GPS: 26 years since the night Clinton switched off the error

The GPS you use today — on your phone, in the car, in a running app — is fundamentally different from the GPS that existed until the end of 30 April 2000. Not in technology, not in satellites, not in infrastructure. The difference was a political decision signed by an American president who, one minute after midnight on 1 May 2000, ordered the shutdown of a deliberate error the United States government had been injecting into civilian GPS signals for years.

Today marks 26 years since that night. And the story deserves telling, because most people who use GPS never knew the signal reaching their phone was worse by design.

GPS was always military first

The American global positioning system — formally called NAVSTAR GPS — grew out of a United States armed forces project in the 1970s. The idea was simple in theory and absurd in execution: put enough satellites in orbit that any point on the planet could be located precisely from the radio signal they emitted. The first Block I satellite launched in 1978. The system only became fully operational, with the 24 satellites required in orbit, in 1995.

From the start, the American Department of Defense kept the best of the system for itself. The military received the PPS signal — Precise Positioning Service — accurate to within a few metres. The civilian signal, called SPS — Standard Positioning Service — was given an intentional built-in error. That error was called Selective Availability, or simply SA.

How a deliberate error works

SA worked through a process called dithering — a deliberate corruption of the clock data transmitted by the satellites. GPS works by triangulating the time radio signals take to travel from multiple satellites to the receiver. If the time is correct, the calculated position is accurate. If the time is slightly altered in a random, controlled way, the calculated position lands somewhere inside a bubble of uncertainty.

With SA switched on, that bubble had a radius of around 100 metres. In some conditions it reached 300 metres. In practice, that meant your GPS receiver knew you were somewhere within a block and a half, not exactly where you were. For military use with access to the PPS signal, no problem. For anyone trying to navigate a motorway or pinpoint an exact location, it was enough to put you on the wrong street — or on a parallel road.

The Pentagon's argument was reasonable for the Cold War: if enemies could use civilian GPS with military precision, the American advantage disappeared. SA was the solution. In theory.

Why the error stopped making sense

By the late 1990s, SA had become more of a problem for the United States itself than for any hypothetical enemy. Three pressures converged at once.

First, the civilian GPS market was exploding. Commercial aviation, maritime navigation, surveying, precision agriculture, logistics — all depended on GPS, and all suffered from the 100 metres of imprecision. American companies were being hurt by a system the US government itself operated.

Second, the European Union was developing Galileo, its own satellite navigation system, promising a civilian signal with no intentional degradation. If GPS stayed worse by design, the market would migrate to the competitor.

Third — and perhaps the most embarrassing — differential GPS techniques already made SA practically useless as a military tool. Ground reference stations published real-time corrections that cancelled out the SA error, restoring accuracy. Anyone with the right equipment could work around the degradation. SA stopped ordinary civilians from having good GPS, but it didn't stop sophisticated actors from having it. It was the worst of both worlds.

Midnight, 1 May 2000

Bill Clinton signed the order on 1 May 2000. Just after midnight Washington time — a little past 04:00 UTC — the operators of the GPS Control Segment in Colorado Springs executed the command. Dithering was switched off on every satellite simultaneously.

The effect was immediate and measurable. GPS receivers all over the world saw positional uncertainty drop from 100 metres to under 20 metres. In ideal conditions, to under 5 metres. Accuracy improved by a factor of 5 to 10 in a matter of minutes. Without swapping out a single satellite, without installing any new hardware, without any firmware update. The error simply stopped being injected.

Clinton described the decision as a measure to make GPS "more responsive to civil and commercial users worldwide". The Department of Defense confirmed it had developed the capability to deny GPS signal to adversaries in specific regions without needing to degrade the system globally — making SA not just unnecessary but counterproductive.

The world that was born after that night

In 2000, civilian GPS receivers cost between $500 and $1,500 and were dedicated devices — you bought a handheld unit or had one fitted to the car's dashboard. But with the signal now ten times more accurate, the market moved up a level fast.

The first integrated in-car navigation systems — what we used to call a car GPS — started gaining real traction from 2001 and 2002. The difference between 100 metres and 10-20 metres was exactly what separated "knows which street you're on" from "knows which lane of the street you're in". For urban navigation, that difference is everything.

When smartphones started integrating GPS chips from 2007, it was the post-SA signal that went into everybody's pocket. Waze, founded in 2008, was born into a world where civilian GPS had already had Clinton's correction for eight years. So was Google Maps, which began integrating GPS on mobile devices in that same period. The accuracy you take for granted when you open any navigation app today was released in the small hours of that morning in 2000.

Precision agriculture, fleet tracking, commercial drones, automatic aircraft landing systems, telecoms network synchronisation — almost every piece of infrastructure that depends on precise location in today's world was born or matured after the end of SA.

The permanent legacy

In 2020, the Department of Defense announced that the new GPS Block III satellites, in production since 2010, no longer include the hardware required to implement SA. It isn't just that SA is switched off — on the newest satellites it literally doesn't exist any more. The decision of 2000 was made permanent in the hardware itself.

Europe's Galileo became operational in 2016 without ever having implemented intentional degradation. Russia's GLONASS, China's BeiDou — no civilian satellite navigation system launched after 2000 has included the idea of degrading its own signal as standard policy. SA became a Cold War artefact that didn't survive the turn of the millennium.

Next time Maps drops the pin on the right pavement — not the building on the corner, not the street behind, but exactly where you are — remember that isn't space technology. It's a political decision taken at two in the morning on 1 May 2000, when somebody in Colorado Springs ran a command that nobody outside the civilian GPS world noticed at the time, but which changed the way the entire planet moves.

If you want to understand how other invisible technologies were deliberately built to stay that way, the piece on Mark Weiser and ubiquitous computing goes straight at that idea — the man who in the 1990s was already describing a future where technology disappears from view precisely when it works best.