The road to the one-room schoolhouse in Sutherland, Virginia, was three miles long, crossing railroad tracks and route 460, lined with honeysuckle and pines. An African American girl named Gladys Brown, the daughter of sharecroppers, walked the three miles to school each morning, often accompanied by a handful of friends. It was the late 1930s, but other than the occasional automobile passing the children on the highway, the world the girl lived in could have belonged to the 19th century. The small farmhouse where she lived had no indoor plumbing or electricity; when school was out, she would help her mother scrubbing clothes clean on a washboard, pressing them with an iron heated directly from a wood fire. “I dreamed of one day living somewhere bigger, prettier, and different,” she would later write, “but the dreams were vague because I had not been anywhere outside of Sutherland.”
Even at an early age, Gladys West (née Brown) was a promising student and a hard worker, sensing that a solid education would be crucial if she wanted to ultimately explore a wider world beyond rural Virginia. She had an aptitude for math, though the resources for actually nurturing those skills were limited, given that the one teacher at the Butterwood Road School—for “colored” students only—had to simultaneously teach a class composed of students ranging from first to seventh grades. And so Gladys West began to nurture her talents on her own. “I counted fenceposts along the road and across the fields as we walked to school, just to have something different to do,” she later recalled. “I became good at it, never realizing that I was sharpening some skills and abilities that I would one day utilize to help calculate the hypothetical shape of the Earth.”
You might well have benefited from those calculations several times today, while you were following the directions of your car’s navigation system, or looking for the nearest Italian restaurant on your phone—or anything else that relies on our modern-day ability to determine our exact geographic location using computers and satellites. Eight-year-old Gladys West wouldn’t have been able to even imagine such a thing, counting fenceposts on her way to the Butterwood Road school, but the path she was on would eventually lead to the creation of one of the modern world’s true miracles: the Global Positioning System, commonly known as GPS.
Finding space in time
Like many of the core technologies that define the digital age, GPS was an offshoot of American Cold War competition with the Soviet Union and the threat of nuclear annihilation. After the Soviets launched Sputnik, the first man-made satellite to orbit Earth, in October of 1957, a pair of graduate students at the Applied Physics Laboratory in Maryland devised an ingenious system for tracking Sputnik’s location by analyzing slight variations in the microwave signals the satellite was transmitting, in effect using an antenna’s known location on Earth to calculate the satellite’s unknown location in orbit. Impressed with the demo, the grad students’ supervisor suggested that the approach could be reversed: if you knew the exact location of at least three satellites in orbit above you, by triangulating their various signals you could theoretically determine your location on the ground—or, if you happened to be on one of the new Polaris nuclear submarines the military was building, you could use it to determine your location in the middle of the ocean.
By 1964, the Navy had deployed the first satellite-based navigation system, a predecessor of GPS called Transit. The system used five satellites and could generate a location that was accurate to within about 150 meters, though it would usually take as long as an hour to obtain a result. In the 1970s, the military began plans for a more advanced system that could produce location data almost instantaneously. But a fundamental problem limited their ability to generate the kind of precise location data that we enjoy today with GPS, which is generally accurate to about one meter. The problem was fundamental in a cosmic sense: it was embedded in the very nature of Einstein’s theory of relativity.
The initial need for the satellite navigation system was to provide accurate location information on Polaris ballistic missile submarines that the Navy was developing at the time. Sponsored by the Navy, Transit was developed jointly by DARPA and the Johns Hopkins Applied Physics Laboratory. The first prototype, launched in 1959, failed to reach orbit, and it was the following year that the second satellite successfully went through all the tests. Except for submarine tracking, Transit was also used as a navigation system by the surface ships, as well as for hydrographic survey and geodetic surveying, and later for civilian use as well.
The ironic thing about GPS is that it is a technology designed to locate your position spatially that is enabled largely by an ultra-precise measurement of time. When your phone picks up signals from GPS satellites, the information it’s receiving is a time-stamp marking of the exact moment each satellite sent out its signal. (Each satellite contains a fantastically precise atomic clock.) Because radio waves take time to travel over long distances, a receiver on the ground can calculate its distance from each satellite by calculating the difference between the transmission time of the signal and its reception. (A satellite farther away from you will have an earlier transmission time, a satellite closer to you will have a later transmission time.) Because the satellites have predictable orbits, a GPS receiver can determine its location if it can get an accurate reading of its distance from four satellites. In a world of Newtonian physics, this would all work perfectly. But Einstein proved that time was distorted by gravity—the technical term for it is “time dilation”—and Earth’s gravitational fields are wildly inconsistent, depending on where you happen to be on the planet. When you are measuring microseconds to determine your exact location using GPS, those gravitational variations can make a massive difference—the difference between landing your plane on the runway and crashing it into the airport parking garage.
One of the main concepts of the theory of relativity by Albert Einstein, time dilation describes the difference in the elapsed time as measured by two clocks, either due to a relative velocity between them, or to a difference in gravitational potential between their locations. In simple terms, the time slows down when the object moves really fast.
All of this would be relatively easy to calculate if Earth itself happened to be a perfect sphere. But while it looks to the naked eye from space like an exact circle of Euclidean perfection, in reality our planet bulges out slightly at the equator, while the poles are flatter than they would be in a true sphere. And Earth’s gravitational fields are also distorted by many of its distinguishing features: tides, mountain ranges, and ocean trenches. To make GPS work, you didn’t just need satellites and atomic clocks. You also needed to know the true shape of Earth’s gravity.
And that is what Gladys West figured out.
Albert Einstein And Gladys West.”