echoes can be determined as well as the polarizations of 
the characteristic ordinary and extraordinary waves. These measurements will determine the 
locations of the magnetopause and plasmapause, and the associated electron density 
profiles in response to changes in the solar wind. 

Based on the same principles, a TOPside Automated Doppler Sounder (TOPADS) is being 
developed to obtain reliable electron density profiles above the F2 layer peak in the 
ionosphere. All previously flown topside sounders lacked the capability to measure the 
Doppler shift and angle-of arrival of the echoes leading to errors in the profile and the 
height of the F2 layer peak. Automatic processing of the ionogram data will provide real 
time electron density profiles. Originally planned for the Ukrainian WARNING mission, 
TOPADS is also being considered for the U.S. NPOES program. Three orthogonal 20 m 
tip-to-tip dipole antennas will be used for reception, one of them for transmission. 
Operating as a HF radar, TOPADS will provide for the first time topside plasma velocity 
measurements by tracking the motions of plasma irregularities.