10GPS, the city's hidden clock

The short version: GPS is a US government system of satellites about 20,200 km (12,550 miles) up, each one broadcasting where it is and what time it is by its on-board atomic clocks. Your phone times those signals to work out where it is - typically within 4.9 m (16 ft) under open sky, and worse among tall buildings. The same signals quietly set the clocks of cell networks, power grids and trading systems, which is one reason a GPS jammer has no lawful use in the US. The "atomic" clock on your wall gets its time from a different radio signal entirely, a 60 kHz broadcast from Colorado called WWVB.

Who runs GPS, and how many satellites are up there?

What you see: A blue dot on a map, and maybe a "satellites in view" number in a GPS test app.

GPS belongs to the US government. In its own words, "the U.S. Space Force develops, maintains, and operates" it, and the public site GPS.gov is run by the National Coordination Office for Space-Based Positioning, Navigation, and Timing.¹⁵⁷

The US commits to keeping at least 24 operational satellites available 95% of the time. As of July 3, 2023, there were 31 operational satellites, not counting retired spares still in orbit.¹⁵⁸ They fly in medium Earth orbit at about 20,200 km (12,550 miles) and each one circles the planet twice a day.¹⁵⁸

The baseline design is 24 slots in six evenly spaced orbital planes, four slots per plane, so that a user "can view at least four satellites from virtually any point on the planet." In June 2011 a change called "Expandable 24" made it effectively a 27-slot constellation.¹⁵⁸

The current fleet spans Block IIR (launched from 1997) to GPS III, first launched in 2018 with a 15-year design life.¹⁵⁸ The first four Navstar satellites, the start of it all, went up in 1978.¹⁵⁹

As of October 2026, the Coast Guard Navigation Center's constellation status table listed 32 GPS satellites, two of them flagged as unusable.¹⁶⁰

How does my phone know where it is?

What you see: The blue dot snapping into place a few seconds after you open a map, often with a widening circle while it thinks.

Each GPS satellite is a very precise clock with a radio attached. Block IIF satellites, for example, carry "advanced atomic clocks."¹⁶¹ The satellite broadcasts its position and the time it sent the signal. Your phone notes when the signal arrived. Radio travels at the speed of light, so the delay tells the phone how far away that satellite is.

Several such distances narrow your location to one point. This is trilateration: finding a position from measured distances to known points.

The catch is that your phone's clock is cheap quartz, nowhere near as good as the satellites'. So the phone treats its own clock error as a fourth unknown, alongside latitude, longitude and altitude. Four unknowns need four measurements, which is why the constellation is laid out so that at least four satellites are visible from virtually anywhere.¹⁵⁸

What frequencies does GPS use?

What you see: Nothing on the street. In a phone spec sheet, "dual-frequency GPS (L1 + L5)."

GPS broadcasts on three carriers in the L band, around 1.2 to 1.6 GHz:¹⁶²

Sources: frequencies from the 2006 L2C fact sheet;¹⁶² signal status from GPS.gov.¹⁶³

L1 C/A is the legacy signal almost every receiver uses. L5 is "designed to meet demanding requirements for safety-of-life transportation," and L1C is designed to work alongside other countries' satellite systems.¹⁶³ A second frequency matters because the upper atmosphere slows radio signals by an amount that depends on frequency. Comparing two lets a dual-frequency receiver correct for that ionospheric distortion.¹⁵⁷ As far back as 2006 the government said L2C would enable "lower-cost, dual-frequency civil GPS receivers."¹⁶²

As of October 2026, GPS.gov still describes both L2C and L5 as pre-operational, to be "employed at the user's own risk until it is declared operational."¹⁶⁴ Civil navigation messages have been broadcast on both signals since April 2014.¹⁶⁴

How accurate is GPS on my phone?

What you see: A blue dot with a pale circle around it. The circle is the phone's own guess at its error.

