Radar Doppler measurements of the cislunar medium

During 1962 and the first half of 1963 more than three hundred hours of data on radar echoes from the moon have been collected and analyzed. This letter is a preliminary report on the principal features of the studies of the ‘Doppler excess’ frequency, the amount by which the measured Doppler frequency (received frequency minus transmitted frequency) exceeds what it would have been if the propagation medium had been free space. In the present experiment the Doppler excess frequency is almost entirely due to the time rate of change of the electron content in a column between the earth and the moon. A limited number of comparisons with Faraday polarization and with other ionospheric measurements reveals that the Doppler excess effects are largely due to changes that take place in the ionosphere, the rest of the cislunar medium being fairly stable. The Doppler measurements show the expected morning increase and evening decay of ionospheric electron content, together with irregularities in the ionosphere that become more pronounced during magnetic storms. However, on occasion it appears that the Doppler excess frequency shows changes that differ from those deduced by the Faraday method and which therefore must occur in that part of the cislunar medium that lies beyond the region near the earth where both the magnetic field strength and electron density are relatively high. Presumably these changes are in the interplanetary gas density between the earth and the moon, although they may possibly be very marked changes in the electron content of the magnetosphere above several thousand kilometers.