Observations at the Mullard Radio Astronomy Observatory, Cambridge

When the news of the launching of the first satellite was received at Cambridge, we were not aware of any plans for radio observations in this country. Such observa­tions seemed likely to be important: ( a ) In order to provide approximate information on the orbit so that accurate visual and radar observations might be made. ( b ) Because the presence of a transmitter at varying heights and zenith angles offered possibilities for ionospheric investigation. We therefore planned a series of observations which eventually engaged most of the radio astronomy group, and a large part of the ionosphere group of the Cavendish Laboratory. The first observations were aimed at the determination of an approximate orbit, and an interferometer normally used for observing radio stars at a frequency of 38 Mc/s was modified to receive the 40 Mc/s satellite transmission. This instrument was in operation on the night of 5 to 6 October, and a 40 Mc/s receiver for measuring the Doppler shift was installed the following day. A 20 Mc/s interferometer was added for ionospheric investigation, and receivers for measuring the apparent changes of intensity were later installed for 20, 40 and 80 Mc/s, the last being used on the second harmonic of the 40 Mc/s transmitter.