I am not a lawyer. And this post concerns US law only. I did attend a conference last fall, that was not open to the general public, where the legality of counter-UAS measures was a significant focus. I can't reveal what was said there but I can say it is very complicated subject. I can mention some things based on public sources, however.
If you think flying a drone is a legal mess, any sort of active drone countermeasures is a big legal mess; there are many non-drone laws and regulations that in combination basically prevent all forms of active drone countermeasures.
Active drone countermeasures can potentially do more damage than the drone itself.
Shooting drones at an airport is addressing the wrong problem in the first place. There have been about two confirmed drone strikes on aircraft in the US in the 14 years that multirotor UASs have been flying; during the same period and region, there have been an estimated 150,000 recorded bird strikes. (had to fill in the last few years since data isn't available yet). Averaged over the last 40 years, there have been 8.5 human fatalities per year due to bird strikes which would be about 119 fatalities in a similar period. 2000-2015 (not quite the same years as covered above) had 26 fatalities and 400 injuries (FAA Wildlife Strike ReporT). And during the same period drones were flying, an aircraft with 155 people on board made an emergency landing on the hudson river without fatalities. If that hadn't happened to be the "most successful ditching in aviation history", the body count might have been much higher. So even if you shot down all the UASs near all airports, you would only be making the skies safer for manned aircraft by a tiny fraction of 1 percent. And the number of bird strikes is rising substantially. And you would be creating significant safety hazards in the process. Also, while UAS may present some hazards, think about how much less risk a UAS poses compared to a manned aircraft doing the same job.
Shooting a drone is a lot like shooting a person. Except there are specific exceptions under the law for shooting a person in self defense. And you could get up to 20 years in prison just for shooting the drone even if you don't kill a person in the process, though you probably wouldn't get the maximum sentence. Any form of active drone countermeasures are likely to break multiple laws and in many cases they are serious felony charges carrying 1-20 year maximum sentences, each. While a prosecutor might choose not to pursue the case, if they do you are looking at a messy legal battle.
The military, coast guard, DOJ, and homeland security can use drone countermeasures in certain circumstances.
The FAA reauthorization act of 2018 has several sections pertaining to drone countermeasures which pave the way for some future changes - mostly requiring studies and proposals.
Anybody else is probably breaking one or more laws with any known active counter-UAS technology.
Here are some of the laws and issues:
- UAS are covered by the Aircraft Sabotage Act. damaging any aircraft, including unmanned aircraft, is against the law. So is using a destructive device in proximity to an aircraft, endangering the safety of the flight or interfering with the flight crew.
- Even if you could grab the drone out of the air without damage, altering the flight path in any way has safety ramifications which could come under the sabotage act, civil liability, etc.
- Transmitting any interfering signal (jamming or taking over control) is against the law.
- Transmitting without a license is against the law
- selling uncertified transmitters is against the law.
- EMP would likely violate FCC regs and might also be considered a weapon of mass destruction.
- Interfering with GPS breaks multiple laws.
- Even receiving the radio control/telemetry signal is potentially covered under the wiretap act and the pin register act which between them cover both the "content" and "non-content" (think letter vs envelope) portions of the signal. Thus just acquiring the information you need to take over in flight could be illegal even before you actually take over. Even drone detection may be a problem.
- hacking a computer is illegal and the flight controller is a computer.
- Various firearm and other weapon laws apply to some countermeasures.
- Any projectile may cause significant harm.
- You probably need a Remote Pilot Certificate if you take over control of a drone flight
- You are liable for civil lawsuits from a number of angles including damage to the drone, damage/injury caused by the drone, and damage/injury caused by your counter-UAS projectiles, and civil-rights violations.
- Many counter-UAS approaches have the potential to also interfere with manned aircraft and air traffic control.
- Bullets straight up into the air might typically land with less than lethal (though still potentially injurious) force but fired at an angle they may be lethal when they come back down.
- In many cases, just testing counter-UAS technologies on your own drone could be a serious offense.
- airports could be defunded for testing counter-UAS technology without specific FAA approval.
- Propellers which aren't spinning aren't necessarily less of a threat than ones that are because the greatest injury is likely caused by stabbing rather than slicing. A rotating prop is stopped on impact with flesh and then the kinetic energy of the aircraft drives it in. Plus a falling drone can do considerable damage even without a blade. So a net doesn't necessarily help much and could make things much worse.
- If you try to take over a drone, you might end up with neither pilot in control. Who is liable then?
- Using a countermeasure near an airport may lead to harm: interference with air traffic control, GPS, radars, and transponders. Often a populated area. Often an area with more radio signals to be interfered with and to get in the way of countermeasures.
- A number of the laws may still be broken even if you fail to affect the drone in any way.
- and more
Ruprecht law has an article on the subject, that includes links to most of the laws I have mentioned.
7 Big Problems with Counter Drone Technology (Drone Jammer, Anti Drone Gun, Etc.)
About the only legal recourse you have for drone countermeasures is the necessity defense, in which you admit that you did break the law but there was a significant imminent danger that you were not in any way responsible for, that there was an immediate need to act, that you had no legal alternatives (including advance preparation), that you broke the law to the minimum extent necessary, and that the harm caused was less than the harm avoided.
Necessity can be trickier than it looks. It is hard to satisfy all of those conditions simultaneously. For example, If you are buying counter-UAS equipment to protect someone on the ground, then you anticipated the threat and probably could have used some other means of protection that was legal, though not necessarily desirable, such as bullet resistant shields, body armor, using an indoor venue, etc. What if it is learned afterwards that the drone was not in any way malicious and that the harm you prevented was zero and the harm you caused was non-zero? Also, necessity can be incompatible with other defenses (such as lack of evidence, tainted evidence, fruit of the poison tree) as you prove the prosecution's case by making an affirmative defense.