A small surveillance drone flies over an Austin stadium, diligently following a series of GPS waypoints that have been programmed into its flight computer. By all appearances, the mission is routine.

Suddenly, the drone veers dramatically off course, careering eastward from its intended flight path. A few moments later, it is clear something is seriously wrong as the drone makes a hard right turn, streaking toward the south. Then, as if some phantom has given the drone a self-destruct order, it hurtles toward the ground. Just a few feet from certain catastrophe, a safety pilot with a radio control saves the drone from crashing into the field.

From the sidelines, there are smiles all around over this near-disaster. Professor Todd Humphreys and his team at the University of Texas at Austin's Radionavigation Laboratory have just completed a successful experiment: illuminating a gaping hole in the government’s plan to open US airspace to thousands of drones.

They could be turned into weapons.

“Spoofing a GPS receiver on a UAV is just another way of hijacking a plane,” Humphreys told Fox News.

In other words, with the right equipment, anyone can take control of a GPS-guided drone and make it do anything they want it to.

“Spoofing” is a relatively new concern in the world of GPS navigation. Until now, the main problem has been GPS jammers, readily available over the Internet, which people use to, for example, hide illicit use of a GPS-tracked company van. It’s also believed Iran brought down that U.S. spy drone last December by jamming its GPS, forcing it into an automatic landing mode after it lost its bearings.

'Spoofing a GPS receiver on a UAV is just another way of hijacking a plane.' — University of Texas Radio Navigation Laboratory researcher Todd Humphreys

While jammers can cause problems by muddling GPS signals, spoofers are a giant leap forward in technology; they can actually manipulate navigation computers with false information that looks real. With his device -- what Humphreys calls the most advanced spoofer ever built (at a cost of just $1,000) -- he infiltrates the GPS system of the drone with a signal more powerful than the one coming down from the satellites orbiting high above the earth.

Initially, his signal matches that of the GPS system so the drone thinks nothing is amiss. That’s when he attacks -- sending his own commands to the onboard computer, putting the drone at his beck and call.

Humphreys says the implications are very serious. “In 5 or 10 years you have 30,000 drones in the airspace,” he told Fox News. “Each one of these could be a potential missile used against us.”

Drones have been in widespread use in places like Iraq, Afghanistan and Yemen, but so far, GPS-guided unmanned aerial vehicles have been limited to the battlefield or southern border patrols and not allowed to fly broadly in U.S. airspace.

In February, under pressure from the Pentagon and drone manufacturers, Congress ordered the FAA to come up with rules to allow government and commercial use of UAVs over American soil by 2015. The plan could eventually see police drones keeping watch over U.S. cities, UAVs monitoring transmission lines for power companies, or cargo plane-size drones guided by GPS pilotlessly delivering packages across the country. FedEx founder Fred Smith has said he would like to add unmanned drones to his fleet as soon as possible.

The new rules have raised privacy concerns about a "surveillance society," with UAVs tirelessly watching our every move 24/7. But Humphreys’ experiments have put an entirely new twist on the anxiety over drones.

“What if you could take down one of these drones delivering FedEx packages and use that as your missile? That’s the same mentality the 9-11 attackers had,” Humphreys told Fox News.

It’s something the government is acutely aware of. Last Tuesday, in the barren desert of the White Sands Missile Range in New Mexico, officials from the FAA and Department of Homeland Security watched as Humphrey’s team repeatedly took control of a drone from a remote hilltop. The results were every bit as dramatic as the test at the UT stadium a few days earlier.

DHS is attempting to identify and mitigate GPS interference through its new “Patriot Watch” and “Patriot Shield” programs, but the effort is poorly funded, still in its infancy, and is mostly geared toward finding people using jammers, not spoofers.

The potential consequences of GPS spoofing are nothing short of chilling. Humphreys warns that a terrorist group could match his technology, and in crowded U.S. airspace, cause havoc.

“I’m worried about them crashing into other planes,” he told Fox News. “I’m worried about them crashing into buildings. We could get collisions in the air and there could be loss of life, so we want to prevent this and get out in front of the problem.”

Unlike military UAVs, which use an encrypted GPS system, most drones that will fly over the U.S. will rely on civilian GPS, which is not encrypted and wide open to infiltration. Humphreys warns it is crucial that the government address this vulnerability before it allows unmanned aerial vehicles broad access to U.S. airspace.

“It just shows that the kind of mentality that we got after 9-11, where we reinforced the cockpit door to prevent people hijacking planes -- well, we need to adopt that mentality as far as the navigation systems for these UAVs.”