Wind velocity measurement from space

[Edwina Lee]

The Aeolus satellite will measure wind velocities of earth's atmosphere from space using ultra-violet laser light.

But how does the physics work?

This is what the article said about it:-

• Aeolus will fire an ultraviolet laser through the atmosphere and measure the return signal using a large telescope
• The light beam gets scattered back off air molecules and small particles moving in the wind at different altitudes

https://www.bbc.co.uk/news/science-environment-45100090

 

[Edwina Lee]

Here is my thinking of how it might work.

Small particles & air molecules move on average at the velocity of the wind.

Hence, any photons bounced off an air molecule would exhibit the Doppler effect, and so would photons reflected from a particle.

(Now, I do not know the physics of light, but I get the idea that photons have no mass, and so does not affect the velocity of molecules and particles.)

So, velocity can in theory be deduced from measuring the Doppler effect of photons coming back.

But how far away?

(Here again, I do not know the physics of light, but I get the idea that light reflection, and light re-admission are instantaneous.. 

So, if the time at which a laser fired, perhaps one photon at a time can be measured, and the time of the corresponding photon can be measured, then the distance can be calculated.

Or perhaps, the laser fires a range of UV frequency photon short bursts, then there is no need to go all the way down to one photon at a time.

But I bet the technologies which made these possible are very advanced indeed. Or perhaps not given the many properties of light & materials.

 

 

[Strange]

1 hour ago, Edwina Lee said:

Here is my thinking of how it might work.

Small particles & air molecules move on average at the velocity of the wind.

Hence, any photons bounced off an air molecule would exhibit the Doppler effect, and so would photons reflected from a particle.

You are absolutely right: it uses the Doppler effect to measure the speed of particles that light is reflected from. (You can think of this in tiers of photons, or just light as a wave, the result is the same.)

I know people like videos to explain things so: https://www.youtube.com/watch?v=q1nbaE0tpJs*)

 

1 hour ago, Edwina Lee said:

(Now, I do not know the physics of light, but I get the idea that photons have no mass, and so does not affect the velocity of molecules and particles.)

Actually, although light (and photons) have no mass, it does have momentum and so it can impart a pressure on things. But it will be insignificant compared to the pressure of the wind.

1 hour ago, Edwina Lee said:

Or perhaps, the laser fires a range of UV frequency photon short bursts, then there is no need to go all the way down to one photon at a time.

This is more likely. And then measure the difference in frequency of the received light compared to the laser.