The Vehicle Dismount and Exploitation Radar, known as Vader, operated from a Predator surveillance drone 5 miles in the sky, shows U.S. Border Patrol agents apprehended 1,874 of 3,836 people who illegally crossed into a 150-square-mile stretch of the Sonora Desert in southern Arizona from Oct. 1 to Jan. 17, the reports cited by the Los Angeles Times indicate.
This means the radar system, developed to track Taliban fighters in Afghanistan, spotted an additional 1,962 people in the same area who evaded arrest and disappeared into the United States, the Times said.
The 49 percent capture rate contrasts with the Government Accountability Office's January estimate the Border Patrol caught 64 percent of those who illegally crossed into the Tucson sector in 2011.
The GAO is the audit, evaluation and investigative arm of Congress.
The drone's tally could cause difficulties for White House efforts to pass comprehensive immigration reform after Congress returns from recess next week, the Times said.
The Obama administration has argued the U.S. borders are the most strictly policed in history, with nearly 365,000 apprehensions last year.
Republicans have demanded more guards, drones, fencing and other security measures before they would approve legal status for the estimated 11 million people believed to have entered the United States illegally or overstayed their visas.
President Barack Obama is to visit Mexico in four weeks, where he and President Enrique Pena Nieto are expected to talk about efforts to maintain rigorous border security.
Vader, borrowed from the Army's Aberdeen Proving Ground in Maryland, has been used in Arizona since March 2012 at a cost of about $5 million a year.
It displays people in real time as moving black dots on a map.
The Border Patrol, a federal police agency that's part of the Department of Homeland Security, has asked Congress to allocate money to purchase two more Vader systems, the Times said.
This pleases House Homeland Security Committee Chairman Michael McCaul, R-Texas, who told the Times, "That is the kind of technology we would like to see all across the border."
"You can't measure what you can't see," he said. "There is an awful lot we're not seeing."
But some agents are unhappy to get a precise headcount of how many migrants and smugglers they're missing, a former U.S. law enforcement official told the newspaper.
"The rank-and-file guys are afraid it will make them look bad," the official said.