GPS satellites do not know where you are. Each one broadcasts what time it is from 20,200 km up, over and over, forever — and your phone turns four of those timestamps into a place. This is the map those clocks draw across the Earth, and the small network of ground stations that keeps them honest.
Thirty-one satellites were flying in the GPS constellation as of July 3, 2023 — comfortably above the 24-slot baseline the U.S. government commits to. They fly in six equally spaced orbital planes, four baseline slots each, in medium Earth orbit. Every one carries an atomic clock. Source: GPS.gov, Space Segment.
Each cyan thread is the path one satellite traces over the ground during a single 11 h 58 m orbit. Six planes, six threads, woven into a lattice — and the weave stops dead at 55° north and south. Control-site locations are real; coastlines and tracks are drawn schematically from published orbital parameters.
The constellation is defined as 24 slots in six equally spaced planes — four baseline slots per plane, arranged so that at least four satellites are in view from virtually any point on Earth. In June 2011 the Air Force repositioned six satellites into an “Expandable 24” configuration: three slots were split, so GPS now effectively runs as a 27-slot constellation with better geometry over most of the world.
Four independent global systems, stacked inside a shell barely 4,100 km thick. GLONASS deliberately flies lowest and steepest — 64.8° of inclination buys coverage over the Arctic, which matters when much of your country sits above 60° north.
Every GPS plane is tilted 55° from the equator. A satellite's ground track is therefore a sine wave that peaks at 55° N and bottoms at 55° S and never goes further — which is exactly what you see on the map above. The orbital period is 11 hours 58 minutes 2 seconds, half a sidereal day, at roughly 14,000 km/h. Two laps per day, and the track lays itself back down over the same ground tomorrow.
Orbits drift and atomic clocks wander. A ground network measures every satellite as it passes, predicts where it will be and what its clock will read, and uploads the corrections that each satellite then rebroadcasts in its navigation message. The master control station is at Schriever Space Force Base, Colorado, with an alternate at Vandenberg, California.
Look at where the original six monitor stations sit on the map: Colorado, Cape Canaveral, Hawaii, Ascension, Diego Garcia, Kwajalein. Strung around the equator, mostly on specks of island, because those are the only pieces of land under the busiest part of the lattice. Six more sites were added in 2005 — in Argentina, Bahrain, the United Kingdom, Ecuador, Washington DC and Australia — to close the remaining gaps.
Position falls out of timing. Your receiver measures how long each signal took to arrive, multiplies by the speed of light, and solves for the one point where four spheres intersect. That makes the clock the whole ballgame — and clocks in orbit do not run at the same rate as clocks on the ground.
20,200 km at the speed of light. Computed, not measured.
The onboard atomic clocks keep time to roughly four billionths of a second a day.
Fainter than the thermal noise it arrives buried in — the receiver digs it out by correlation.
One nanosecond of clock error is about 30 centimetres of position error (computed from the speed of light). Leave the 38 µs/day relativistic gain uncorrected and the system would walk off by roughly 11 kilometres a day.
The U.S. government commits to a global average user range error of 7.8 metres or better, 95% of the time. The signal that actually goes out is roughly ten times better than that. And until the year 2000, a deliberate error called Selective Availability made civilian GPS an order of magnitude worse again.
Selective Availability ended a few minutes past midnight EDT after May 1, 2000, simultaneously across the whole constellation. In September 2007 the government went further and ordered the GPS III satellites built without the capability at all — the degradation switch no longer exists in orbit.
GPS has no subscription, no meter and no invoice, which makes its value easy to miss until it stops. A 2019 study for the U.S. National Institute of Standards and Technology put numbers on it.
Across ten industries — agriculture, electricity, finance, location services, mining, maritime, oil & gas, surveying, telecoms, telematics. About 90% of it accrued after 2010.
Modelled estimate, not an observed figure.
Agriculture alone. Also a modelled scenario.
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