Another step forward for EGNOS
The EUROCONTROL EGNOS Data Collection Network (EDCN), managed by the Spanish company PILDO LABS, has recently expanded its capability and is now able to monitor signals from the GALILEO satellites.
The EDCN was initially set up to perform independent performance monitoring of the EGNOS service. This continuous monitoring of EGNOS performance has made an important contribution to the certification of the EGNOS service provider. Performance evaluations (accuracy, integrity, availability and continuity) are made, both locally and centrally, using PEGASUS, a dedicated tool developed by EUROCONTROL.
EDCN can also track the newly extended GPS signals. Indeed, the USA has begun to extend the capabilities of their Global Positioning System (GPS), launching new satellites that broadcast a second civil signal on the so-called “L5” frequency. This new frequency will provide a more accurate and robust navigation capability to suitably equipped users.
Since March 2011, when the EGNOS signal was declared available for Safety of Life operations, EDCN has continued to provide independent evidence of EGNOS compliance with the aviation requirements.
The EDCN is a standardized data collection environment established by EUROCONTROL to perform continuous EGNOS performance monitoring. This network was created by 2001 in preparation for the EGNOS SIS Validation, and it was an important contribution for the EGNOS Service provider Certification process coordinated with the European Commission. Nowadays the network is evolving towards multi-constellation and multi-frequency monitoring. PILDO LABS has been in charge for the managing of the EDCN since its first steps.
Key Performance Indicators for performance monitoring
The performance of the EGNOS system is measured, using PEGASUS, through four Key Performance Indicators: Accuracy, Integrity, Availability and Continuity.
PEGASUS has evolved from a purely GPS performance assessment tool, to a multi-mode application assessment tool including all GPS augmentation systems under development in Europe, in particular SBAS (Satellite Based Augmentation Service) and GBAS (Ground Based Augmentation System).
The GNSS position error is the difference between the estimated position and the actual position. For the sake of clarity and readability all these results are synthesised in global accuracy indicators. The statistics are collected independently for each EGNOS SIS channel (PRN) in order to isolate error scenarios caused by an EGNOS faulty signal. Each site collects daily statistics per EGNOS SIS channel showing the distribution of the errors over the day.
The integrity is a measure of the trust that can be placed in the correctness of the information supplied by the total system. The necessary level of integrity for each operation is established with respect to specific horizontal and vertical alert limits and when the integrity estimates exceed these limits, the user should be alerted within the prescribed time period.
The EGNOS integrity requirements include both an alert limit in horizontal and vertical dimensions and an allocated time to warn the user (Time To Alert) corresponding to the intended level of service.
Since the availability performance is significantly affected by local effects and that a network with a limited number of stations will not allow the mapping of these performances over the whole service volume, the availability analysis is based upon a consolidated set of EGNOS messages and assessment techniques based on estimation tools using the actual parameters sent to users through the EGNOS SIS.
The estimation method consists of computing the availability of service at a given location, using the real SBAS data broadcast collected and the real GPS ephemeris and almanacs, without having collected measured data on the field.
In line with the Availability results, the Continuity risk presented is also based on the estimated performance tools using the actual parameters sent to users through the EGNOS SIS.
The algorithm used for the computation of the continuity risk is based on the 15 seconds sliding window computation technique. The continuity performance is combined using the GEO switching method and the assessment is done on a monthly basis.
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