How GPS Determines a Location

TL;DR. The blue dot on a phone's map looks like magic: open the app, and within a second or two it knows exactly where you are, anywhere on the planet, for free. Underneath, a receiver in the phone is doing a timing experiment with light-speed precision, measuring how long radio signals took to arrive from several satellites 20,200 kilometers overhead and solving four equations at once. That timing experiment only works because those satellites carry atomic clocks accurate to a few billionths of a second, and because engineers had to build a correction for Einstein's relativity directly into the system, or the whole thing would drift by kilometers a day. The satellites themselves are a US military constellation, opened to civilians after a Cold War tragedy, whose free global signal now quietly times cell networks, stock trades, and the power grid, not just maps.

Key takeaways

  • A GPS receiver doesn't get told its location. It calculates it, by timing how long radio signals took to travel from at least four satellites and converting each delay into a distance, a process called trilateration.
  • That calculation depends on satellite clocks accurate to nanoseconds, because at the speed of light, a timing error of one microsecond becomes a position error of about 300 meters. GPS satellites carry atomic clocks for exactly this reason.
  • Relativity is not a rounding error here. Satellite clocks run about 38 microseconds a day faster than clocks on the ground, from a mix of special and general relativity, and GPS engineers correct for it deliberately. Skip the correction and position errors would pile up by about 10 kilometers a day.
  • GPS started as a US military program in the 1970s and became available to civilians largely because of the 1983 shootdown of Korean Air Lines Flight 007, which had strayed off course. Civilian accuracy then jumped overnight in May 2000, when the military turned off a deliberate signal degradation called Selective Availability.
  • A phone rarely relies on GPS alone. It typically blends GPS with three other national satellite networks (Russia's GLONASS, Europe's Galileo, China's BeiDou) plus Wi-Fi and cell tower signals, which is why a fix appears fast even in a city street lined with tall buildings.
  • The same free time signal that GPS provides for navigation also synchronizes cell towers, timestamps financial trades, and keeps sections of the power grid in step, which is why jamming and spoofing incidents, now a documented and growing problem near conflict zones, worry infrastructure planners as much as pilots.

The moment nobody thinks about

A phone comes out of a pocket, a maps app opens, and a small blue dot appears on a street, already centered, already correct, before there's been time to consciously wonder where it is. There's no visible antenna, no dial-up delay, no bill. It works identically whether the phone is in a familiar neighborhood or a city on the other side of the world that its owner has never visited. Compared to almost everything else in this book, that instant, free, planet-wide accuracy is the strangest kind of invisible: it isn't hiding a pipe or a wire running to a specific place. It's hiding a timing measurement being made against objects moving through space at roughly 14,000 kilometers per hour, 20,000 kilometers away.

The mechanism: a stopwatch, not a compass

The single most common misunderstanding about GPS is that satellites somehow "see" a phone and radio its coordinates back down. They don't. A GPS satellite has no idea a particular phone exists. Instead, each satellite continuously broadcasts two things on repeat: its own precise position in space (called its ephemeris, essentially a very accurate orbital description) and the exact time, down to nanoseconds, at which that specific piece of the signal left the satellite.

A GPS receiver, the chip inside a phone, car, or dedicated GPS unit, listens for that broadcast and compares the time written into the signal against its own clock the instant the signal arrives. That difference is the signal's travel time. Radio waves, like all electromagnetic radiation, travel at the speed of light: about 299,792 kilometers per second. Multiply the travel time by that speed, and the receiver has a distance to that one satellite. Not a direction, not a bearing, just a distance. But a distance to a known point in space is still useful: it means the receiver could be anywhere on the surface of an imaginary sphere with that satellite at the center and that distance as the radius.

One sphere alone isn't enough to fix a location. Do the same calculation against a second satellite, and the receiver now knows it sits somewhere on the circle where two spheres intersect. A third satellite narrows that circle down to just two points, usually far enough apart (one of them often deep underground or in space) that context throws one away instantly. This process, using measured distances rather than measured angles to pin down a position, is called trilateration.

Don't be confused: trilateration is not triangulation. Triangulation calculates a position from angles, the way an old ship's navigator might sight two lighthouses and work out where their bearing lines cross. GPS never measures an angle. It measures the time a signal took to arrive and converts that into a distance. Three known distances to three known points is trilateration, and it's the actual math running inside every GPS chip, even though "triangulating your position" has become the everyday phrase people use for the whole idea.