GPS.gov says a GPS-enabled smartphone is "typically accurate to within a 4.9 m (16 ft.) radius under open sky."¹⁵⁷

The government's formal promise is about the signal, not your phone. It commits to a daily global average user range error of 2.0 m (6.6 ft) or less, 95% of the time. That is how far off the satellite's own distance information is, before your receiver adds any errors of its own. The measured figure on April 20, 2021 was 0.643 m or less, 95% of the time.¹⁵⁷ At the other end, high-end dual-frequency receivers with correction services can get to a few centimeters in real time.¹⁵⁷

GPS.gov lists what makes things worse:¹⁵⁷

• satellites blocked by buildings, bridges and trees
• indoor or underground use
• signals reflected off buildings or walls
• atmospheric conditions
• receiver design and quality
• less often: radio interference or jamming, major solar storms, satellite maintenance, and noncompliant devices

It also notes that mapping errors are often mistaken for GPS errors.¹⁵⁷

Why is my GPS wrong downtown?

What you see: Walking between towers, the dot drifts across the street, lands you inside a building, or swings a block away and back.

Two of GPS.gov's degrading factors team up in dense downtowns: blockage by buildings and multipath, signals reflected off buildings or walls.¹⁵⁷ A street lined with tall buildings is called an urban canyon. From the bottom of one you see a narrow strip of sky, so fewer satellites are in direct view, and the ones you do see tend to sit in a line along the street, which makes for weak geometry.

Multipath is the sneakier problem. A signal that bounces off a glass tower before reaching you has traveled farther than the straight-line distance. The phone reads that extra distance as real, and your computed position shifts. If the direct signal is blocked entirely and only the reflection gets through, the error can be large.

Phones fight back with more satellite systems (next item), Wi-Fi and cell-tower positioning, and second frequencies. None of it fully cures a canyon.

A 2026 preprint by researchers at the University of Notre Dame and Sebelas Maret University logged the accuracy estimates Android phones report about themselves (a 68 percent confidence radius, not a check against surveyed positions).¹⁶⁵ Driving through high-rise downtown San Diego, a Samsung S22 reported a median of 10.29 m, against 4.19 m on a more suburban uptown route; walking inside a Las Vegas hotel-casino, medians ran from about 29 m to 48 m across three phones.¹⁶⁵

Is my phone using GPS, or something else?

What you see: A GPS test app listing satellites with little flags or letters: US, RU, EU, CN.

"GPS" has become the everyday word for a whole family of systems. The general term is GNSS, Global Navigation Satellite System. Four are global:

When the European Commission declared Galileo's Initial Services in 2016, 18 satellites were in orbit. The services were a free Open Service, an encrypted Public Regulated Service for government users, and a Search and Rescue service that is part of the Cospas-Sarsat distress-beacon system (more in satellites). ESA's Paul Verhoef called it "the transition from a test system to one that is operational."¹⁶⁶

In a city, more systems means more satellites in view, and better odds that four have a clear line to you between buildings. That is the practical reason modern phones listen to several at once.

Why do cell towers and power grids need GPS?

What you see: A small dome or puck-shaped antenna on a cell site or utility cabinet. (The gear on poles is covered in our guide to utility poles.)

Much of GPS's value is time, not place. A GPS receiver is a cheap way to get a clock that is traceable to official time. GPS.gov commits to timing within 30 nanoseconds of UTC as kept by the US Naval Observatory, 95% of the time, for a specialized fixed receiver.¹⁵⁷

In a 2017 briefing to the federal PNT Advisory Board, E. Powers of the US Naval Observatory laid out who depends on it:¹⁶⁷

• Cell networks: "LTE, CDMA and other Cell Phone technology uses precise timing from GPS (1 microsecond)."
• Power grids: "Small phase inaccuracy will reduce efficiencies and larger errors may result in damage to equipment and power outages."
• Finance: "inaccurate time stamps could result in unfair advantage being gained and loss of revenue."

