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where $v$ is the rate of change of distance (thus the minus sign).

where $f_0$ is the true frequency of the rotor sound, $f$ is the frequency heard at the microphone, and $v$ is the rate of change of distance (thus the minus sign).

$$$\Delta f = \frac{v}{c} f_0$$

$$\Delta f \approx -\frac{v}{c} f_0$$

where $v$ is the rate of change of distance (thus the minus sign).

Getting the absolute rotor frequency to a part per thousand or better from off-line analysis of telemetry is probably possible; the two rotor motors are actively synchronized (phased locked) to each other and probably feedback loops keep them close to a fixed frequency from an internal timebase (I'll look that up after finishing the first draft of this post).

• Mars Environmental Dynamics Analyzer or MEDA
• MEDA
• NASA's Perseverance rover deploys wind sensor on Mars

Getting the absolute rotor frequency to a part per thousand or better from off-line analysis of telemetry is probably possible; the two rotor motors are actively synchronized (phased locked) to each other and probably feedback loops keep them close to a fixed frequency from an internal timebase I'll look that up after finishing the first draft of this post, update: I couldn't find anything so far: Wikipedia' Ingenuity (helicopter) links to Mars Helicopter Technology Demonstrator (Canham et al. 2018, AIAA Atmospheric Flight Mechanics Conference, also here and archived)

• Mars Environmental Dynamics Analyzer or MEDA
• MEDA
• NASA's Perseverance rover deploys wind sensor on Mars
• Mars Weather Report from Perseverance