So why does GPS need a fourth satellite, when three spheres already narrow things down to a point? Because the math above assumed the receiver's own clock was perfect, and it isn't. A satellite's atomic clock costs hundreds of thousands of dollars and is accurate to a few billionths of a second. The quartz clock inside a $30 GPS chip is not, and it drifts by amounts that would be useless for this kind of calculation on its own. So a real GPS receiver treats its own clock error as a fourth unknown, alongside its three position coordinates, and uses a fourth satellite's signal to solve for all four values at once: latitude, longitude, altitude, and exactly how far off its own internal clock currently is. That's the elegant part of the design. The receiver doesn't need an atomic clock of its own; it borrows the precision of four satellites' atomic clocks and cancels its own timing error out of the equations entirely.

The precision required is unforgiving. Since radio signals travel at the speed of light, a clock error of just one microsecond, one millionth of a second, translates into a position error of roughly 300 meters. A GPS satellite's atomic clock is accurate to around three nanoseconds, three billionths of a second, which is exactly why it takes an atomic clock and not an ordinary quartz oscillator to make any of this work at the accuracy people expect from a phone.

The correction Einstein made necessary

Atomic clocks are precise enough to expose something stranger than manufacturing tolerances: the clocks aboard GPS satellites run at a genuinely different rate than identical clocks sitting on the ground, because of relativity. Two separate effects are involved, and they pull in opposite directions. Special relativity says that a clock moving fast relative to an observer ticks slightly slower, and GPS satellites orbit at about 14,000 kilometers per hour, which by itself would make their clocks lose about 7 microseconds a day compared to a ground clock. General relativity says that a clock sitting in a weaker gravitational field, meaning farther from Earth's mass, ticks slightly faster, and at an altitude of 20,200 kilometers, that effect adds about 45 microseconds a day. Combined, the general relativistic gain wins out over the special relativistic loss, and a GPS satellite's onboard clock runs fast by a net of about 38 microseconds every day compared to an identical clock on the ground.

That sounds tiny until it's converted into distance. Left uncorrected, a 38-microsecond daily clock drift would make GPS position calculations inaccurate by roughly 10 kilometers a day, and the error would keep growing. A navigation system that's off by 10 kilometers after one day, and worse the next, is not a navigation system. GPS engineers solved this before the satellites ever launched, not by writing patches later: each satellite's atomic clock is deliberately built and tuned to run very slightly slow before launch, at a rate calculated so that once the satellite is in orbit and the relativistic effects kick in, the clock ends up ticking at exactly the correct rate as seen from the ground. It's one of the few pieces of everyday consumer technology where a correction for general relativity isn't a theoretical curiosity. It's a specific number, engineered into a satellite, that a phone in a pocket depends on every time it opens a map.

The complete picture: a constellation, a control room, and a phone doing the rest

The satellites making all of this possible form what's officially called the GPS space segment: as of recent counts, 31 operational satellites, with a few more spares kept in reserve, orbiting at roughly 20,200 kilometers in what's called medium Earth orbit. They're arranged across six evenly spaced orbital planes, angled 55 degrees to the equator, with several satellites in each plane. That specific geometry isn't an accident. It's designed so that from literally any point on Earth's surface, at least four satellites are above the horizon and visible at any moment, which is the minimum a receiver needs to solve those four unknowns.

The satellites can't run themselves. A ground control segment, led by a Master Control Station at Schriever Space Force Base in Colorado, along with an alternate control station and a global network of monitoring antennas positioned from Cape Canaveral to Hawaii to Ascension Island to Diego Garcia, continuously tracks every satellite's actual orbit and clock behavior. Satellites drift slightly from their intended orbits over time, and their atomic clocks, however good, need periodic correction against a master time reference. Ground controllers upload updated orbital and clock data to each satellite regularly, which is what lets that satellite keep broadcasting an accurate ephemeris and time signal to every receiver on Earth below it.

A modern phone, though, rarely relies on raw GPS signals alone, and that's a big part of why a location fix appears in a second or two rather than the minute or more that older, GPS-only devices sometimes needed. GPS satellites transmit at low power, roughly comparable to a car headlight's wattage spread over thousands of kilometers, and their signals are easily blocked by buildings, weakened by clouds, or lost entirely indoors. So phones combine several sources at once.

