Satellites can reveal themselves in unexpected, even astonishing ways, and not only by flaring in the sun of an evening horizon, like artificial stars. Their presence can also be inferred indirectly—by causing subtle, nearly undetectable problems elsewhere, such as a briefly wandering tractor.

In Pinpoint, his new book about GPS, the satellite-based Ground-Positioning System, author Greg Milner explores, among other things, the world of precision agriculture, where semi-automated field equipment is guided not by a farmer’s steady hand but by signals sent from machines in space.

GPS, of course, was not developed for farming but as a military tool, and a military tool it remains. It is controlled by the U.S. Air Force and still today has differential levels of precision: there is a signal open for use by citizens of any country, and then there is an encrypted signal used by U.S. military systems and guided weaponry. In other words, GPS enables not just your smartphone to find its way through Central Park, but a lights-out military helicopter to navigate the deserts of another nation at 1 a.m.

Several years ago, my wife, writer Nicola Twilley, and I had an unusual opportunity to visit the military facility in Colorado from which the GPS constellation is controlled. Located on a huge, windy plot of land just east of the Rocky Mountains, Schriever Air Force Base is a covert gateway to space: ground control for the constellation of satellites by which we—and our weapons—remain oriented. Indeed, Master of Space is how the 50th Space Wing, a group of soldiers stationed on the base, describes itself, and this is not just an expression of hubris.

The satellites the soldiers control are overseen within a fortified command facility; if the door to the central room is left open for more than 15 seconds, an armed team will respond, as if to a foreign invasion. GPS, after all, does not only guide your Uber driver to the nearest Irish bar. It also helps U.S. cruise missiles and Predator drones alike find their way to an explosive battlefield rendezvous. It time stamps financial transactions on Wall Street. It synchronizes New York City’s traffic light system. It helps land passenger airplanes. It is worth protecting.

As Milner explains in Pinpoint, however, this functionality extends even to laughably mundane activities in the civilian realm. This includes beet farming.

Growing beets, Milner writes, requires exceptional precision: the plants are small enough that even a tiny error can throw off an entire field of seedlings, and the exact paths taken by farming equipment thus assume undue importance. To achieve the necessary level of geographic exactitude, a Colorado beet farmer named Troy Seaworth, of Seaworth Farms, doesn’t only use GPS, he tells Milner. He also relies upon a convenient but competitive back-up system: the Russian-controlled GPS rival known as the GLObal NAvigation Satellite System, or GLONASS.

GLONASS, with its own 24 satellites, was first developed in the 1970s and later launched in the 1980s as a Soviet alternative to GPS. While GLONASS has been shown to be slightly more accurate in the northern latitudes, its main advantage for global customers is simply the fact that it is not controlled by the U.S.; rather, it is controlled by the Russian military.

Farms like Seaworth’s are what Milner calls “the terrestrial end of an obsessively precise space-based positioning system.” Using a combination of GLONASS and GPS, Seaworth—as well as countless other farmers like him around the world—is able to achieve “sub-inch accuracy.” This means that an automated tractor can rumble down a highly specific route, and that individual planting, fertilizing, and watering machines can all do their jobs, without missing a beet.

If both satellite systems are not running smoothly, however, or their signals are not received in tandem by Seaworth’s machines, things begin to creep slowly but surely out of control.

Milner explained this to me over the phone with a slight laugh. He described a slow parade of agricultural equipment that almost appears drunk or, more precisely, as if they have developed the jitters. “Usually, we’re right around twelve satellites,” Seaworth explains to Milner. “If you get below six or eight, you can tell.” It is a kind of satellite withdrawal, shaky-hand syndrome for signal-addicted machines.

This odd behavior can even be the first way a farmer might notice that something has gone wrong with the satellites. In other words, even before news breaks that GLONASS is down, or that there has been a regional GPS interruption, the wobbly machines shivering out of line across a family’s beet field will betray evidence of malfunctions in the sky.

Even space weather—such as solar flares—can have strange, indirect effects on our everyday world. The next beets you buy could bear subtle traces of this astral influence, by being just slightly under-fertilized or drastically over-watered. You could blame a bad beet salad on satellites.

Interestingly, however, these sorts of effects need not only be detrimental. In one long section of Milner’s book, he discusses the early use of GPS by the U.S. military for prosecuting its various campaigns overseas. Milner points out that, in those years, the practice known as “selective availability”—by which the military’s GPS signal would be substantially more accurate than the civilian one—would be temporarily put on hold during war operations abroad. “When American troops intervened to remove Haiti’s military government in 1994,” Milner writes, “attentive GPS users worldwide noticed that their receivers gave more accurate readings.”

Although it is anachronistic to suggest as much, due to the rarity of everyday GPS-controlled machinery back in 1994, Milner’s Haiti anecdote nonetheless implies that, for the duration of that military campaign, construction and farming equipment would have been just a bit more accurate. It would have been a bit steadier, a bit more precise. Like a particularly good year for wine, 1994 could thus have produced prize-winning beets—or exceptionally aligned rail bridge footings.

Future architectural historians could perhaps someday find that meticulously constructed examples of civil infrastructure—or agronomists researching surprisingly robust yields on farms around the world—can be correlated back to these brief pulses of geographic precision, only made possible as an indirect side-effect of a U.S. military operation.

The next time your family’s tractor begins to work with unusual precision, in other words, perhaps check your Twitter feed for reports of war.