A panel of federal appeals court judges on Thursday appeared to have differing views on whether an aerial surveillance program in Baltimore should continue to be tested as a crime-fighting tool.

The three-judge panel of the U.S. Court of Appeals for the Fourth Circuit in Richmond, Virginia, heard oral arguments remotely in an appeal from a federal district court decision denying a request from the American Civil Liberties Union to temporarily block the pilot program of the Baltimore Police Department.

The ACLU, on behalf of a grassroots organization and area activists, sued the department and its leader in April to stop the test from kicking off.

The arguments at Thursday's hearing centered on whether the program violates people’s reasonable expectation of privacy regarding movement, and results in indiscriminate searches without a warrant.

Under the six-month test that began in May, aircraft outfitted with wide-angle cameras are sweeping over Baltimore during the day to capture images at a rate of one per second. The images can then be stitched together to create a continuous visual record of the city.

Police have said the resolution of the images is not sharp enough to identify a person’s face, ethnicity, gender and clothing or a vehicle’s color, make, model and license plate. Instead, people and vehicles appear on the imagery as dots, which can then be identified when the visual record is paired up with street-level cameras, license plate readers and gunfire sound detectors.

“If I understand correctly, this data that are collected represent a person as one pixel. I’ve looked at the pictures, I can’t even identify the persons myself in the pictures,” Judge Paul V. Niemeyer said during the hearing. “... This is so generalized I’m not sure it even fits within the language of the Fourth amendment protecting the privacy of individuals or protecting people in their houses and so forth.”

Brett Max Kaufman, senior staff attorney with the ACLU, argued the program constitutes a search. He said that while looking at the dots on an image makes this seem "a completely harmless case,” the system is designed to identify people. Otherwise, he said, police would not be using it.

“That is exactly the goal. That is exactly the capability,” he said. He called it the “most ambitious mass surveillance program ever deployed in an American city.”

A private company, Ohio-based Persistent Surveillance Systems, is operating the surveillance system and analyzing the images for police. The project is being funded with roughly $3.7 million from the nonprofit of Texas billionaires Laura and John Arnold.

Baltimore Police Commissioner Michael Harrison has said the images can only be used to investigate homicides, nonfatal shootings, armed robberies and carjackings. The department's attorney, Rachel Simmonsen, told the judges the images have assisted police in investigating two cases.

“It’s not a pixel that has a constitutional right, it is an individual person that has a constitutional right, and the Fourth Amendment’s reasonable expectation can only be invaded by a person,” Simmonsen said, arguing that the plaintiffs lack standing in the case. “Here, the specific harm that they are alleging is that they will be identified, but even that is highly unlikely.”

But the plaintiffs have said that just knowing that their movements are being recorded makes people reluctant to participate in community events.

Chief Judge Roger L. Gregory appeared to disagree with Simmonsen's arguments over the data collection. He said the program is more invasive than the use of records that reveal where cellphone users have been present, which the U.S. Supreme Court previously ruled police generally require a warrant to access.

“It's even more invasive because with a phone, somebody else may have my phone and you will get the same data,” Gregory said. “But no one else has me. So, you will always get me ... If I walk out, I can't hide myself.”

Persistent Surveillance secretly tested the technology in Baltimore in 2016, as crime soared after the death in police custody of a young black man, Freddie Gray. Residents and top city officials were unaware of the test, approved by a different police commissioner, until the media revealed it.