SIERRA VISTA, Ariz. -

U.S. drones fly over the border to catch illegal activity, but critics maintain they are not doing enough.

Those who fly them say unmanned aerial vehicles can do more, but they lack the funding from congress.

U.S. Customs & Border Protection near the southern Arizona border has four drones, and could get more through Congressional mandates within immigration reform proposals.

"Right now, we have more planes than we can technically fly," said David Gasho, who directs Arizona's CBP air operations.

Gasho said only three of its four unmanned aerial vehicles can be flown simultaneously. "Right now, giving us some more airplanes would actually compound the challenge of putting more in the air," Gasho said.

Gasho said as it is now, they are staffed five days a week, 16 hours a day. "We're not here all the time. We're not funded to do that," Gasho said.

He also said UAVs are not all-weather airplanes, but the capabilities they bring to border security are indispensable.

Gasho has flown UAVs for seven years and said technology has improved in that time. "We see more things, process more information than we did before," he said.

Gasho said he is aware of UAV assessments that show their performance is not reflected in higher apprehension numbers.

"If we were round-the-clock with the technology we now have, what we'd see would go up exponentially," Gasho said.

He said they are able to scan up to 70 miles below from 19,000 feet or higher, as well as into Mexico without violating its air space.

"We provide a lot of intelligence that could affect us in the next couple of hours or couple of days," he said. 

Gasho said although many images are real-time, interpreting images from each "orbit" can take up to 160 people. "Our challenge is affecting an end game to everything we see."

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