In the last 10 years the INRIM time scale and algorithm group has been deeply involved in the development of the European satellite navigation system, named Galileo . Galileo is a project of the European Union and the European Space Agency. Timekeeping and correct time measurement is the heart of a satellite navigation system as the estimation of a user position is obtained by the geometrical intersection of at least 3 spheres whose centre is a satellite distributing electromagnetic signals and the radius is the distance between that satellite and the user receiver. A schematic of the necessary measurements is illustrated in fig. 1 in the case of a 2-dimensional space.
Fig 1 Simplified schematic of the pseudo-range measurements necessary to estimate a user position (the vessel).
The satellite distributes a message that informs the user of the satellite position and of the time reading of its on board clock, the receivers measure the epoch of arrivals of the satellite signal and, by the difference between the two clock readings, the time of flight is measured and this corresponds to the distance travelled by the signal, and thus the radius of the sphere. This measurement is repeated with at least 3 other satellites and the position is estimated. The more the clocks and the time measures are affected by errors, the worse is the position estimate. Fig. 2 illustrates the importance of good clocks, good measurement systems, and good reference time scales for achieving the best level of accuracy in determining the user position. In addition a time dissemination service may also be added keeping the navigation system reference time closely linked to the international time reference the Universal Time Coordinated.
Fig 2 In the space and ground segments of a navigation system as well as in the connection to the metrological community, clocks, time scales, and time measurements play a fundamental role.
Since 1999 INRIM has been involved in all the different aspects of timing in Galileo and particularly in recent years four main projects are in progress: the contribution to the GIOVE Mission, to the Precise Time Facility, to the Time Service Provider, and to the ESA Advanced Integrity Algorithms.
Since 2006 an experimental phase called GIOVE Mission has been launched supported by ESA. INRIM contributes by hosting a Galileo prototype receiver connected to a reference clock (fig. 3), and analyzing the data from the first experimental Galileo satellites in order to characterize the on board clocks.
Fig 3 The prototype Galileo receiver hosted at INRIM and connected to the Italian reference time scale.
Working on the received satellite signals it has been possible to estimate the noise of the Galileo overall measurement system (Orbit Determination and Time Synchronisation ODTS), which is at the level of the state of the art time transfer systems [1]. Fig. 4 shows an example comparing the capability of the Galileo measurement system to estimate two ground H masers versus a Precise Point Positioning (PPP) estimate of the same clock difference. The impressive agreement is at the level of 100 ps.
Fig. 4 Galileo (ODTS) (blue) and NRCan PPP (pink) comparison for the two H masers located in INRIM and Washington DC at the USNO [1].
Analysis of the onboard clocks has opened the way to understanding clock behaviour in space and to the development of new statistical tools inspired by the space situation and of interest also to metrological timekeeping. For example in fig. 5 the Dynamic Allan Deviation extends the classical concept of Allan deviation estimating the noise of a clock as a function of the observation interval, to the case of non stationary behaviour adding a third axis, the epoch, representing the evolution in time of the Allan deviation and hence of the clock instability. This algorithm is currently under development also in the framework of the ESA project on Advanced Integrity Algorithms.
Fig. 5 The Dynamic Allan Deviation extends the classical 2 dimensional analysis to a time variant system.
To ensure the interoperability of the Galileo system with the existing US Global Positioning System (GPS), it is necessary to measure the GPS to Galileo Time Offset (GGTO). First results (fig. 6) with different measurement techniques allows an estimation at the level of 5 ns accuracy as required by a user receiver combining the GPS and Galileo signals together [3].
Fig. 6 GPS to Galileo Time Offset estimates [3].
In addition INRIM is involved in the development of the final Galileo timing system (fig. 7) by designing the time scale algorithm of one of the Galileo ground time laboratories named Precise Timing Facility [4].
Fig. 7 The Galileo timing system based on 2 internal Precise Timing Facilities generating the Galileo System Time and an external Time Service Provider involving the time metrology community (ESA archive).
INRIM also contributes to a consortium of different industries and UTC(k) laboratories currently developing the prototype of the Galileo Time Service Provider, supported by the European Union, with the aim of providing the Galileo system with the necessary corrections to maintain the Galileo system time in agreement with the international time reference UTC [5].
[1] P. Waller, F. Gonzalez, J. Hahn, S. Binda, I. Hidalgo, R. Piriz, A. Mozo, G. Tobias, I. Sesia, P. Tavella, G. Cerretto: "In-Orbit Performance Assesment of GIOVE Clocks ", Proc. of 40th PTTI Meeting, Reston, VA, USA, 2008.
[2] L. Galleani, P. Tavella: "Dynamic Allan variance", UFFC IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 56, no. 3, pp. 450-464, March 2009.
[3] R. Píriz, A. Mozo, G. Tobías, V. Fernandez, P. Tavella, I. Sesia, G. Cerretto, J. Hahn: "GNSS Interoperability: Offset between reference Time Scales and Timing Biases", Metrologia, Vol. 45, no. 6, pp. 87-102, 2008.
[4] R. Zanello, M. Mascarello, L. Galleani, P. Tavella, E. Detoma, A. Bellotti: "The Galileo Precise Timing Facility", Proc. of EFTF and IEEE-FCS - TimeNav07, Geneva, Switzerland, 2007.
[4] J. Achkar et al.: "Fidelity - Progress report on delivering the prototype Galileo Time Service Provider", Proc. 21th EFTF, Geneva, Switzerland, 2007.