Anyone who owned a GPS receiver in the 1990s and stood perfectly still could watch the machine lie. The little arrow drifted. Left twenty meters, then back, then off in some other direction, wandering a slow circle around a point it never quite settled on, as if the device were unsure of a fact it should have known cold. It was not broken. It was being lied to, on purpose, by the United States government.
The lie had a name: Selective Availability. And a few minutes past midnight, Eastern time, on the night of May 1, 2000, someone turned it off.
A military system, hurried into public life
The Global Positioning System was never built for drivers or hikers. It was a Cold War military project — a constellation of satellites, each carrying an atomic clock, broadcasting the time so precisely that a receiver on the ground could work out its own position from the tiny differences in when each signal arrived. The Department of Defense built it to guide troops, ships, and bombs.
A civilian channel had been in the plan from early on — the program’s 1974 management plan already specified that the coarse signal would “provide a navigation signal in the clear to both the military and civil user,” and a company was selling receivers to land surveyors by 1982. What a single night in 1983 supplied was urgency. A Soviet fighter shot down Korean Air Lines Flight 007 after the airliner drifted hundreds of miles off course into Soviet airspace, killing all 269 people aboard. Two weeks later, the Reagan administration announced that once GPS was operational, its signal would be made available to civilian aircraft, free of charge, so that no navigator would ever again wander blind into the wrong sky.
It was a generous promise, and the military kept it with a caveat. Civilians could have the signal. They could not have the good version.
The deliberate error
Full accuracy, the Pentagon reasoned, was a weapon. If a hostile force could get a cheap receiver and know its position to within a few meters, it could aim its own missiles with American precision. So the system was split. Military receivers, using an encrypted code, got the truth. Everyone else got a degraded signal — degraded on purpose.
Selective Availability, switched on for the civilian signal on March 25, 1990, did this by tampering with what the satellites broadcast. The main trick was dithering the atomic clocks: introducing a wandering error into the timestamps, small enough to be deniable, large enough to matter. Since GPS turns timing into distance, a nanosecond of clock error becomes meters of position error. The published civilian specification promised only that a receiver would land within 100 meters horizontally — roughly a football field in any direction, and that only about 95 percent of the time — with the vertical figure looser still, 156 meters. That was the drift people watched on their screens. Not a glitch. A policy, running in real time.
The irony arrived fast. When the Persian Gulf War loomed in 1990, the American military discovered it did not own nearly enough of its own encrypted receivers. It rushed to buy commercial units off the shelf — the same civilian units the dither was designed to cripple. So the Pentagon temporarily switched Selective Availability off, degrading no one, to let its own troops navigate the desert with the equipment it had been sabotaging. On July 1, 1991, months after the fighting stopped, the error came back on.
The workaround that made it pointless
Through the 1990s, the case for the lie steadily collapsed, because civilians simply engineered their way around it.
The method was called differential GPS. The insight was elegant: if you put a receiver at a surveyed location whose coordinates you already know exactly, that receiver can compare where the satellites say it is against where it actually is, measure the error in real time, and broadcast the correction to everyone nearby. Because Selective Availability applied the same wandering error across a whole region, a moving receiver could subtract it right back out. The U.S. Coast Guard built exactly such a network of correction stations along the coasts. Others followed. The deliberate 100-meter smear could be undone to a few meters by anyone who cared enough to listen to a second signal.
By the late 1990s the situation was faintly absurd. The government was spending effort to degrade a signal that surveyors, farmers, and shippers were routinely, legally correcting anyway. Meanwhile the military had developed the ability to jam or degrade GPS over a specific battlefield — a scalpel, where Selective Availability was a blanket thrown over the entire planet. The one real justification for the global lie had a better replacement.
Zero
On May 1, 2000, the White House announced that Selective Availability would end that night. A threat assessment had concluded that switching it off would do little to harm national security, now that regional denial was possible. President Bill Clinton’s statement put it plainly: civilian users would be able to pinpoint locations up to ten times more accurately than they could now.
Just past midnight, in the first hours of May 2, the error term was set to zero.
There was no ceremony visible to the public, no signal on any screen that a decision had been made in Washington. There was only the effect. A receiver that had spent years wandering in a lazy circle went still. The government had promised ordinary civilian users twenty meters or better; the tracking data came back better than the promise. At one monitored site in Erlanger, Kentucky, the horizontal error that had held 95 percent of fixes inside 45 meters on the last full day of Selective Availability held them inside 6.3 meters on the first full day without it. And as those errors collapsed, so did the whole texture of what the device could be trusted to do. A car navigation system that had drifted a block or two off the road now sat on the actual street. An emergency dispatcher locating a caller went from a football field of uncertainty to something close to a doorstep. Surveyors, pilots on instrument approaches, hikers in unmarked country, boats threading a channel at night — all of them woke up on the morning of May 2 to a machine that had, overnight, stopped hedging.
The permanent version
For seven years the switch could, in principle, have been flipped back. The satellites still carried the machinery to degrade their own signal; the United States had merely promised not to use it. That reassurance was not quite enough for a world now building its economy on the blue dot.
So in September 2007, the government went further and announced that the next generation of satellites, GPS III, would be built without the capability to impose Selective Availability at all. Not switched off — absent. The lie was not just retired but designed out. The satellites already in orbit still carried the machinery, and would until they were retired one by one — as did the last few of the old design still waiting on the ground, the final one going up in August 2009. The hardware itself disappeared even sooner than the policy required: the very next batch, the Block IIF satellites that started going up in 2010, was already built without it, years before the first true GPS III reached orbit in 2018.
The drift is gone now, and almost no one remembers it was ever there. The phone that shows you exactly where you are standing, to within a few strides, is running on a signal that a government once spent real effort to blur, until the night it decided the truth was more useful than the error.