What is the GPS Clock? | GPS Timing

The GPS Clock is a satellite system that provides a very precise timing service through the use of atomic clocks.

This provides everyone on Earth with low-cost access to international atomic time standards, without needing an expensive local atomic clock.

The GPS system is based on atomic clock technology.

Each GPS satellite is equipped with multiple atomic clocks, synchronized to a ground-based master clock.

The GPS Clock provides precise timing based on atomic clock technology.

The Need For Accurate Electronic Timekeeping

Basic electronic clocks are mass-produced items often built using low-cost components. Computer real time clock chip and oscillator components are no different. Consequently, they tend to suffer from poor stability and therefore do not maintain accurate time, losing typically 5 minutes of accuracy per month.

The rate of which a computers time will drift varies depending on the quality of the computer clock crystal. A reasonably good computer clock crystal has a stability of 100 parts per million (ppm). This equates to a drift of 1 second in every 10,000 seconds, or roughly 5 minutes per month. So, if two computers are set to the same correct time at the start of a month, they could have a significant difference by the end. Many PC clocks are far worse. A variation of this amount is therefore unacceptable for time critical applications and processes.

The Astonishing Accuracy Of Atomic Clocks

A solution to this problem is the use of atomic clocks which provide astonishing levels of accuracy. There are multiple types of atomic clocks.

Caesium clocks use a magnetic field to separate a beam of Caesium atoms of different energy levels. Hydrogen atomic clocks utilize high-energy hydrogen atoms. Rubidium clocks use rubidium gas, contained in a glass cell, which changes light absorption at a specific frequency.

The best atomic clocks can maintain an evaluated frequency uncertainty of approaching 2.3 × 10−16. This equates to losing less than one second in 138 million years!

GPS Timing

The GPS system is a constellation of 31 orbiting satellites. The primary purpose of the system is to provide a precise positioning and navigation service, however the satellites also transmit very accurate timing information continuously.
Each GPS satellite has an integral atomic clock which is synchronized periodically to a ground based master clock maintained by the U.S. Naval Observatory (USNO). USNO maintains synchronization of the entire GPS system to international standards.

The GPS system maintains a time transfer accuracy of less than 40 nanoseconds relative to UTC, 95% of the time. Everyone on Earth now has access to atomic time standards without needing a local atomic clock.

The Difference Between GPS and UTC Time

GPS time is the time scale used by the GPS satellites and the ground-stations that control and synchronize them.

GPS timing does not adjust for the periodic leap second insertions to UTC time. UTC adjusts with leaps seconds in order to stay synchronised with the Earth’s rotation. Therefore, GPS time is ahead of Coordinated Universal Time (UTC) by 18 seconds (as of July 2025).

The satellite system does however provide an offset between GPS and UTC, so that ground based receivers can easily compute UTC time. Local time in a particular location, can be calculated by simply adding a time zone and daylight saving offset to UTC time.

Synchronizing To Traceable National Time Standards

Network time servers and master clocks can receive and process accurate time from GPS. The timing information is then used to synchronize an internal clock and provide a time resource to network time clients and slave clocks.

In this manner clients can synchronize to within a few milliseconds of national time standards (UTC). This provides systems with a legally traceable source of time.

Alternatives to the GPS Clock

GPS is a US military developed and maintained Global Navigation Satellite System (GNSS). However, there are a number of alternative GNSS systems available, operated by various countries to rival GPS. Reception of GNSS satellite systems is free to air and subscription free.

Galileo

Galileo is a rival civilian system developed and maintained by the European Union. Galileo satellite signals can be received world-wide. The system provides an excellent alternative to the GPS system. It has the added advantage of being a civilian system, rather than being military.

GLONASS and BeiDou

GLONASS is the acronym for GLObalnaya NAvigatsionnaya Sputnikovaya Sistema in Russian.

The BeiDou Navigation Satellite System is a satellite-based radio navigation system owned and operated by the China National Space Administration.

These systems are less widely used as timing references for Western based organisations due to recent military conflicts and trade disagreements.

QZSS

The Quasi-Zenith Satellite System (QZSS), is a four-satellite regional navigation satellite system developed by the Japanese government. The system is only available in the Asia-Oceania regions, with a focus on Japan. It is less useful in areas outside the Asia-Oceania region.

GPS Clock Synchronization Of Network Time Servers

TimeTools network time servers integrate GPS and GNSS timing receivers to use the GPS clock for network time synchronization. Applications include: network timing, monitoring and control systems and time critical processes. They can be used anywhere that a legally traceable source of time is required.

NTP servers such as TimeTools TA610 model, can concurrently receive multiple GNSS satellite systems to provide enhanced reliability and redundancy.

Learn more about GPS NTP Time Servers here.
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