How does a laser rangefinder measure distances up to 20 km?

[DariusP]

Hello, I've known about average laser rangefinders that we can use for domestic needs. They're relatively cheap and can measure up to 50 meters but that's about it.

However, I've read that military laser rangefinders are able to measure distances using light up to 20 kilometers. How is that possible? By Beer's law light should be attenuated exponentially when traversing some medium, so how does a light travel 40 kilometers and can still be deciphered out of noise? Surely they're not firing high intensity pulses for these measurements?

I've read about it here in the applications section: https://en.wikipedia.org/wiki/Laser_rangefinder

 

[A.T.]

how does a light travel 40 kilometers and can still be deciphered out of noise?

Under ideal conditions you can see 40km or further:
https://en.wikipedia.org/wiki/Visibility

 

[davenn]

Summary: Laser rangefinder ability to measure long distances up to 20 km

However, I've read that military laser rangefinders are able to measure distances using light up to 20 kilometers.

easy ... they can fire a laser at the moon and get a good reflected signal to measure the distance accurately
over a return distance of more than 760,000 km, so why would 20 km be so difficult ?

Gear I work with can do an accuracy of 3mm over 1km ( 2km return path) with a 2mW laserDave

 

[davenn]

Summary: Laser rangefinder ability to measure long distances up to 20 km

so we never got a response from you
do you now see any reason for no difficulty measuring 20km ?