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.