Don't be confused: GPS is one system; GNSS is the whole category. "GPS" specifically refers to the US system. Russia operates its own equivalent, GLONASS. The European Union operates Galileo. China operates BeiDou. Collectively, these are called GNSS, Global Navigation Satellite Systems, and nearly every modern smartphone chip listens to signals from several of them simultaneously, not just GPS, which is why people use "GPS" loosely to mean "satellite navigation in general" even when the actual satellite doing the work overhead is Russian, European, or Chinese.

Alongside GNSS signals, phones also use assisted GPS (A-GPS), where the carrier network sends the phone satellite position data over the cellular connection instead of making it wait to download that data directly from a satellite, which speeds up the very first fix substantially. Indoors or between tall buildings, where satellite signals barely reach, phones fall back on known Wi-Fi router locations (crowdsourced over years by phones that passed by them with GPS working) and on the signal strength and identity of nearby cell towers, a method sometimes called cell tower triangulation. None of these substitutes is as precise as a clear-sky GPS fix, but blended together, they're why a phone in a downtown office building can still place a blue dot on the right block.

Who keeps the satellites talking

The workforce behind this system is smaller and more concentrated than most in this book, because it's almost entirely a US federal military and aerospace operation. Inside the Space Force, the unit responsible for day-to-day command and control of the GPS constellation is Mission Delta 31, a Positioning, Navigation, and Timing unit stood up in 2023, which absorbed duties previously spread across the Air Force's old satellite operations squadrons and Space Delta 8. Within it, the 2nd Space Operations Squadron (sometimes referred to for its navigation warfare role as the 2nd Navigation Warfare Squadron) at Schriever Space Force Base actually commands the satellites hour to hour, monitoring health telemetry, uploading navigation data, and repositioning spacecraft when one needs to move within its orbital slot.

Building the satellites in the first place is a separate, contractor-run effort: aerospace engineers and technicians at firms like Lockheed Martin design, assemble, and test each new generation of GPS satellite, which then launches on a rocket (recent launches have flown on SpaceX Falcon 9 boosters) and undergoes weeks of on-orbit testing before it's declared healthy and added to the operational constellation. Beneath all of that, monitor station technicians at a handful of sites scattered from the Atlantic to the Indian Ocean to the South Pacific quietly track every satellite's signal, feeding the data that lets Schriever's control room correct each satellite's clock and orbit before ordinary drift becomes a navigation problem anyone would notice.

Where this came from

GPS began as a Cold War military problem, not a consumer convenience. In the early 1970s, the US Department of Defense set out to replace a patchwork of older navigation systems with a single, unified satellite network that could give ships, aircraft, and eventually ground troops a precise position anywhere on Earth. The program, first named Navstar, launched its first developmental satellite in February 1978, with three more following that same year. Building out a full constellation took a long time; the system wasn't declared to have reached the 24-satellite configuration needed for full global operation until the early to mid-1990s.

Civilian access to GPS did not arrive because the military decided consumer navigation would be profitable. It arrived because of a specific disaster. On September 1, 1983, Korean Air Lines Flight 007, a Boeing 747 flying from Anchorage to Seoul, drifted far off its planned route into Soviet airspace, apparently because of a navigational error, and was shot down by a Soviet interceptor, killing all 269 people aboard. Within weeks, President Ronald Reagan announced that once GPS was complete, it would be made available to civilian aircraft and ships worldwide, free of charge, explicitly to help prevent exactly this kind of navigational tragedy from happening again. It is one of the clearer, more direct lines in modern infrastructure history from a specific catastrophe to a specific, lasting policy change.

For years afterward, though, civilian GPS was deliberately made worse than military GPS. The Pentagon ran a feature called Selective Availability, which intentionally injected timing errors into the civilian GPS signal, degrading its accuracy to roughly 100 meters, out of concern that adversaries could otherwise use the free signal for precision weapons guidance. That changed abruptly at midnight at the start of May 2, 2000, when the US government, under President Bill Clinton, switched Selective Availability off across the entire constellation simultaneously, after the military developed newer, more targeted ways to deny GPS accuracy to a specific adversary in a specific region without degrading it for everyone else on Earth. Civilian accuracy improved roughly tenfold overnight, from about 100 meters to closer to 10 to 20 meters, essentially with the flip of a switch on satellites already in orbit, and it's the single biggest one-day accuracy jump in the system's history.

