New Approach for The Real Time Detection of Scintillations and Potential Applications
Hernández, C.; Catalán, C.; Rodriguez, I.; 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 and its propagation effects. 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 analysis of the quality of the broadcast signals and the propagation effects has to be done in real-time.

In the frame of the Galileo satellite navigation system design, a new algorithmic approach for analyzing in real-time the carrier phase behaviour in terms of measurement noise and stability 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 quality of the carrier phase measurements is a fundamental aspect for the integrity monitoring scheme, and therefore it has to be assessed continuously in order to prevent the integrity algorithms to work out of specifications. The first step of the new algorithmic approach is based on the cycle slip detection and repair algorithm. As part of this process, the incoming carrier phase measurement is compared with a prediction, which is built based on the carrier phase measurements received in the last few seconds plus a certain propagation model derived from the expected dynamics. According to the theoretical noise covariance model propagation that has been built, the difference between the predicted carrier phase measurement and the incoming one has a direct and known relationship with the true carrier phase noise. The second step is the derivation of an observable based on doing a covariance analysis of the mentioned parameter, which is strongly related to the "sigma_phi" parameter used for scintillation characterization. Finally, an internal alarm is raised whenever the observable exceeds a threshold, established according to the continuity and integrity requirements, and the carrier phase measurements are no longer used until the situation is recovered.

Preliminary results, based on processing real GPS data from a set of IGS sensor stations worldwide distributed, seem to be very promising, although further tests are required according to the experimentation test plan in order to achieve a full characterization.

The objective of this paper is therefore twofold: to provide a description of the algorithms defined for the real-time detection of scintillations including the theoretical modeling and covariance analysis; and to present the planned test campaign with real GPS and Galileo simulated data, together with the most relevant results.

This new outstanding algorithm for real-time detection of scintillations could be used in other fields of applications, mainly for those users demanding a high-degree of reliability and for which the carrier phase measurements play an important role, etc. It is important to note that one of the main features of the proposed algorithm is that the level of complexity is compatible with the hardware and software limitations that can be found in most of the applications, which makes it even more interesting.