Abstract
Abstract. System integrity (i.e. the capability of self-monitoring) and the reliability of the positions provided need to be ensured within all safety critical applications of the GPS technology. For the sake of such applications, GPS augmentations, for example Space Based Augmentation Systems (SBAS) are to be applied to achieve the required level of integrity. SBAS provides integrity in a multi-step procedure that is laid out in the Radio Technical Commission for Aeronautics (RTCA) Minimum Operational Performance Standards (MOPS) for airborne navigation equipment using GPS. Besides integrity, SBAS also improves accuracy of positioning via broadcasting corrections to reduce the most important systematic errors on standalone positioning. To quantify integrity, the protection level is defined, which is calculated from the standard deviation of the models broadcast in SBAS. Air Navigation Service Providers, airspace users and aviation authorities need to evaluate the performance of GPS systems and their augmentations. This is a necessary step to define the conditions under which GPS systems can be operationally used and which operations can be supported. For this evaluation two proprietary software are used widely in Europe: Pegasus from Eurocontrol (Butzmühlen et al., 2001) and magicGemini from GMV. Both tools provide several functionalities such as computation of position simulating MOPS-compliant receivers and determination of GNSS augmentation attributes like accuracy, integrity, continuity and availability. RTKLIB is an open source GNSS data processing and analysis tool (Takasu, 2009). The actual version (2.4.3) of RTKLIB has SBAS augmented positioning mode, but no protection level calculation is included. There is an open source project on GitHub3, a fork of RTKLIB 2.4.2 version with an option for WAAS MOPS compliant position calculation, including protection level calculation, too. This was developed by the Houghton Associates, Inc. and tested on Cygwin platform. Their development was finished in 2014. We have merged the WAAS MOPS position calculation into the newer RTKLIB release (2.4.3 beta) and made closer integration into the original RTKLIB utility program RNX2RTKP. Our enhanced RTKLIB version is also available on GitHub4 as a fork of the original RTKLIB project of Tomoji Takasu. This enhanced version was developed and tested on Ubuntu Linux 14.04 and 16.04. Raw static and kinematic data were post-processed by our enhanced RTKLIB version. Calculated SBAS positions and protection levels were compared to the results of Pegasus and magicGemini. Although the RTCA standard defines the exact formula to calculate protection levels, the numerical results of the tested software are slightly different. Accurate tests regarding the possible sources of this kind of discrepancies were carried on in order to validate our open source solution. The aim of our work is to provide an open source alternative to the available proprietary software. The open source solution might be a good basis for the evaluation of GPS and SBAS performance monitoring.
Highlights
Due to the utmost importance attached to its security perspective, aviation is one of the most conservative technologies, being especially careful with the uptake of novel solutions
International and European standards and guidance material promote the market uptake of these solutions. It is the International Civil Aviation Organization, which encourages the introduction of the so-called performance based navigation solutions (PBN), in its PBN Manual (ICAO Doc 9613)
Protection level is a notion introduced to be able to assign a numerical value to the reliability of navigation
Summary
Due to the utmost importance attached to its security perspective, aviation is one of the most conservative technologies, being especially careful with the uptake of novel solutions. The ICAO Global Air Navigation Plan (ICAO Doc 9570) treats PBN implementation as a top priority, while the SESAR (Single European Sky ATM [Air Traffic Management] Research) European ATM Master Plan puts special emphasis on the introduction of PBN as well (SESAR, 2015) This focused attention is especially needed in the short term, because it is not evident how to bridge the gap between the present situation of legacy navigation equipment and that of the long-term goal. In order to reap the fruits of a fully deployed satellite based navigation network, unilaterally from airport, operator and ANSP (air navigation service provider) side, the stakeholders would ideally need to take concerted action and invest in the new technological solution unanimously This proves difficult due to the large number of very diverse players, just as well as the highly cost sensitive environment where anyone who moves first, will inevitably encounter a temporary financial disadvantage.
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