Stopping drug smugglers on the ground is one thing. You can build a fence, send more Border Patrol agents and put up more cameras. But it's a whole other thing to stop Mexico's cartels from using tiny planes that are nearly impossible to catch.

That's why the U.S. Customs and Border Protection (CBP) is spending $100 million on new sensors that can detect ultralight aircraft. The giant contract – awarded to New York defense company SRCTec earlier this month – comes as the cartels have been using more of the planes to elude Border Patrol agents. The cartels also seem to have become pretty good at it. The Air Force has chased them with jets, and the Border Patrol has pursued them with Black Hawk helicopters.

Closer to gliders than complete planes; ultralight planes are small, cheap and their engines are relatively quiet. They move slowly, but are flown low to blend in with the southwest border's rugged and hilly terrain, which the smugglers use to hide from radar. The last available data on ultralight incursions is from 2011, when the CBP detected 223 flights, double from two years prior. It stands to reason the real number is much higher, owing to the diminutive aircraft's sneakiness.

According to government's original solicitation for the ultralight detectors, the sensors are intended to counter "the high-priority threat presented by small, low-flying aircraft transiting across United States borders." They should be able to work in hills and mountains, while simultaneously tracking two dozen ultralights from as low as 33 feet above ground level out to a range of 20 kilometers. They need to be built tough to withstand extreme weather conditions, and they should be easily transported across rough terrain.

SRCTec would not comment to Danger Room about the contract. But the company has developed a sensor, called VantagePoint, that appears to fit what the CBP is looking for. According to SRCTec's product description, VantagePoint uses sensors and cameras to "detect and track people, vehicles, and low and slow aircraft, such as [unmanned aerial vehicles] and ultralights." SRCTec claims the sensor works in rough terrain, packs into a box takes only a few hours to set up. It can also communicate with operators remotely.

Which all sounds good. One problem: there's no telling how the Border Patrol plans to stop the planes once they detect them. The agency can't legally shoot down ultralight planes just for carrying weed and blow. The ultralights don't even have to land, either. Instead, the ultralights often fly low, and then drop their dope – in packages that can weigh up to 250 pounds – at a pre-planned location for an awaiting pick-up crew. Even if the government detects the aircraft, the pick-up crew on the ground is likely to be long gone by the time Border Patrol agents arrive. But if they can be tracked, then it gives agents a little more time to stop the shipment when it hits the ground.

The terrain may pose the biggest challenge, and has been a recurring problem for the Border Patrol, and not just with ultralight aircraft. The Department of Homeland Security, which oversees the Border Patrol, spent $1 billion on a virtual fence of surveillance-radar towers – called SBInet – that ended up on the chopping block after the towers didn't work. Terrain interference messed with their sensors, and maintenance issues caused the towers to break down.

The ultralight sensor plan was originally underneath SBInet, and has apparently survived its collapse. It's now listed as a separate program apart from SBInet's successor, the slimmed-down and more mobile Arizona Border Surveillance Technology Plan, which uses truck-mounted radars instead of fixed towers. At the same time, the Arizona plan has run into similar problems.

Like the Arizona plan, the ultralight sensors are supposed to be easily transportable. According to the solicitation, the sensors should be able to operate under battery power for up to four days at a time when not plugged into the U.S. electrical grid. Also, the agency wants the sensors to send data remotely to the CBP's Air and Marine Operations Center in Riverside, California.

One reason the cartels use ultralight planes is because they're really cheap. Prices can range from as low as $3,000 for an ultralight, and the planes can be manufactured at home. For a bigger ultralight, it can set you back $30,000. Drug prices are hard to estimate, but using figures from the U.S. Department of Health, a 250-pound load of marijuana could be worth more than $400,000. A similar weight in cocaine, could be worth more than $16 million wholesale.

That means absent cocaine, the cartels could make more money on marijuana flights in one year – just using ultralight aircraft – than the government will spend on its efforts to stop them. And the cartels can always buy more planes.