What you need to know about GNSS leap seconds

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1 What you need to know about GNSS leap seconds

2 Testing to ensure GPS receivers keep track of leap seconds 1 The importance of time For many people, time is the ultimate paradigm. Financial markets, military campaigns, transport networks and industrial processes are just a few of the activities that depend on timekeeping that must be both 100% accurate and simultaneously available to all participants. Global navigation satellite systems are no exception. Their proper function depends on all ground control stations, satellites and receivers keeping exactly the same time. Page 2

2 UTC The global standard for time is UTC, or Co-ordinated Universal Time, maintained by the International Bureau of Weights and Measures (BIPM) in France, which collects data from more than 200

3 2 UTC The global standard for time is UTC, or Co-ordinated Universal Time, maintained by the International Bureau of Weights and Measures (BIPM) in France, which collects data from more than 200 atomic clocks and a few primary absolute frequency standards from more than 50 institutions around the world. Each month the BIPM publishes standard international references for frequency and time, International Atomic Time (TAI) and UTC, which is equal in rate to TAI, but adjusted every so often by an integer number of seconds to account for variations in the rotation of the Earth. These adjustments are known as leap seconds.

3 GPS time GPS time is a composite timescale defined on the basis of measurements from a number of atomic frequency standards in use at monitoring stations and onboard the satellites of the GPS

4 3 GPS time GPS time is a composite timescale defined on the basis of measurements from a number of atomic frequency standards in use at monitoring stations and onboard the satellites of the GPS constellation. GPS time is continuous, and so has no leapsecond insertions, but is steered to be within 1 microsecond of UTC. As a result, GPS time is always a number of whole seconds plus a fraction of a microsecond different to UTC. This offset is declared in the GPS navigation message, allowing all receivers to provide time to their users according to UTC. Page 4

5 4 Leap second insertion Leap second events can happen at any time during the year, although to date they have generally occurred at midnight on 31st December or 30th June. The event is communicated to all receivers in advance within the normal navigation message by changing four parameters: The week number in which the change will occur The day number on which the change will occur The current leap seconds offset The new leap seconds offset How is a GPS signal made up? Consider just the L1 signal from the satellite... The satellite transmits a PRN (Pseudo Random Noise) ranging code The PRN is unique to each satellite and is the radio tape-measure

6 5 How receivers should respond When a leap second event occurs there are three phases through which every receiver must pass. First, up to six hours before the event, receivers should continue to use the existing UTC-to-GPS offset. Then, during the six hours before and six hours after the event, receivers will begin a transition from the existing UTC-to-GPS offset and the revised one. Finally, six hours after the event, all receivers will use the new UTC-to-GPS offset. Page 6

6 The importance of testing Clearly, as time is such an important factor in satellite navigation, it is essential that any GNSS receiver will cope with a leap second insertion event in the correct

7 6 The importance of testing Clearly, as time is such an important factor in satellite navigation, it is essential that any GNSS receiver will cope with a leap second insertion event in the correct manner. And the only way a manufacturer of GNSS receivers can be confident that a receiver design will behave correctly is to test the unit and observe the results. However, as leap second insertions are such rare events, there is no practical way that this testing can be performed in the field using a live satellite navigation message. Page 7

7 Simulating leap second insertions The solution to the test problem is to use an RF simulator in the controlled environment of the test laboratory.

8 7 Simulating leap second insertions The solution to the test problem is to use an RF simulator in the controlled environment of the test laboratory. The ease of setting up a leap second insertion test will depend on the software used by the simulator, and there may be several alternative methods, either by creating data files or by inputting the required data via a graphical user interface. Each method will allow the user to modify the contents of the simulated navigation message, and create the leap second event by changing the week number, the day number, the current UTC-to-GPS offset and the future UTC-to- GPS offset. Page 8

8 Analysing the results When the test is run, a correctly functioning receiver that is navigating from the simulator s output signal should detect and apply the leap second insertion.

9 8 Analysing the results When the test is run, a correctly functioning receiver that is navigating from the simulator s output signal should detect and apply the leap second insertion. The results can be observed either by directly monitoring the receiver s UTC time data output or by logging the receiver s UTC time data to a file for subsequent analysis. For a single leap second insertion the time count will appear to stop for 1 second before continuing. For example, , where 7 was the time at which the leap second event occurred. Page 9

10 9 The complete solution An RF constellation simulator reproduces the environment of a GNSS receiver on a dynamic platform by modelling satellite motion, signal characteristics, atmospheric and other effects, so that the receiver will actually navigate, in the lab, according to the parameters of the test scenario. As with other GNSS receiver quality assurance tasks, a suitably specified RF simulator will allow leap second insertion tests to be performed with total accuracy and full repeatability. Therefore, should a receiver fail any test, the design can be modified and the revised unit can be subjected to exactly the same test conditions. Page 10

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Time Scales and Time Keeping in the 21 st Century

Time Scales and Time Keeping in the 21 st Century Peter Whibberley NPL Time & Frequency User Club 3 December 2008 Overview How we got where we are from GMT to UTC How UTC is produced Dissemination of UTC