Nº 7 2013 > The future of time
To abolish or not to abolish the leap second?
Galileo and Coordinated Universal Time leap seconds
Jörg Hahn, ESA/ESTEC, Galileo System Engineering Manager
Galileo is Europe’s own global navigation satellite system designed to provide a highly accurate global positioning service under civilian control. It will be interoperable with the current Global Positioning System (GPS) of the United States and GLONASS of the Russian Federation.
The first two of four operational satellites designed to validate the Galileo concept in both space and on Earth were launched on 21 October 2011. The two others followed on 12 October 2012. This in-orbit validation phase will be followed by additional satellite launches to reach initial operational capability by the middle of this decade.
In the initial stage of Galileo’s operation, preliminary versions of the open service, search and rescue service, and public regulated service will be available. Then as the constellation is built up beyond that, new services will be tested and made available until the system reaches full operational capability.
Two Galileo control centres in Europe — one in Fucino (Italy) and the other in Oberpfaffenhofen (Germany) — control the satellites and manage navigation. The data provided by a global network of Galileo sensor stations will be sent to the Galileo control centres through a redundant communication network. The control centres will use the data from these stations to compute the integrity information and to synchronize the time signal of all satellites with the ground station clocks. The exchange of data between control centres and satellites will be performed through up-link stations.
Galileo System Time
Satellite navigation requires highly accurate measurement of signal travel times. Galileo’s own internal reference time system is known as Galileo System Time (GST). It is used for synchronizing all Galileo clocks, including those in the ground segment, on satellites, and in receivers. The broadcast navigation messages are also time-tagged with GST.
GST is a continuous time-scale steered to Coordinated Universal Time (UTC), modulo 1 second. GST is not subject to leap seconds. The Galileo Time Service Provider links GST to UTC, relying on data from the European timing laboratories.
How Galileo System Time is generated
GST is generated by the Galileo ground mission segment from a set of high-precision atomic clocks in Fucino and Oberpfaffenhofen in the two Precise Time Facilities operating in hot redundancy. Each Precise Time Facility is equipped with two active hydrogen masers and four high-performance caesium clocks.
The Galileo Time Service Provider collaborates with the European timing laboratories to transform GST into UTC for dissemination purposes, and keeps Galileo informed of the difference between the International Atomic Time (TAI) and UTC, as well as of any leap-second announcements.
During the in-orbit validation phase, the Galileo Timing Validation Facility took on the role of time service provider. This facility is located at the National Institute of Metrological Research (Istituto Nazionale di Ricerca Metrologica), Turin, Italy, and is supported by the UTC laboratories of Germany, France, the United Kingdom and Spain. GST is continuously compared with the national realizations of UTC via the two-way satellite time and frequency transfer (TWSTFT), and the GPS Common View service.
The key performance requirements are that the difference between GST and UTC shall be estimated with an accuracy better than 28 nanoseconds (95 per cent of the time), and shall be less than 50 nanoseconds (modulo 1 second) for 95 per cent of the year. Some early performance results show that, between February and March 2013, the difference between GST and UTC was kept within a few nanoseconds.
Dissemination of UTC through Galileo
Galileo provides both positioning and timing capability. It disseminates UTC in accordance with Recommendation ITU–R TF.460-6. Thus the Galileo navigation message includes GST-to-UTC conversion parameters, including the total number of leap seconds, an announcement of the introduction of any new leap seconds with the associated date, and the fractional GST-UTC offset. The GST-to-UTC transformation parameters are computed and updated daily by Galileo facilities.
Galileo users will be able to estimate GST from the signal-in-space and, by applying the transformation parameters, obtain UTC for time-tagging their applications. For the vast majority of navigation and timing users, the user position needs to be expressed in Earth-fixed coordinates and the corresponding time tag will be expressed in UTC. Some specialized applications, such as astronomy, may require access to Universal Time.
Impact of leap seconds on Galileo
Galileo facilities are synchronized to GST, which is also used for time tagging most of the Galileo data. However, Galileo still uses UTC in time tagging where required by international formats, for example the GPS Common View data, or the ITU format for TWSTFT data.
Most of the data provided to Galileo by external service facilities — such as the Time Service Provider, the Geodetic Reference Service Provider, and the Return Link Service Provider — are time tagged on UTC. These external service facilities also use UTC for synchronizing their computer networks. Galileo operators use UTC as the reference time for operational planning and event recording.
Galileo is designed to work with leap seconds, and their application is automated. Nevertheless, a new leap second still involves several human actions, for example: updating the default configuration of all elements of the system using the UTC conversion protocol; verifying that the leap second is applied in all elements of the system, including synchronization to UTC of individual pieces of equipment; and verifying that the leap second is applied in the operator’s environment.
These actions may involve human errors, jeopardizing the reliability of the system. Abolishing the leap second would simplify system operations and make them more robust. Leap seconds are undesirable in terms of system operations. If leap seconds were discontinued, however, Galileo would have to update the corresponding interfaces.
Whatever the outcome regarding the leap second, however, Galileo would continue to follow international standards and recommendations.