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Drones are causing airports to close temporarily. Is there any way that they can be detected? For example helicopters are detected by radar as simultaneous Doppler shifts both up and down because of the advancing and retreating blades. This is made more difficult with drones because small plastic blades don't show up too well on radar. Is there an optical equivalent of sufficient sensitivity to detect a quadrotor drone?

I'm not 100% on their methods exactly but you can detect if someone is using a specific radio frequency and by monitoring all channels a drone can operate on you can detect if someone is flying an RC aircraft within proximity to the airport. its possible to use an automatic detection program that does just that so a human doesn't need to stare at a monitor all day or look out the tower window with binoculars. its also possible to detect the direction the radio frequency is emitted from by measuring which direction gives the strongest signal.

(you can also do the same thing in your house and see a spike at 60hz which is given off by your homes electrical system. all those long wires running panel to light/outlet can act as an antenna.)

What if it's autonomous?

In that case it visual, acoustic, or radar detection since there is no RF signal being emitted.

Acoustic detection: Uses microphones to detect the distinct buzzing noise using several microphones you can triangulate the direction, but its limited to 100ft and background noise will make detection difficult. this is likely not used.

Radar can be tuned to detect small objects like drones but it depends on the shape and if signal can bounce back. its dependent but not impossible to detect drones with radar. its not solely reliant on propellers as jet planes and even mortar shells or birds can be detected.

Usually a combination of systems will be used making the chance of detection higher.

if your asking so you can fool detection i would advise against it and fly somewhere else. luckily i live 20mi from any airport so i dont need to ask permission to fly in my yard.

I hasten to say that I do not fly drones. My interest is purely academic,

In my original question I mentioned the use of doppler to detect helicopters because this has upper AND lower sidebands caused by advancibg and retreating blades,

A drone has small plastic blades which are unlikely to have radar signature, but might respond to optical methods.

ps - Unrelated - Thank you John Henry for showing me thingiverse, Interesting.

you're welcome. I spend a lot of time researching things in my free time this happened to be one of them.

You did a username search in google didn't you? lol i'm a bit surprised but thought it was funny. I know because i just did and my thingiverse account was the first link. I do CAD too but most of my stuff isn't uploaded anywhere.

Yes, I wanted to know where your note of caution came from and whether to expect a visit from certain quarters.

To reciprocate, I'm http://mike-wsm.org.uk/ Now stuck in nursing home.

oh im just a hobbyist. I just like to inform people while suggesting they stay out of certain trouble. so no worries. Nice website looks clean and simple. i understand, you probably have a lot of free time and you're just curious. I start to feel stagnant if i dont keep learning things myself. have a good time.