Free to use, coordinated by treaty, open to any manufacturer

GPS is, by explicit US government policy, provided as a global public service, free of any direct user fee for civilian, commercial, or scientific use anywhere in the world. Nobody pays a subscription to receive a GPS signal; the cost is absorbed by the US federal government, split mostly between the Department of Defense, which built and runs the constellation for military purposes, and the Department of Transportation, which represents civilian interests in how the system is operated.

That openness only works at the technical level because the interface between the satellites and any receiver is published as a public specification, currently called IS-GPS-200 (an evolution of the earlier ICD-GPS-200 document). Any company, anywhere, can read that specification and build a chip that correctly decodes GPS signals, which is exactly why GPS receivers show up inside products from hundreds of manufacturers with no licensing relationship to the US government at all.

Because GPS is no longer the only satellite navigation system in the sky, international coordination has become its own layer of standards work. The International Committee on Global Navigation Satellite Systems (ICG), convened under the United Nations since 2005, brings together the operators of GPS, Russia's GLONASS, the European Union's Galileo, and China's BeiDou to work out compatibility and interoperability, so that a single receiver chip can combine signals from several of these systems without them interfering with each other or using incompatible formats. That quiet diplomatic and engineering coordination is a large part of why a phone's multi-constellation location fix simply works, regardless of which country built which satellite overhead at that moment.

Keeping the constellation current

GPS satellites don't last forever, and the constellation has always been run as a rolling replacement program rather than a fixed set of hardware. Older satellite generations, known as Block II and its successors, are gradually being replaced by GPS III satellites, the first of which launched in December 2018, with a further generation called GPS IIIF (Follow-On) now extending the modernization program past thirty planned satellites total. Each new generation adds meaningful improvements: GPS III satellites are reported to offer roughly three times the accuracy and substantially stronger resistance to jamming compared to the oldest satellites still in service. Because satellites are launched to replace others gradually rather than all at once, the constellation at any given moment is a mix of generations of different ages, and it's routine for satellites to keep operating years, sometimes over a decade, past their original design lifetime, quietly extending their service until a replacement is ready.

The ground side gets its own modernization cycle. The control segment's core software and hardware, the systems at Schriever that track every satellite and compute the corrections uploaded to them, has gone through a multi-year, famously difficult modernization program (known as OCX) to handle the more complex signals the newer satellites broadcast. None of this maintenance is visible to a phone user; it shows up only as a signal that keeps working and, gradually, keeps getting a little more accurate and a little more resistant to interference.

When the signal lies, or doesn't arrive

GPS can fail in two distinct ways, and they aren't the same problem. Jamming simply overpowers or drowns out the real GPS signal with noise on the same frequency, so a receiver loses its fix entirely and, ideally, knows it has lost it. Spoofing is worse: it broadcasts a fake but convincing GPS signal, tricking a receiver into calculating a wrong position or time while displaying full confidence that it's correct. Both have moved from theoretical concerns to a well-documented, rapidly growing problem, concentrated heavily around active conflict zones. Aviation trade publications tracking the issue recorded more than 430,000 GNSS jamming and spoofing incidents affecting flights in 2024 alone, with the rate of GPS signal loss per flight climbing sharply year over year. The Baltic Sea region logged roughly 46,000 reported interference incidents between August 2023 and April 2024, most attributed to jamming believed to originate from Russian territory, with commercial ships and aircraft in the area regularly reporting lost or corrupted position signals.

The starkest documented case involves Azerbaijan Airlines Flight 8243, which crashed near Aktau, Kazakhstan, on December 25, 2024, killing 38 people. Flight-tracking data showed the aircraft's GPS signal disappearing over Grozny, in an area experiencing heavy electronic warfare activity related to regional drone attacks, followed by a period of transmitted position data that didn't match the plane's actual location. Subsequent investigation concluded the aircraft had been struck by a Russian air-defense missile during that period of GPS disruption and confusion, and in 2026 Russia and Azerbaijan reached a settlement acknowledging the strike as an unintentional result of the air-defense response. It stands as one of the clearest real-world illustrations of how GPS jamming and spoofing near a conflict zone can compound into a genuine aviation disaster rather than a mere inconvenience.

