> The moon is illuminated by the sun, so not really a fair comparison. Take a drone and paint it the color of the sky and it's going to be a lot more difficult than “a quarter the size of the moon” would imply. A drone can also fly much higher than 300 feet, and it's not a solid object.

A good point. I think flying above 400 ft is already illegal though, so dealing with that might be somewhat simplified.

> All drones will have transponders I assume?

I think that might be a fair assumption. We require license plates for vehicles on the road, requiring transponders for use in populated areas sounds like a good way to track and license (if your transponder is issued, there you go).

> So now the camera needs a radio, and access to a national database of transponder codes.

Well, yeah. If it's a system responsible for tracking and ensuring licensing compliance and flight permission, it would, just as bridge toll systems now often use license plates to track fees. It's not all that hard in this day and age.

> Assuming drones become more popular how does the camera know which transponder code belongs to which drone? Signal strength? This will be different depending on environment. Signal direction? You need multiple receivers for this.

You already likely need multiple systems to get good visual and radio coverage. But even then, it doesn't need to be a perfectly self-contained system, just a piece of the puzzle. If you see six objects and receive 5 signals, eliminate the ones you can, and send someone to investigate with directional equipment.

We don't expect red-light cameras to pull us over, but we do expect they do something. It's not crazy to expect they also might have specific circumstances that flag to additional review for possible dispatch of an officer.

> How do you stop a drone from emulating another drone’s transponder code? A challenge response system with a public/private key pair?

How do you keep one car from using another's license plate? You make the sale of fake license plates illegal, you make sure the only valid plates are issued by the government, and you make it illegal to tamper with them.

> And the most important question yet. What do you do when you spot an unauthorized drone? If a cop spots a car he can chase it.

Already covered a little above, but there's already solutions appearing to help with this. There's anti-drone ammunition[1] and trained animals[2] on the simple end, up to military grade anti-drone solutions.

> How does the interceptor drone bring down the unauthorized drone without causing any collateral damage? What happens if there are multiple unauthorized drones in an area? How do you trace the unauthorized drone back to the owner? Unlike a car, the operator isn't in the vehicle.

What do we do with vehicles or trailers without clearly defined ownership we find on the street? It's towed to an impound lot. If someone wants to claim property, they they come for it.

If downing it for legitimate reasons ensues, the owner of the drone would be responsible for damage, and if the owner can't be determined, I'm sure there's a legal precedent to fall back on. It's not that wild a situation.

> All of that just to say yeah it’s possible, but it’s a lot harder and a lot more expensive than it seems. Much too hard and much too expensive to feasible anytime soon. Unless drones prove to be a lot more popular and effective than truck, car, and suitcase bombs, than I think they will, no one is going to pay for all of this.

I don't think it's nearly that expensive, as outlined above. If there isn't something like this, I expect drone terrorism would definitely be the go-to solution. This wouldn't stop that, but it does put a hurdle in the way (in the same way that doors and locks and security systems put a hurdle in the way of a home invader, but in the end only slow and dissuade, and do not actually prevent in most cases).

1: https://www.thefirearmblog.com/blog/2017/03/23/usaf-anti-dro...

2: http://fortune.com/2017/02/22/drones-eagles-france/