Subrahmanyan Chandrasekhar

“ We have seen how the number of waves counted on a record of the beat phenomenon during a certain interval of time enables us to derive only the position of the projectile at the time from a knowledge of its position at the time . Since the position of the projectile at the instant of firing can be assumed known we should in principle be able to follow the projectile in its trajectory. ”

“ We allow the micro waves of a given frequency emitted by a transmitter to be reflected from a projectile in motion. On account of the Doppler effect these reflected waves have frequencies slightly different from that of the incident waves (see equation [1] below) . Consequently, if we now superpose the reflected wave trains on a wave train having the same frequency as the incident waves we shall obtain a beat phenomenon. It is evident that quantitative measures based on the existence of this beat phenomenon should enable us to determine the dynamical characteristics of the trajectory. ”

“ Before proceeding further with the interpretation of when is a function of time we should perhaps remark that in order that the procedure may at all have an unambiguous meaning it is necessary that two conditions be satisfied. ”