It was just a few years since the launch of the first GPS satellite. As the constellation grew—and, along with it, a method of tracking that further enhanced the ability of humans to observe and gather data—the question of sensory augmentation and the law would grow more urgent.

For an airport of its size, Newark Liberty is squeezed into a fairly small footprint. The team that set up the GBAS receivers found that the only spot at the airport that provided full coverage was next to a runway at the airport’s eastern edge. Each receiver sat within 200 meters of the New Jersey Turnpike.

The freeway’s centrality guarantees that a sizable amount of its traffic is always commercial vehicles. A significant number of these, it turned out, are driven by people who possess “personal privacy devices,” a euphemistic term for GPS jammers. The drivers carried these jammers to foil the tracking apparatuses set up by their bosses to monitor the whereabouts of employees. Although operating one is illegal, they are cheap—often under $100—and easy to obtain from Internet vendors. Small and innocuous, many plug right into a vehicle’s dashboard cigarette lighter. They form a little interference bubble in the area surrounding the jammer, stopping the GPS signal from getting through. This interference was disrupting the GBAS receivers.

The discovery came as a shock to the GBAS team. “We were generally aware of people with jammers in the cars,” says Sam Pullen, leader of the Stanford GPS Lab’s GBAS research group. “But we knew those devices were low-power. So we suspected there would be fewer bumps and that we’d rarely see them. We didn’t think about it that closely, but there was no reason to think that car-based jammers by themselves would be much more frequent than what we had seen previously.”

Authorities began a stakeout of the Turnpike. But pinpointing the source of the jammers was difficult, so the GBAS outages continued. On April 29, the effort finally paid off. Police were positioned on the Turnpike, near the runway, when the interference began. It seemed to coincide with the passing of a certain truck. They raced after it and motioned the driver to pull over. The jammer was right there on the dashboard. The driver made no attempt to hide it. In exchange for handing it over, the police let the driver go on his way with a warning.

Moving the GBAS array was not an option. No other spot could so effectively cover the airport. Instead, the team made some adjustments to the software, which mitigated the problem a bit, but not completely. Cars and trucks with GPS jammers continued to use the Turnpike, and sometimes they jammed the GBAS. Throughout the summer months, the team monitored the problem—the monotonous jamming blips from trucks passing through America’s most dangerous two miles.

Until one day in August, when the static suddenly got worse. By the second half of the 1980s, companies like Trimble and Qualcomm were exploring the market for GPS  tracking devices that would allow trucking companies to monitor their drivers’ whereabouts. A GPS tracker is simply a GPS receiver integrated into some kind of communication device that periodically transmits or records that location. The problem for early fleet management systems was not so much the GPS aspect, as it was finding radio frequencies usable over wide geographic areas to transmit the coordinates back to the monitoring centers. But the customers were there. Three years after it debuted, Qualcomm’s Omnitracs system had signed up about 100 trucking companies and was monitoring the whereabouts of 15,000 trucks. Drivers were required to enter messages onto a dashboard-mounted terminal. Satellite dishes on the trucks relayed the information to Qualcomm’s San Diego headquarters, which forwarded it to the driver’s dispatchers.

As cellular networks developed and the cost of access decreased, GPS tracking became a more realistic proposition. Between 1992 and 1997, U.S. Census Bureau “vehicle inventory and use” surveys found that the percentage of commercial trucks on the road being tracked electronically rose from roughly one in ten to nearly one in four. The process of tracking via GPS became much simpler. “A GPS tracking device is like a mini-cell phone, more or less,” says Ryan Driscoll, the marketing manager at GPS Insight, an Arizona-based company that designs GPS tracking software. At regular intervals, the device transmits the GPS reading to a monitor, either a live human or a computer that gathers and archives the location data.

By 2005, companies and government organizations were using GPS to track 1.3 million fleet vehicles. Analysts projected that the North American fleet management market alone would grow to $7 billion over the next few years. More than half of all fleets with 100 or more vehicles now use a GPS fleet management system—for companies with more than 350 vehicles, the adoption rate is approaching 60 percent. The worldwide fleet management industry, valued at $12 billion in 2014, is on track to be worth more than $35 billion by 2019, with Asian and Pacific markets showing the highest growth rates. In Delhi, a public-private partnership between the Indian government and an infrastructure investment company has installed GPS trackers on all 60,000 public rickshaws, to prevent drivers from gouging tourists by taking longer routes. In China, GPS tracking is used to control the black market in “gutter oil,” recycled cooking oil that is combined with industrial waste and other effluents, which may account for 10 percent of all cooking oil used in the country. GPS trackers on garbage trucks keep track of where oil is collected, so that officials can ensure that the amount collected does not mysteriously decrease before disposal.

Fleet management is one of the fastest-growing segments of the overall GPS industry, and the largest segment of the telematics industry—companies that specialize in processing and transmitting real-time data from vehicles. It has proven itself to be recession-proof. “Our businesses thrive in a bad economy,” Driscoll says. “When expenses are tight, they need this. They can’t afford to waste fuel. They can’t afford to pay drivers for wasted labor hours. They’ve got to shave as many costs around the entire operation as possible.”

Among the private trucking fleets that use portable GPS tracking device, two-thirds integrate the system with other data gathering, to analyze job performance. The most extreme example may be United Parcel Service, whose drivers—“among the most regimented workers in the country outside those on an assembly line,” according to labor journalist Jane Slaughter—use trucks outfitted with 200 telematics sensors, recording such metrics as how hard a driver brakes and how long it takes to deliver a package and get a signature. In much the same way that Todd Humphreys described GPS in a drone as the “bulwark,” surrounded by and supporting other navigation tools, GPS location tracking in fleets is the foundation supporting many kinds of telematics data. Perhaps a clue as to why vehicle GPS tracking device is so seductive as a gateway to other information is that fleet managers overwhelmingly rate “vehicle location” as the principal benefit of GPS tracking, far ahead of fuel consumption and cost control. As the sales manager of a GPS fleet management company put it, summing up the prevailing attitude of his clients, “I want to know my employees are working, not at the Circle K having a soda.”