- The GPS system is based on atomic clock technology.
- Each GPS satellite has multiple atomic clocks, synchronized to a ground-based master clock.
- The GPS clock provides everyone on Earth with access to atomic time standards without needing a local atomic clock.
- GPS time transfer accuracy is within 40 nanoseconds of UTC.
Electronic Clocks – Low Cost But Poor Timekeepers
Electronic clocks are mass-produced items that are built to a cost. The components used are often low-cost items. 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.
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, two computers set to the correct time at the start of a month could differ significantly at the end of the month. Many PC clocks are far worse. For time critical applications and processes, this variation is obviously unacceptable.
Atomic Clocks – Expensive, But Provide Astonishing Accuracy
Luckily there are clocks that provide astonishing levels of accuracy – atomic clocks. There are a number of 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!
The GPS system is a constellation of 24 orbiting satellites. The system is primarily intended to provide a precise positioning and navigation service. However, very accurate time information is continuously transmitted by the satellites.
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. However, it does not account for periodic leap second insertions to UTC time. Therefore, GPS time is ahead of Coordinated Universal Time (UTC) by 18 seconds (as of May 2018).
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.
Synchronization To 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 GPS
A number of alternative Global Navigation Satellite Systems (GNSS) are available to rival GPS. GLONASS is the Russian GPS system, while Galileo is the European Unions rival system. Both systems have world-wide availability. BeiDou is the Chinese GNSS system which provides services to the Asia-Pacific region.
Some time servers and master clocks can receive multiple GNSS systems to provide enhanced reliability and redundancy.
Network Time Servers For GPS Clock Synchronization
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. Ideal anywhere a legally traceable source of time is required.