In 1977, three years before GPS launched, the engineers building the satellites had to make a decision.

The clocks they were about to put in orbit were going to run faster than the clocks on the ground. By about 38 microseconds per day.

That sounds like nothing, but over 24 hours of GPS operation, an uncorrected clock would put you 11 kilometers off your actual position.

They had two options:

1. Adjust the time signal on the ground, applying the correction as the data came back down.
2. Pre-tune the clocks on the satellites to run slow by exactly the right amount, so that by the time relativity sped them back up, they’d tick at the right rate.

GPS chose option two.

They built the clocks to run at 10.22999999543 MHz instead of the nominal 10.23 MHz, so that orbital relativity speeds them up to ~10.23 MHz by the time the signal hits your phone.

The correction is baked in.

That’s what putting clocks in space looks like. One decision, and now everyone on Earth gets both navigation and time from the same signal.

This post is about the impact of that decision.

Why Clocks in Orbit Run Faster

Two relativistic effects act on a GPS satellite clock, and they push in opposite directions.

Special relativity slows the satellite clock down because it’s moving fast. General relativity speeds it up because it sits in weaker gravity than the ground. Gravity wins. Net result: the satellite clock gains about 38 microseconds per day.

Sounds like nothing. But uncorrected, that 38 microseconds drifts your GPS position by 11 km in 24 hours. Within a day of launch, GPS would be useless for anything more precise than “are you in the right country.”

This was known before launch. It was tested. It works.

What GPS Time Actually Is

GPS time is its own scale, started at midnight on January 6, 1980, and ticking continuously since. No leap seconds. No time zones.

The relationship to the other scales is fixed and simple:

GPS = TAI − 19 seconds        (constant since launch)
GPS = UTC + 18 seconds        (today)

GPS−TAI never changes. GPS−UTC grows every time UTC gets a leap second, and freezes after the 2035 leap-second abolition.

It is, in every meaningful sense, the most accurate clock in your daily life. And you’ve never seen it.

What It’s Used For

GPS time runs almost everything that needs precise timing in modern civilization, but it’s invisible because nobody consumes it directly.

• Finance. US and EU regulators (MiFID II, SEC) require trading firms to timestamp orders to microsecond precision. GPS-disciplined oscillators are how.
• Telecom. Cellular base stations need their carrier frequencies aligned across the network. GPS clocks them. Without GPS, your phone would struggle to hand off between towers.
• Power grid. Phasor Measurement Units monitor the AC waveform across the entire grid, synchronized to GPS. This is how grid operators detect instabilities before they cascade into blackouts.
• Datacenters. Stratum-1 NTP servers are typically GPS-disciplined. Every clock you’ve ever checked on a computer ultimately traces back, through several layers of network sync, to a GPS receiver in someone’s rack.
• Aviation, surveying, autonomous vehicles, drones, scientific instruments, particle physics. Anything built since 1995 that needs accurate timing or positioning, which is essentially everything.

The civilian world runs on GPS time. It just doesn’t admit it.

What It Didn’t Solve

Putting clocks in space solved navigation.

It did not solve timekeeping.

Your watch is still on local time. Your calendar uses civic dates with leap seconds buried in the UTC. You’re reading a clock face anchored to a Roman calendar, a Babylonian 24-hour day, and an Earth rotation that nobody can predict.

GPS time is great if you are a satellite, a financial trader, a power-grid engineer, a fighter jet, or a cell tower.

It is not great if you are trying to know what time to pick up your kid from school.

For that, you still need wall time, which still needs UTC, which still needs leap seconds, which still needs Earth’s wobbling rotation.

We built absurdly precise atomic clocks. We launched them into orbit. We baked relativity corrections into the silicon. We covered the planet in time signals accurate to nanoseconds.

And your meeting is still at 3 PM on Tuesday.

GPS quietly handles the part it needs to handle. But all of this assumes you’re on Earth.

Where This Goes

Earth orbit needs relativity corrections. The Moon needs more. Mars needs different ones still.

The further you get from Earth, the more “GPS-style time” stops being a solution.

Tomorrow: if an hour is an Earth measurement, so how do you tell time on a planet that doesn’t have them?

Sources

• Error analysis for the Global Positioning System — Wikipedia
• GPS time — Wikipedia
• Schriever Space Force Base — Wikipedia
• Phasor measurement unit — Wikipedia

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