Baltimore police tested a city-scale aerial surveillance system for months before the public—and even key city officials—were told.

In 2016, investigative reporting revealed that the Baltimore Police Department had been working with Persistent Surveillance Systems to fly camera-equipped aircraft over the city. The wide-area system could photograph roughly 32 square miles at once and allow analysts to move backward and forward through archived imagery to trace vehicles or pedestrians across the city.

The pilot had begun months earlier and was financed through private donations rather than the ordinary city contracting process. Contemporary reporting said Baltimore's mayor, city council and Board of Estimates had not been informed before the program became public.

Baltimore police defended the program as a trial intended to help investigate violent crime. The dispute was not merely about whether an airplane could see a person standing outside; it was about building an archive that could later reconstruct where people had traveled.

Sources: Bloomberg — 2016 disclosure · The Guardian — Baltimore acknowledgement