When GPS signal is lost or corrupted, well-designed systems don't simply go blank. Aircraft and ships fall back on inertial navigation systems, which use onboard motion sensors to estimate position by tracking acceleration and direction from the last known good fix, a technique called dead reckoning that slowly loses accuracy the longer it runs without a fresh satellite fix to correct it. Phones, similarly, fall back toward Wi-Fi and cell-tower positioning, which is far less precise but still generally points to the right neighborhood rather than nothing at all.

The scale underneath the blue dot

The GPS constellation itself is a modest piece of hardware, on the order of 31 satellites, yet it underlies a genuinely enormous dependent economy. Industry estimates put the number of devices worldwide relying on GNSS signals (GPS combined with the other constellations) at more than 25 billion, spanning smartphones, vehicles, drones, ships, farm equipment, and countless industrial sensors, with roughly 1.5 billion new GNSS-equipped devices manufactured every year, the overwhelming majority of them smartphones.

The clearest attempt to price all of this came from a 2019 study commissioned by the National Institute of Standards and Technology (NIST) and carried out by RTI International, which estimated that GPS had generated about $1.4 trillion in economic benefit to the US private sector since it became available in the 1980s, with roughly 90 percent of that value created after 2010, once smartphones and GPS-dependent logistics became widespread. The same study modeled what a GPS outage would cost: roughly $1 billion a day in direct economic impact nationally, rising as high as an estimated $45 billion for a 30-day outage striking during a critical agricultural planting season, when GPS-guided farm equipment and precision fertilizer application depend on the signal most heavily.

What else is quietly built on GPS: the hidden clock

Almost everything described so far treats GPS as a way to answer "where am I." But GPS satellites broadcast something arguably even more valuable than position: an extremely precise, freely available, globally synchronized time signal, and huge sections of infrastructure depend on that time signal without doing any navigation at all. Cell phone towers use GPS-disciplined clocks to keep their transmission timing synchronized with neighboring towers, which is part of what lets a call hand off cleanly from one cell to the next as a phone moves. Financial exchanges and trading firms use GPS time to timestamp transactions with enough precision to reconstruct, after the fact, the exact sequence in which trades occurred across different systems, a requirement that has become tighter as regulators demand more precise trade-timing records. Electric utilities install GPS-timed devices called phasor measurement units across the power grid, which timestamp voltage and current readings precisely enough (to within a couple of microseconds) to detect instability building up across a wide region before it turns into a cascading blackout.

None of these systems are using GPS to find out where they are. They already know where they are; they're using it because a free, globally consistent clock, accurate to billionths of a second, turns out to be one of the most useful pieces of infrastructure a modern network can plug into, and GPS was, for decades, the only source of it that was both free and precise enough. That's also exactly why GPS jamming and spoofing worry infrastructure planners for reasons that have nothing to do with maps: an attack that degrades the GPS timing signal in a region could, in principle, disrupt cell networks or grid monitoring in that area even if not a single person there is trying to navigate anywhere.

Trade-offs and what comes next

The dependency this chapter has been describing, a huge share of modern infrastructure quietly leaning on one free, US-government-run satellite signal for both location and time, has become widely recognized as a concentration risk rather than an efficiency win. In 2020, the US issued Executive Order 13905, directing federal agencies to reduce over-reliance on GPS specifically and to develop complementary positioning and timing sources that don't share GPS's vulnerabilities. Congress separately required the Department of Transportation, under the National Timing Resilience and Security Act of 2018, to establish a land-based backup timing system, leading to a multi-year demonstration program testing roughly a dozen candidate technologies, including a modernized version of an older radio-navigation system called eLoran, which broadcasts from powerful ground-based transmitters rather than satellites and is, by design, far harder to jam or spoof than a faint signal arriving from 20,200 kilometers away.

