# Red/Blue Shift in EM Waves

A Doppler weather radar operates at a frequency of 3.40 GHz. The wave from this radar system reflects from an approaching weather system moving with a speed of 39.0 m/s. What is the difference in frequency between the outgoing and returning waves? Ans: 884 Hz

How do I compute this? I did

$$f = f_s (1 \pm u/c)$$

$$f = 3.4\times 10^9 (1 + \frac{39}{3 \times 10^8}) = 442 Hz$$

I suppose thats the incoming frequency? How do I know the outgoing frequency difference? Looks like its just times 2? Why?

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