O-Strand Radar Practice Test

State the Doppler shift equation for a target moving with radial velocity v and wavelength λ.

f_d = v / λ f_d = 2 v / λ f_d = v f_0 / c f_d = 2 c / λ

Correct Answer:

f_d = 2 v / λ

Explanation:

In radar, the Doppler effect causes a frequency shift on the returned signal that is twice the one-way shift because the wave experiences the Doppler effect on both the outbound trip and the inbound echo. For a target moving with radial velocity v and a wavelength λ, the one-way Doppler shift is Δf = v/λ. Double that for the round trip, giving Δf = 2 v/λ. This can also be written using f0 = c/λ as Δf = 2 f0 v / c. So the correct relationship is f_d = 2 v / λ. The other forms miss the factor of two (they reduce to v/λ) or mix definitions.

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