National Guardsmen posted to the Mexico border will provide added aerial surveillance to border security efforts, according to Kevin McAleenan, the commissioner of Customs and Border Protection, told members of Congress last week.

"What the National Guard is going to do is bring significant aviation assets that can close our gap for that surveillance piece at the immediate border," McAleenan said. "It is supportive but it also tells us what's crossing so we can interdict."

The troops are explicitly barred from helping to arrest or deport immigrants, but they can use drones and light-, medium- and heavy-lift helicopters during their deployment, and they can assist with surveillance systems such as cameras and blimps, Business Insider reports.

Beyond that, the troops will perform maintenance work on roads and facilities.