Echo Arrival Measurements with RPI on the IMAGE Satellite

G.S.Sales, B.W.Reinisch. D.M.Haines, G.C.Cheney, U. Massachusetts Lowell
J.L.Green, S.F.Fung, R.F.Benson, W.W.L.Taylor, NASA GSFC

NASA's IMAGE satellite was launched was launched on 25 March 2000 from Vandenburg AFB, CA. 
into a highly elliptical polar orbit with an altitude of 7 Re at apogee and 1000 km at 
perigee. A radio sounder on IMAGE, the Radio Plasma Imager (RPI), transmits radio pulses 
with frequencies between 3 kHz and 3 MHz into all directions to obtain information about 
the plasma distribution in the magnetosphere, including the magnetopause, the cusp, and 
the plasmasphere. The received echo signals are displayed in a "plasmagram" showing the 
echo amplitudes as function of frequency and range similar to the ionograms used in 
ionospheric sounding, except that the echo ranges extend to several Re. To correctly 
interpret the different echo traces in the plasmagram it is necessary to measure the 
arrival angle of the different echoes. RPI has three orthogonal dipole antennas, two 500-m 
tip-to-tip dipoles in the spin plane, and a 20-m tip-to-tip dipole along the spin axis. 
One of the long dipoles is used for transmission of the radio pulses, and all three 
antennas are used for the arrival angle measurement. The intermediate frequency at the 
output of the three receivers is quadrature sampled, i.e., samples are taken at omega*t = 0 
and omega*t = pi/2. This sampling technique measures two instantaneous E field vectors in 
the polarization plane, E and E', and the vector product E x E' gives the direction of 
arrival of the echo. During IMAGE's first few months in orbit, RPI has recorded echoes from 
the magnetopause, the plasmasphere, and the cusp. The direction finding technique is 
discussed using some of these echoes. It is shown that the accuracy of the calculated 
angles is critically dependent on the calibration of the three receivers.

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Presented at the URSI meeting, Boulder, Colorado, U.S.A., Jan 2001