Scintillation effects on Galileo service performance

Presentation Overview Galileo context Scintillation main Characteristics GISM Model Uncertainties Integrity Algorithm Response to Scintillation Scintillation Impact on Galileo performance (SISMA & User Availability) Conclusions

Galileo context (1/3) Galileo Performance are required to be met at Sun Spot Number equal to 250. Corresponds to the maximum monthly SSN identified over the last 60 years. Such specification can appear very stringent, However: It makes sense when looking at it from a user service point of view Indeed it is difficult to imagine that the safety of life service is no longer available every peak solar cycles.

Galileo context (2/3) Galileo also specified to provide a worldwide safety of life service. This requires the deployment of a worldwide 40 GSS stations network. Out of 40, 14 stations are within the scintillation area (+/- 20 degrees around the Magnetic Equator. 4 to 5 stations may be affected at the same time for several hours Affected means that one line of sight has a S4 larger than 0.7. Galileo Sensor Station Network

Galileo context (3/3) The main differentiator of Galileo system with respect to GPS is the capability to provide real time integrity information to the user The service to be provided by Galileo is tailored for civil aviation applications. It needs then to be available quasi 100% of the time. The monitoring needs to be continuous in the sense that when the service is available at T0, it shall remain available until T0 plus 15 seconds in order to enable the completion of critical phase of flight. High level impact on Galileo system performance: Orbit Determination, Time Synchronisation function Not critical thanks to the redundancy of stations sized for integrity performance purpose. Integrity determination function Continuity of service: Scintillation is one of the main drivers Loss of lock and Cycle slips at receiver level makes GSS data not proper for integrity monitoring IPF (Integrity Processing Facility) discards the data as soon as the phase noise gets significant Availability of service: Same situation as continuity.

Scintillation main characteristics (1/2) Scintillation phenomena is statistically characterised by two parameters: S4, linked to the standard deviation of the normalised intensity variation Characterised the C/N variations in presence of scintillation The amplitude variation follows a Nakagami distribution with m=S4-2 sf, which is the standard deviation of the phase scintillation variation. The scintillation phase error is considered to follow a zero-mean normal Probability Density Function

Scintillation main characteristics (2/2) A key parameter to take into account at Galileo performance assessment is that scintillation has a very specific time and geographical distribution It mainly affects stations that are localised on the geo magnetic equator (+/- 20 degrees) and to certain extent stations in polar areas. Stations in the equatorial regions are not affected all the time. The scintillation phenomenon lasts only a few hours after sunset (following TEC gradient) When a station is affected, all the lines of sight are not impacted the same way. Half of the satellite in sight may be affected.

GISM Model Uncertainties (1/4) S4 Amplitude The following figure shows the S4 parameters obtained with the GISM configured with a solar flux of 300 according to universal daytime at Fucino (Italy). Such area is a priory not subject to strong scintillation since located in mid latitude area. However the GISM provides unexpected results since it can be noticed that the S4 parameters reaches 1.65. Literature usually mentions that the S4 shall not exceed 1.4 or even 1 as stated in the SIPIC Attempt to produce time series with S4 larger than 1 with the GISM have been unsuccessful

Model Uncertainties (2/4) Frequency Correlation An critical point to clarify in order to assess GNSS robustness to scintillation is the correlation of the phase noise between frequency. If the error is correlated with 1/f² frequency factor it can be fully removed with ionosphere free combination. Otherwise the error is even amplified by the “gamma” factor. The current model does not allow assessing the correlation between two frequencies. Indeed only one frequency can be simulated at the same time, and two consecutive runs with exactly the same configuration show different results. The following picture shows three example of time series obtained with three successive runs of the GISM with the exact same configuration file

Model Uncertainties (3/4) Phase Time series (Wrapped or Unwrapped) Current models for time series generation seem to artificialy maintain the phase error in between +/- 180°. When the conditions is suppressed, the phase error is drastically increased with no longer gaussian behviour. Error can go up to 60 cm !

Model Uncertainties (4/4) Fading dynamic (receiver model) Fading dynamic drives the response of the receiver in terms of loss of lock. Current formula assumes a fading constant over the loop bandwidth. This may be valid for wide loop bandwidth (10 to 100 Hz) used for on user receivers but not for the ground receivers (1 Hz). Improvement of the receiver model in order to better characterise receiver response and hence, GNSS system performance is really needed.

Integrity Algorithm response to scintillation Impact of Scintillation on Galileo integrity capability cannot be derived directly from ground receivers response. Response of the integrity algorithm processing those data is also a performance driver. Algorithms response is correlated with the quality of the received data Loss of Lock When the receiver looses lock, the measurement is no longer available to the integrity processing facility. The phenomena is amplified by the integrity algorithm convergence constraints as follows 10 mn are necessary for a single line of sight to be reintroduced in the integrity computations. If more than half of the line of sight belonging to a same station are discarded, the whole station is considered as faulty, and all the lines of sight are rejected by the Integrity Algorithm 40 mn are necessary to reintroduced a rejected station in the integrity computations.

Integrity Algorithm response to scintillation Phase Noise / Cycle sips Even if tracking is maintained by the receiver, the data can be rejected at Integrity algorithm level if considered too corrupted for integrity monitoring assessment. Integrity algorithm makes an extensive use of phase measurements. Barrier are implemented to reject data with excessive phase noise and cycle slips. Threshold set to 1.32 radians, correlates with a rejection of the data at S4 around 0.7

Integrity Algorithm response to scintillation Test with Real GPS data (collected from the IGS network) Phase noise measured at Kourou (affected by scintillation with S4 equal to 0.6) Excessive phase noise clearly spotted during scintillation periods Rejection rate us constant to 10% (because of data gaps observed on this real data set) but significantly increased during scintillation period because of phase noise barrier

Scintillation Impact on Integrity Monitoring performance Galileo Integrity monitoring quality is communicated to users through the SISMA (Signal In Space Monitoring Accuracy). In nominal conditions (left), SISMA in between 60 to 95 cm (99.99%) In severe scintillation conditions (right, SF300), SISMA in between 1 to 2.5 m (99.99%)

Scintillation Impact on User Availability Impact at user level depends from the number of stations and user alarm limit value (mainly vertical) With 40 stations and 35 meters vertical alarm limit (LPV200), availability above 99.5% is obtained. Sensitivity analysis to analyse the possibility to reduce the number of stations to 28 In severe scintillation conditions, availability drops down to 90%

Conclusions Galileo system is designed in order to provide high continuity and availability performance in severe scintillation conditions. Key parameters for performance assessment are the receiver response to scintillation, in particular, loss of lock and cycle slips. Integrity Algorithm transfer function At first order, Line of sights affected by scintillations are rejected by the integrity algorithms. Receiver impact is second order since data are rejected because of excessive phase noise before it becomes significant at receiver level (S4<0.7). Scintillation impacts significantly GMS performance but can be recovered by increasing the number of stations. Assessment performed with available models that may have some limitations Severe conditions, polar region. Needs to be confirmed in real conditions