A 2012 paper by Michael Lombardi of NIST gives the numbers behind that. CDMA base stations need clocks accurate to 3 microseconds, CDMA system time is nearly always derived from GPS, and handoffs between towers start to fail as offsets approach 10 microseconds.¹⁶⁸ On the grid, sensors called phasor measurement units, or PMUs, stamp voltage and current readings with GPS-derived time so that measurements from far-apart substations can be compared. The minimum requirement is 26 microseconds, and 1 microsecond is desired. The first prototype PMU, in 1988, used a GPS clock.¹⁶⁸

The same paper is blunt about dependence: when GPS is lost, some industrial clocks go out of tolerance within seconds, and "most will be out of tolerance in less than one hour."¹⁶⁸

Trading rules set their own tolerances. In the US, FINRA Rule 4590 requires member firms' business clocks to be synchronized within one second of the NIST atomic clock, and within 50 milliseconds for computer clocks that record events in NMS securities, including listed options, and OTC equity securities.¹⁶⁹ In the EU, Commission Delegated Regulation (EU) 2017/574 under MiFID II, which applies from January 3, 2018, caps divergence from UTC at 100 microseconds, with timestamps of 1 microsecond or better, for high-frequency algorithmic trading, and at 1 second for voice trading.¹⁷⁰¹⁷¹ The UK carried the regulation into its own law after Brexit, and the UK text on legislation.gov.uk has the same figures.¹⁷¹

Are GPS jammers illegal?

What you see: Usually nothing. Occasionally a news story about a truck driver, or a sale listing for a "privacy" device that plugs into a car lighter socket.

In the US, yes, and broadly. Jamming means drowning out a radio signal with noise so it can't be received. Spoofing means broadcasting fake satellite signals so a receiver computes a false position or time.

The FCC's 2013 order in the Newark case (below) sets out the law:¹⁷²

• 47 U.S.C. 333 says "no person shall willfully or maliciously interfere with or cause interference to any radio communications" of licensed or authorized stations.
• 47 U.S.C. 302(b) and FCC rule 2.803(g) bar making, importing, selling or using noncompliant devices.
• The FCC states jammers have "no lawful use in the United States" and are "per se illegal."

This guide does not describe how jammers or spoofers work beyond those definitions.

In the UK, section 68 of the Wireless Telegraphy Act 2006 "prohibits the use of apparatus (such as a jammer) for the purpose of interfering with wireless telegraphy," according to a UK government call for evidence published April 10, 2026.¹⁷³ The same document says the law "does not currently prohibit possession of a radiofrequency jammer specifically," and it sought views on penalties for possession.¹⁷³

What happened at Newark Airport?

What you see: A pickup truck on an airport access road. That was the whole visible story.

In August 2012, a worker was driving a company pickup at Newark Liberty International Airport with a GPS jammer in it. According to the FCC, it interfered with a ground-based augmentation system (GBAS) that the Port Authority of New York and New Jersey was testing there. GBAS is a set of ground stations at an airport that refine GPS for landing approaches. The FAA complained on August 3, 2012, and the FCC investigated on August 4.¹⁷²

The FCC's Notice of Apparent Liability against Gary P. Bojczak (FCC 13-106) was adopted August 1, 2013 and released August 2, 2013. It proposed a forfeiture of $31,875.¹⁷² A device meant to blind one tracker in one truck reached equipment meant to guide airliners.

Why are planes losing GPS over the Baltic?

What you see: A flight tracker showing a plane flying a strange zigzag, or a news story about airliners losing satellite navigation near a border.

Aviation regulators on both sides of the Atlantic say interference has grown sharply since 2022, much of it near conflict zones. (For how flight trackers get their data, see aviation.)

