Doppler shift refers to a change in the frequency of a signal when the source is moving relative to the observer. If the source is moving towards the observer consecutive waves of the signal are emitted from a point closer to observer, as the speed of the signal does not change this leads to an increased number of waves reaching the observer in the same time interval. As a result the frequency of the signal as perceived by the observer is increased. The reverse is the case if the source is moving away from the observer.
For a signal traveling at a velocity c, the frequency measured by the observer `F_o` is related to the frequency of the signal emitted by the source `F_s` as follows: `F_o = F_s*((c + v_o)/(c + v_s))` where `v_s` and `v_r` are the velocity of the source and receiver respectively.