More than a decade of hearings, studies, and demonstration projects on GPS backup systems have not yet produced a fully deployed national replacement, which is itself a telling fact about how hard it is to walk back a dependency once free infrastructure has become this deeply embedded. Meanwhile, the newest GPS satellites are being built with substantially better jamming resistance than earlier generations, and multi-constellation GNSS receivers, blending GPS with Galileo, GLONASS, and BeiDou, already give ordinary devices some built-in resilience, since spoofing every visible satellite from every constellation at once is a much harder task than spoofing GPS alone. Whether that's enough, given how much of the world's timing infrastructure still assumes a single, mostly uncontested satellite signal will keep arriving correctly, is one of the more consequential open questions in infrastructure resilience right now.

Back to the blue dot

The next time that dot centers itself on a map within a second of opening the app, what actually happened underneath is a receiver timing radio signals from at least four satellites moving at 14,000 kilometers per hour, converting nanosecond-scale delays into distances, correcting for a relativistic clock drift that Einstein's equations predict and engineers built in on purpose, and quietly borrowing a Wi-Fi router's known location or a cell tower's position to fill in the gaps a weak satellite signal leaves behind. It looks instant because a US military constellation, opened to the public after a shootdown that killed 269 people, has spent decades being deliberately engineered to make an extraordinarily hard timing problem disappear behind a small, calm, blue dot.

The leap: what it replaced, and the work behind it

For most of history, not knowing where you were killed people in numbers. On the night of October 22, 1707, a Royal Navy fleet returning from Gibraltar ran onto the rocks of the Isles of Scilly because its navigators, working by dead reckoning, believed they were safely off the coast of Brittany. They were far to the north of where they thought. Four warships went down and somewhere between 1,400 and 2,000 sailors drowned, one of the worst maritime losses in British history. Sailors of that era could find their latitude from the sun, but longitude, their east-west position, could only be guessed by tracking speed and heading over time, and the guess compounded its own error with every hour. The disaster helped push Parliament to pass the Longitude Act of 1714, offering up to £20,000 to anyone who could fix a ship's position at sea. A Yorkshire carpenter named John Harrison spent decades building a clock accurate enough to do it; his H4, tested on an 81-day voyage to the West Indies in 1761, lost only about five seconds. The lesson buried in that story is the one this whole chapter runs on: position is a timing problem, and it always has been.

The leap from Harrison's single hand-built clock to a blue dot on every phone is hard to overstate. He solved longitude for one ship, one voyage, after a lifetime of filing brass by hand. GPS solves it for roughly 25 billion devices at once, for free, continuously, worldwide, and it does so by keeping 31 atomic-clock-carrying satellites healthy in orbit twenty thousand kilometers up. That does not happen on its own. A newer GPS III satellite costs on the order of $250 million to build before it ever launches, and behind each one is a chain of aerospace engineers, launch crews, and the operators at Schriever Space Force Base who upload fresh orbit and clock corrections to every satellite on a repeating cycle so the position it broadcasts stays true. The precision Harrison chased over forty years is now re-established, from scratch, several times a day, by people most GPS users will never picture.

For the reader, the payoff shows up as small mercies that used to be real labor. Driving to an unfamiliar address no longer means printing directions or pulling over to unfold a paper map at a dark intersection. A hiker who takes a wrong fork can see it immediately instead of walking deeper into trouble, the way lost travelers once did when a single missed turn in the desert could turn fatal. A food delivery finds a specific door; a ride arrives at the right curb; a phone left in a taxi can be traced to a street. The morning the signal is jammed or spoofed, which now happens routinely near conflict zones, that whole layer of certainty quietly falls away: the map hesitates, the arrival estimate goes wrong, and drivers rediscover how much attention navigation used to take. The calm blue dot is the visible end of a timing achievement that generations of sailors would have traded almost anything for.

Real-world examples and recent developments

Outside the US military's own constellation and its prime contractor, a whole layer of receiver makers, chipmakers, and new satellite operators shapes how GPS actually reaches a device, and what happens when its signal is jammed.

