Implementation of Galileo time service provider prototype
Background & Objectives
With one nanosecond of time difference on a satellite, the equivalent to about 30 cm on the ground, highly accurate time is essential to the success of any Global Navigation Satellite System (GNSS), including Galileo. Even the best atomic clocks lose time, including those at the core of Galileo. Monitoring and correcting these time drifts enables the fundamental accuracy of Galileo to be maintained. Precision time plays a fundamental but often neglected role not just in navigation, but also in electricity distribution, the functioning of e-mail and the Internet, deep space exploration and telecommunication networks. To address the accurate time needs of Galileo the Fidelity consortium are creating an operational Galileo Time Service Prototype Facility (GTSPF) which will be used by the Fidelity consortium to deliver time services to Galileo for its In-Orbit Validation (IOV), due in 2008. Fidelity was specifically established to undertake this project. The project began in 2005 and has now successfully passed its critical design review and key-point milestones.
The Fidelity consortium is developing the Galileo Time Service Prototype Facility (GTSPF) for the GNSS Supervisory Authority (GSA). The GTSPF will be used by the Fidelity consortium to deliver time services to Galileo to ensure that Galileo System Time (GST) stays aligned to International Atomic Time (TAI).
The Fidelity consortium is coordinated by Helios, a technical and business consultancy specialising in navigation and transport, on behalf of the GSA. The pan-European consortium of nine partners consists of experts in timing, industrial mathematics, software and system engineering. Fidelity is responsible for the design, development and operation of the GTSPF for Galileo IOV before delivering it, complete with software and documentation to the GSA. The GTSPF will comprise commercial off-the-shelf (COTS) hardware and software as well as a specifically developed software application. This application is being developed using the formal structured methodology standardised amongst the European UTC(k) laboratories.
The application at the heart of the GTSPF has, at its core, software containing innovative, complex and unique algorithms. The most sophisticated technical development is focused around two elements which contain the complex mathematics used to provide the time service. The first element is a composite clock to establish a highly accurate and stable virtual timescale using the input of an unlimited number of atomic clocks. The second element contains prediction and steering algorithms which generate the steering corrections needed to maintain a highly aligned and stable Galileo system time. Both these elements incorporate state-of-the-art Kalman filter-based algorithms, whose development has pushed beyond that which was previously possible.
The Fidelity consortium is developing the GTSPF as a means to keep GST aligned to TAI within tight specifications. This is achieved by conducting time transfer measurements between Galileo’s precision time facilities (PTFs) master clocks and the timescales within the core UTC(k) laboratories in the Fidelity consortium. These core UTC(k) laboratories are the National Metrological Institutes of France, Germany, Italy and the United Kingdom. The time transfer measurements are then forwarded to the GTSPF where they are used to predict the retroactively published TAI timescale to the current time.
The GTSPF then measures the deviations between the PTF master clock timescales and the predicted TAI. From these measurements, steering corrections are generated by the GTSPF. These corrections are then provided daily to Galileo, along with the offset between UTC and TAI, for use in steering the PTF master clocks. In addition, the GTSPF will examine the supplied time transfer data for the presence of anomalies (e.g. clock frequency jumps) and issue warnings to the relevant UTC(k) or Galileo if any are detected. The GTSPF will also compile statistical measures of the performance of the various timescales and make these available to designated users. This processing will be carried automatically by the GTSPF on a daily basis without operator intervention.
Work performed & results
The Fidelity consortium will develop the GTSPF and then use it to deliver time services to Galileo for IOV in 2008. The GTSPF will then be delivered to the GSA, including all hardware, software and documentation. As part of the operations of the GTSPF carried out for IOV, the Fidelity consortium will demonstrate the quality of the timing services provided using the GTSPF before the facility is finally delivered to the GSA. In parallel to the technical development of the GTSPF, the Fidelity consortium will also plan and document the operational and institutional migration path from operation of the GTSPF by Fidelity during IOV to the GTSF being operated by the permanent TSP during FOC. This will enable a seamless transition of services as part of the future implementation of the Galileo FOC. In addition, this planning activity is addressing potential uses of the timing services by a wider community including not only other GNSS services but also non-navigation users such as public utilities, financial institutions, etc. In summary, the Fidelity consortium is delivering a key piece of infrastructure for Galileo IOV that will enable the full performance potential of the Galileo system to be realised. It will also develop the planning needed to ensure a smooth transition of timing services to support Galileo FOC together with the