How GPS Relies on Atomic Clocks and Measurement Standards
Source: NIST. Published: 2025-07-15.
GPS Is a Precision Timing System
Most people think of the Global Positioning System as a navigation tool that tells them where they are. At a deeper level, GPS is a precision timing system that determines position by measuring time. Each of the 24 to 31 operational GPS satellites continuously broadcasts a radio signal that includes the precise time of transmission. A GPS receiver on the ground picks up signals from at least four satellites simultaneously, measures the tiny differences in arrival time between them, and uses those differences — combined with the known positions of the satellites and the speed of light — to calculate its own position in three dimensions. The accuracy of the position fix depends directly on the accuracy of the timing, and the timing accuracy depends on the atomic clocks aboard each satellite.
Atomic Clocks in Space
Each GPS satellite carries multiple atomic clocks — typically a combination of rubidium and caesium clocks, with some newer satellites carrying hydrogen masers. These clocks are monitored and corrected by a network of ground control stations and must be steered to agree with GPS Time, the system's internal timescale, to within about 20 nanoseconds. Even at this extraordinary precision, timing errors matter: a one-microsecond error in satellite clock synchronisation produces a position error of approximately 300 metres. The satellites are also affected by relativistic effects — both special relativity (moving clocks run slow) and general relativity (clocks in weaker gravity run fast) — that together cause the satellite clocks to gain about 38 microseconds per day relative to clocks on the ground. The GPS system corrects for this relativistic offset continuously.
The Role of NIST and National Metrology Institutes
GPS Time is not the same as Coordinated Universal Time (UTC), but the two are kept within 19 nanoseconds of each other. UTC is maintained by the BIPM in Paris as a weighted average of approximately 450 atomic clocks operated by about 80 national metrology institutes, including NIST in the United States. NIST's primary frequency standards, NIST-F1 and NIST-F2, contribute to this average and serve as the ultimate reference for the US contribution to UTC. The GPS ground control segment steers GPS Time toward UTC(USNO) — the realisation of UTC maintained by the US Naval Observatory — which is itself kept within nanoseconds of NIST's UTC(NIST). Without this chain of metrological traceability, GPS would drift away from civil time and eventually become unusable for navigation.
Beyond Navigation: Hidden GPS Applications
GPS provides timing services far beyond navigation. The financial sector uses GPS time stamps to synchronise trading systems across exchanges in different cities, ensuring that trades are sequenced correctly to the microsecond and preventing arbitrage. Electric power grids use GPS timing to synchronise phasor measurement units across thousands of kilometres of transmission lines, enabling real-time grid monitoring. Telecommunications networks use GPS to synchronise the frequency of cellular base stations. The internet's time distribution infrastructure ultimately traces to GPS. Scientific research facilities synchronise experiments across continents using GPS. Even the humble timestamp on a digital photograph is rooted in a chain that runs back to atomic clocks aboard orbiting spacecraft.
What Happens When GPS Fails
The dependence of modern infrastructure on GPS timing creates a significant vulnerability. In 2016, a software update error at a GPS ground control station broadcast an incorrect timing offset to approximately 14 % of GPS receivers worldwide for 13 hours, disrupting some telecommunications networks. Jamming and spoofing attacks — in which malicious actors broadcast false GPS signals — have been documented near conflict zones and have affected shipping navigation. Several national programmes, including those in the US, EU, UK, and South Korea, are developing resilient backup timing systems, including terrestrial eLoran networks and fibre-optic time distribution, to reduce critical infrastructure's dependence on any single satellite navigation system.
Measurement Standards and the Unit Master Timezone Converter
The chain from atomic clocks to GPS to your device's clock ultimately determines the offset between time zones that the Unit Master Timezone Converter applies. UTC, maintained by the BIPM, is the origin from which all time zone offsets are measured. Eastern Standard Time is UTC−5; Japan Standard Time is UTC+9. The converter's offset calculations are based on these international standards, which are themselves maintained by the network of atomic clocks described in this article. Understanding this chain helps explain why UTC-based time is reliable to nanoseconds worldwide — and why it matters that organisations like NIST and the BIPM exist to maintain it.
Use the Timezone Converter