GPS is a satellite based Global Positioning System used to provide navigation and precise positioning information to users on the Earth. The system was developed and is operated by the US military (DoD), but is also made available for civilian use, with some restrictions. Some high-end GPS receivers have export restrictions applied by the US Government. Additionally, civilian use is restricted to a less accurate Course/Acquisition (C/A) signal and cannot be corrected for Selective Availability (S/A), which provides a greater degree of precision.
The GPS system consists of 24 orbiting satellites, each with a very accurate atomic clock on-board. A receiver, located anywhere on or near Earth, with a clear view of at least 4 satellites can calculate precise positioning information. Each satellite constantly transmits a weak radio signal that provides very precise timing and location messages. Each message contains information about the exact time and also the position of the satellite when the message was transmitted.
GPS receivers on Earth pick up the radio signals and decode the messages to work out the time taken for each message to reach the receiver. It can then calculate the distance that the satellite is from the receiver. If the receiver can pick up messages from at least four or more satellites, it can use the information to triangulate a very precise location. Many GPS receivers can also provide directional information and speed of travel. However, the accurate timing messages can also be used to provide a reference clock for computers and computer networks. GPS is currently the preferred reference for many NTP network time server appliances and reference clock devices.
The Difference Between GPS Time and UTC Time
GPS time is expressed as two numbers, a week number and the number representing the number of seconds that has elapsed since the start of the week. Week zero corresponds to midnight on the 6th January 1980. Actual time is calculated by working out the number of weeks and seconds into the regular Gregorian calendar.
In theory, GPS time is accurate to around 14 billionths of a second (nanoseconds). However, in practice some latency is introduced by the receivers processing the satellite signals, which means most receivers can provide timing information accurate to around 100 nanoseconds.
GPS time is not corrected to match solar time scales, such as UTC, which takes into account slight variations in the rotation of the Earth. Leap seconds inserted into UTC time are not accounted for in GPS time. Therefore, the GPS system provides a GPS-UTC offset, which is the difference between GPS time and current UTC time. Leap seconds are generally inserted, though can be removed, at the end of June or December. As of November 2013, the difference between GPS time and UTC time is 16 seconds.
Alternative Global Navigation Satellite Systems
There are a number of alternatives to the GPS system, introduced by various nations. GLONASS is a Russian global navigation systems, which is fully operation and available world-wide. Beidou is a satellite navigation system deployed by China, but is a currently limited in scope to the Asian region. The next big advance will be provided by the introduction of Europe’s Galileo system. This is a civilian navigation system, currently under development by the European Union (EU). The full complement of 30 satellites should be in place by 2020. The system hopes to set new standards in Global Navigation Satellite Systems (GNSS) with improved precision and coverage right up to the polar regions.
Network Timing Equipment Using GPS
GPS Network Time Servers, such as TimeTools SR and SC series devices utilise GPS timing information to provide an accurate reference for time critical applications. The appliances utilise an external GPS antenna to receive satellite signals, which are decoded by an integral receiver. By utilising the NTP protocol, they can synchronize servers and workstations to within a few milliseconds of the correct time.
Andrew Everett has worked in the Computer Time and Frequency sector for almost his entire career. He now leads TimeTools development department. Andrew has written many articles that help IT professionals make informed decisions about network and computer systems timing solutions.