Editor’s note: This is the second of a two-part series on security and privacy during the age of drone warfare. Part one is available here.

Technology, as Supreme Court Justice Antonin Scalia wrote in a 2001 Supreme Court opinion, has the power "to shrink the realm of guaranteed privacy." Few other technologies have as much power to do this as drones. Because they can perch hundreds or thousands of meters in the air, drones literally add a new dimension to the ability to eavesdrop. They can see into backyards and into windows that look out onto enclosed spaces not visible from the street. They can monitor wi-fi signals or masquerade as mobile phone base stations, intercepting phone calls before passing them along. Using a network of drones, it would be possible to follow the movements of every vehicle in a city—a capability that would be invaluable to a police department tracking the getaway car in a bank robbery but invasive if used to track a patient driving to a clinic to get treatment for a confidential medical condition.

The growth in nonmilitary use of drones is too recent to have generated a significant body of legal precedents specifically addressing their implications with respect to privacy. But closely related legal cases and evolving legal and societal standards regarding privacy make it clear that the issue will be complex. For example, in 1986 the United States Supreme Court ruled that law enforcement's use of a private plane to view otherwise hidden marijuana plants growing in a California backyard did not constitute a violation of the Fourth Amendment right of freedom from unreasonable search and seizure. The reason? The police observations were made from "public navigable airspace." This might be interpreted to suggest that the owners of drones operated in public airspace will enjoy broad latitude to use them for surveillance in the U.S.

However, drone-based surveillance that is sustained, extensive or systematic enough will at some point almost certainly run afoul of privacy rights, although determining when that point is reached will not be easy. Privacy rights are a complex and evolving patchwork. In the U.S., for example, the right to privacy is found not only through a combination of constitutional amendments, but also in additional protections in some state constitutions and laws. In Europe Article Eight of the European Convention on Human Rights provides an explicit right to "respect for private and family life." An additional challenge to predicting the impact of drones on privacy is that the regulations that govern where, by whom, under what circumstances, and what altitudes drones may be used are in flux. In the U.S., for example, the Federal Aviation Administration has been working to develop new policies governing the use of small UAVs, and is aiming to publish proposed rules in the near future.

Regardless of the details of the eventual regulations adopted by the FAA and by analogous agencies in other countries, drones are certain to have a profound impact on privacy for the simple reason that they make it easy and inexpensive to gather massive amounts of information from above. Ryan Calo of the Center for Internet and Society at Stanford Law School has suggested that the widespread use of drones may in fact benefit privacy law by generating a backlash that will result in increased privacy protections. Even if this occurs, however, it is unclear what forms these new privacy protections might take, and how they would be balanced against the many beneficial uses of information that drones can acquire.