In 1977, three years before GPS launched, the engineers building the satellites had to make a decision.
\nThe 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.
\nThat sounds like nothing, but over 24 hours of GPS operation, an uncorrected clock would put you 11 kilometers off your actual position.
\nThey had two options:
\nGPS chose option two.
\nThey 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.
\nThe correction is baked in.
\nThat’s what putting clocks in space looks like. One decision, and now everyone on Earth gets both navigation and time from the same signal.
\nThis post is about the impact of that decision.
\nTwo relativistic effects act on a GPS satellite clock, and they push in opposite directions.
\nSpecial 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.
\nSounds 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.”
\nThis was known before launch. It was tested. It works.
\nGPS time is its own scale, started at midnight on January 6, 1980, and ticking continuously since. No leap seconds. No time zones.
\nThe relationship to the other scales is fixed and simple:
\nGPS = TAI − 19 seconds (constant since launch)\nGPS = UTC + 18 seconds (today)\nGPS−TAI never changes. GPS−UTC grows every time UTC gets a leap second, and freezes after the 2035 leap-second abolition.
\nIt is, in every meaningful sense, the most accurate clock in your daily life. And you’ve never seen it.
\nGPS time runs almost everything that needs precise timing in modern civilization, but it’s invisible because nobody consumes it directly.
\nThe civilian world runs on GPS time. It just doesn’t admit it.
\nPutting clocks in space solved navigation.
\nIt did not solve timekeeping.
\nYour 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.
\nGPS time is great if you are a satellite, a financial trader, a power-grid engineer, a fighter jet, or a cell tower.
\nIt is not great if you are trying to know what time to pick up your kid from school.
\nFor that, you still need wall time, which still needs UTC, which still needs leap seconds, which still needs Earth’s wobbling rotation.
\nWe built absurdly precise atomic clocks.\nWe launched them into orbit.\nWe baked relativity corrections into the silicon.\nWe covered the planet in time signals accurate to nanoseconds.
\nAnd your meeting is still at 3 PM on Tuesday.
\nGPS quietly handles the part it needs to handle. But all of this assumes you’re on Earth.
\nEarth orbit needs relativity corrections. The Moon needs more. Mars needs different ones still.
\nThe further you get from Earth, the more “GPS-style time” stops being a solution.
\nTomorrow: if an hour is an Earth measurement, so how do you tell time on a planet that doesn’t have them?
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\n", "date_published": "2026-06-03T10:00:00-05:00", "url": "https://llbbl.blog/2026/06/03/day-what-if-we-put.html", "tags": ["Science","Infrastructure","30daysoftime","Timekeeping","Gps","Relativity"] } ] }