Advanced Real-Time Synchronisation for Networks of Sensor Stations with High Accuracy and Integrity
Hernández, C.; Catalán, C.; Curiel, A. M.; Sardón, E.
GMV Aerospace and Defence S.A.

A very important characteristic of any regional or global satellite navigation system providing integrity is the monitoring of the signals broadcast by the satellites by means of a network of stations. These are normally composed of a set of receivers driven by an external clock so, in order to be able to merge the information coming from different stations, they need to be synchronized to a common reference time.

Integrity implies the provision of timely alerts to the users, preventing them from the use of signals from satellites not working within the specifications. This is the so-called concept of Time-To-Alert (TTA). This is in the order of few seconds for most applications; consequently the network of ranging stations used to monitor the quality of the broadcast signals has to be synchronized in real-time.

In the frame of the Galileo satellite navigation system design, a new algorithmic approach for synchronising the GSS (Galileo Sensor Stations) has been established as part of the development of the IPF (Integrity Processing Facility), which is the element of the GMS (Ground Mission Segment) in charge of the computation of Galileo integrity message. This facility is designed and developed by GMV Aerospace and Defence S.A.

The synchronization strategy of the GSS network is based on a two steps approach. A real time algorithm is executed every second, based on a common view technique including both iono-free smoothed pseudorange and carrier phase measurements. This algorithm is steered by a second one that is executed every minute, taking a batch of pseudorange and carrier phase measurements with the objective of solving for certain parameters such as the carrier phase ambiguities and the zenith tropospheric delays. They constitute the state vector that is passed to the real time algorithm.

The ranging measurements used for synchronizing the sensor stations are those that will be used for establishing the status of the broadcast signals and navigation message. Therefore specific mechanisms have to be defined to prevent an excessive correlation between the synchronization errors and the satellite signal errors, which could lead to the missed detection of satellite feared events.

Preliminary results, based on processing real GPS data from a set of IGS sensor stations worldwide distributed, seem to be very promising: an error below 0.4 nanoseconds (67%) has been obtained for this real-time synchronization scheme, together with a zenith tropospheric delay error below 2 cm (67%).

The objective of this paper is therefore twofold: to provide a description of the algorithms defined for synchronizing in real-time the Galileo sensor stations within the integrity message generation; and to present the planned test campaign with real GPS and Galileo simulated data, together with the most relevant results.

This new outstanding synchronization algorithm providing high accurate and reliable clock biases in real time with respect to a reference timescale could be used in other fields of applications, such as in the frame of the timing community, geodesy, etc.