  • Trimble Inc. (1984): Founded in Sunnyvale, California, in 1978, Trimble built the world's first commercial GPS receiver in 1984, turning a system still years from full military deployment into a tool for surveyors and, later, farmers and construction crews. Trimble Inc.
  • Garmin (1990): Founded in Lenexa, Kansas, in 1989 by engineers Gary Burrell and Min Kao, the company's first product, a marine GPS receiver called the GPS 100, debuted in 1990 and drew 5,000 orders, an early sign that GPS receivers could be a consumer product and not only a military or surveying tool. Garmin, company history
  • u-blox (1997 and 2000): A GNSS chipmaker spun out of ETH Zurich in 1997, u-blox supplied the receiver module used in the world's first GPS-enabled mobile phone in 2000, and its chips are still built into cars, drones, and fitness trackers today. Wikipedia, u-blox
  • Xona Space Systems (2024 to 2025): A US startup building Pulsar, a low-Earth-orbit satellite network meant to provide positioning and timing independent of GPS, Xona launched its first production-class satellite, Pulsar-0, and in February 2025 won a $4.6 million US Air Force Research Laboratory contract to test the signal in GPS-denied environments. GPS World, Xona satellite begins tests

Recent developments

  • Space Force's Resilient GPS program (September 2024): The Space Systems Command awarded four contracts, to Sierra Space, L3Harris, Astranis, and Axient, to design small, cheaply produced satellites that would broadcast core GPS signals as a hedge against jamming or an attack on the main constellation, part of a roughly $1 billion, five-year effort. SpaceNews, Space Force taps four companies
  • A jamming and spoofing surge tied to the June 2025 Iran-Israel war: GNSS monitoring firms recorded more than 12,000 spoofing incidents affecting over 3,000 vessels worldwide between June 13 and June 24, 2025, and interference was blamed for the grounding of the container ship Antonia near Jeddah, Saudi Arabia, that May. Foreign Policy, The Epidemic of GPS Jamming

Glossary

Trilateration. Calculating a position from measured distances to several known points, as opposed to triangulation, which uses angles.

Pseudorange. The distance a GPS receiver calculates to a satellite based on signal travel time, called "pseudo" because it still contains the receiver's own uncorrected clock error until the final calculation.

Ephemeris. The precise description of a satellite's own position and orbit, broadcast continuously as part of its navigation signal.

Atomic clock. An extremely precise clock that keeps time based on the natural vibration frequency of atoms, accurate to within a few nanoseconds a day, standard equipment on every GPS satellite.

Time dilation. The relativistic effect by which a clock's rate depends on its speed (special relativity) and the strength of gravity around it (general relativity), both of which measurably affect GPS satellite clocks.

Medium Earth orbit (MEO). The orbital band, roughly 2,000 to 35,000 kilometers up, where GPS satellites operate at about 20,200 kilometers.

GNSS (Global Navigation Satellite System). The general category covering GPS (United States), GLONASS (Russia), Galileo (European Union), and BeiDou (China), all of which modern phones can use together.

A-GPS (assisted GPS). A method where a cellular network supplies satellite position data to a phone over the internet, speeding up a receiver's first location fix.

Selective Availability. A deliberate degradation the US military applied to civilian GPS signals for security reasons, in effect from the 1980s until it was switched off in May 2000.

Master Control Station. The Schriever Space Force Base facility that monitors every GPS satellite's health, orbit, and clock, and uploads correction data to keep the constellation accurate.

GPS jamming. Overpowering or blocking the real GPS signal with radio noise, causing a receiver to lose its position fix.

GPS spoofing. Broadcasting a fake but convincing GPS signal to trick a receiver into calculating a wrong position or time without any obvious warning.

PNT (Positioning, Navigation, and Timing). The three related services GPS and other GNSS systems provide, a term used across government and industry because the timing function matters as much as the location function.

Sources and notes

Open questions

  • Public figures for how many devices worldwide rely on GNSS vary widely by source and year (estimates commonly range from the low billions to more than 25 billion once every embedded sensor and industrial device is counted); treat any single number as an order-of-magnitude estimate.
  • The investigation into Azerbaijan Airlines Flight 8243 involved competing early accounts before Russia and Azerbaijan reached their 2026 settlement; some technical details of exactly how the jamming and spoofing unfolded moment to moment remain based on flight-tracking analysis rather than a fully public official accident report.
  • Despite years of hearings, executive orders, and demonstration programs, no nationwide GPS backup timing or positioning system had been fully deployed in the United States at the time of writing, so the practical resilience picture described here may shift as that policy work continues.

Knowing where you are is only useful if everyone agrees on what time it is and how far a meter really goes. How clocks, measurements, and standards coordinate modern life 👉