doppler frequency shift

The doppler frequency shift is fDn(t)=vcosθ(t)λ,

If the transmitter transmits at fc then the recieved frequency is fc+fD.

So, if v is positive (distance between transmitter and reciever is reducing), then the recieved frequency is higher.

θ refers to angle between reciever to transmitter and direction of motion. So, if reciever moves perpendicular to transmitter, i.e θ=90 then there is no doppler shift.

Doppler has a greater effect on very small wavelength/very high frequency.