• EASA (Europe): "Since February 2022, there has been a notable increase in GNSS jamming and spoofing," with "the growing severity and sophistication" of incidents.¹⁷⁴ Its Safety Information Bulletin 2022-02, "GNSS Outage and Alterations Leading to Navigation / Surveillance Degradation," was first issued March 17, 2022. Revision 3 followed on July 5, 2024 and Revision 4 on July 3, 2026; Revision 4 adds standard pilot-controller phrasing and builds on a joint EASA-EUROCONTROL task force.¹⁷⁵
• FAA (US): Safety Alert for Operators 24002, "Recognizing and Mitigating GPS/GNSS Disruptions," dated January 25, 2024, cites "the increasing frequency of GPS/GNSS disruptions," often near conflict zones, military operations areas and counter-drone protection areas. Listed effects include loss of satellite-based approach capability, unreliable terrain warnings, wrong map positions and ADS-B position errors; some crews have diverted.¹⁷⁶
• FAA resource guide: According to an April 2026 report by GlobalAir.com, the FAA's GPS/GNSS Interference Resource Guide (first published December 2025, v1.1 in 2026) says the rate of GPS signal loss per 1,000 flights rose 65% in the first half of 2024 over the first half of 2023. Hot spots it names include the Eastern Mediterranean, the Black Sea, Russia and the Baltic, the India-Pakistan border, Iraq and Iran, the Korean Peninsula and the area around Beijing. It also describes a 2022 disruption near Denver International Airport traced to an unauthorized transmitter.¹⁷⁷ Those figures are GlobalAir's summary of the FAA guide.

How does an "atomic" clock set itself?

What you see: A wall clock or watch labeled "atomic" with a little tower icon that fills in overnight.

That clock contains no atom-counting hardware. It has a small radio receiver tuned to WWVB, a station run by the National Institute of Standards and Technology (NIST) near Fort Collins, Colorado, which broadcasts a time code continuously on a 60 kHz carrier.¹⁷⁸ NIST itself says these are better called radio-controlled clocks. They typically sync once overnight and run on ordinary quartz in between. WWVB covers most of the continental US by day, and coverage expands at night,¹⁷⁸ which is why the clocks try at night.

If yours won't sync, give it a night or two: that overnight window is when it listens.

What is that ticking voice on shortwave?

What you see: On a shortwave radio, a steady tick each second and a man's voice announcing "Coordinated Universal Time."

That is WWV, broadcast from the same Fort Collins area, or its sister WWVH on Kauai, Hawaii. NIST lists WWV on 2.5, 5, 10, 15 and 20 MHz and WWVH on 2.5, 5, 10 and 15 MHz.¹⁷⁸ The BIPM, the international time bureau, also lists WWV on 25 MHz.¹⁷⁹ Any shortwave receiver will pick them up (see hobby radio for listening), and you can listen without a radio by phone: 303-499-7111 for WWV and 808-335-4363 for WWVH.¹⁷⁸

Both stations were still broadcasting in 2026. NIST's outage log, last updated April 15, 2026, records one WWV outage, on August 15, 2023, and none after it, and eleven WWVH outages between September 2025 and April 2026, most of them on 2.5 MHz.¹⁸⁰ NIST's fiscal 2019 budget request had proposed ending the broadcasts from Fort Collins and Hawaii, including WWVB; it was a proposal, and Congress had the final say.¹⁸¹

In Boston

There is no widely cited Boston study of GPS error, but the Financial District and the streets around Downtown Crossing fit GPS.gov's description of trouble closely: tall buildings blocking sky and reflective walls bouncing signals.¹⁵⁷ A step into an open plaza can steady a wandering rideshare pin. More local radio is in Boston.

The best-known US jamming case, at Newark,¹⁷² happened roughly 200 miles (320 km) away.

For wall clocks, NIST says WWVB reaches most of the continental US by day,¹⁷⁸ which includes Boston, at the far end of a long path from Colorado.

Elsewhere in the world

GPS is free to use worldwide, and receivers need no per-country license. Every region also has access to GLONASS, Galileo and BeiDou (see the table above).

Radio-controlled clocks are a national affair. The stations use different frequencies, so a clock built for 60 kHz will not hear Germany's 77.5 kHz signal. The BIPM's table of time-signal stations (information received February-March 2026) lists:¹⁷⁹

Canada's CHU broadcasts on shortwave only, with no long-wave time code of its own in the table.¹⁷⁹

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