Every satellite in the Global Positioning System carries an atomic clock, and every one of those clocks is deliberately adjusted before launch so that it runs slightly slow. The reason is not a manufacturing flaw but a prediction of Albert Einstein’s theories of relativity: time itself ticks at a different rate on an orbiting satellite than it does on the ground, and unless engineers account for the difference, the whole navigation system would quickly become useless.
The scale of the problem is easy to underestimate. The timing errors involved are measured in millionths of a second per day, yet because GPS turns time into distance, those tiny discrepancies would translate into positioning errors of miles within a single day if left uncorrected.
How GPS turns time into position
A GPS receiver works by measuring how long signals take to travel from several satellites overhead. Because those signals move at the speed of light, an incredibly precise clock is essential: light covers about 300,000 kilometers every second, so an error of even a millionth of a second corresponds to roughly 300 meters of distance. The reference material on GPS error analysis lays out how timing precision is the foundation of the entire system, and why even minute clock drift compounds into meaningful location error.
This is what makes relativity a practical engineering concern rather than an abstraction. When the accuracy of a location depends on nanosecond-level timing, effects that shift clocks by microseconds cannot be ignored.
Two relativistic effects pulling in opposite directions
The clock aboard a GPS satellite is subject to two separate relativistic effects that push in opposite directions. The first comes from special relativity, which holds that a moving clock, seen from the ground, runs slow. A GPS satellite travels at several kilometers per second, and this motion alone would cause its clock to lose about 7 microseconds per day relative to a clock on Earth.
The second comes from general relativity, which holds that clocks run faster where gravity is weaker. A GPS satellite orbits roughly 20,000 kilometers above the surface, far from the strongest part of Earth’s gravitational field, and this effect alone would cause its clock to gain about 45 microseconds per day. The two effects do not cancel; they combine.
Why the net figure is about 38 microseconds a day
Adding the two together gives the number engineers actually design around. The general-relativistic gain of roughly 45 microseconds outweighs the special-relativistic loss of about 7 microseconds, leaving a net gain of approximately 38 microseconds per day as seen from the ground. A university course on GPS and satellite navigation hosted by Penn State walks through how these opposing effects are tallied to produce that combined correction.
Thirty-eight microseconds sounds trivial, but at the speed of light it is anything but. Left unaccounted for, that daily drift would accumulate into a navigation error of about 10 kilometers per day, rendering the system worthless for anything requiring real precision.
How engineers build the correction into the system
The fix is elegant in its simplicity. Rather than trying to speed up or slow down the satellite clocks in orbit, engineers set their frequency slightly low before launch, offsetting the expected relativistic gain so that, once in orbit, the clocks tick at the correct rate as measured from the ground. Additional smaller corrections handle the fact that satellite orbits are not perfectly circular, which causes the relativistic effect to vary slightly around each loop.
The result is a system in which relativity is not a nuisance to be tolerated but a factor engineered into the hardware from the start. The clocks are wrong on the ground on purpose so that they will be right in space.
What GPS demonstrates about relativity
Beyond its practical importance, GPS stands as one of the most routine confirmations of Einstein’s physics ever built. Predictions once tested only in delicate laboratory experiments are now validated continuously by billions of navigation devices, every one of which depends on the corrections being exactly right. A theory developed in the early twentieth century, describing how motion and gravity warp the passage of time, turns out to be indispensable to finding a street address, and the daily 38-microsecond adjustment is the quiet proof that the physics holds.
Other timing errors engineers must manage
Relativity is the most famous correction, but it is not the only one that keeps satellite navigation honest. Signals from the satellites slow slightly as they pass through the charged upper atmosphere and the moist lower atmosphere, introducing delays that receivers and ground stations must estimate and remove. The precise position of each satellite in its orbit has to be tracked and broadcast, since any error in where a satellite is believed to be translates directly into a positioning error on the ground. Even the atomic clocks themselves drift by tiny amounts and are monitored and adjusted from control stations. Relativity stands out because it is large, constant, and predictable, a correction that can be built into the hardware once rather than chased continuously, which is why it so cleanly illustrates the physics at work.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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