GAST C and GAST D Performance Analysis Based on BeiDou Navigation Satellite System

Z. Wang, J. Zhang, Y. Zhu, R. Xue

Abstract:	BeiDou Navigation Satellite System is called BeiDou System for short, with the abbreviation as BDS. When fully deployed, the space constellation of BDS consists of five Geostationary Earth Orbit (GEO) satellites, twenty-seven Medium Earth Orbit (MEO) satellites and three Inclined Geosynchronous Satellite Orbit (IGSO) satellites. By the end of 2012, there are five GEO, four MEO and five IGSO BeiDou navigation satellites in orbit. BDS Signal-In-Space Interface Control Document (ICD) is issued by the China Satellite Navigation Office on December 27, 2012. In China, BDS will be used as the primary navigation system for civil aviation, especially for the approach and landing operation section. Ground-based augmentation systems (GBAS) for satellite navigation are intended to replace the Instrument Landing System (ILS) for precision approach of aircraft into an airport in the near future. In China, many airports will install GBAS ground system, especially for the plateau airports such as LinZhi airport, which is surrounded by high mountains that are clouded with mist. Aircraft take off and land in narrow windy valleys: the distance between the flight path and the sides of the valley at the narrowest points is less than 4 km. Hence, the accuracy and the coverage of the traditional navigation aids, such as the ILS, are limited. Moreover, strong winds and a large temperature difference between day and night at the airport location significantly shorten the lifespan of the equipment. Therefore, it is urgent and important to develop GBAS based on BDS in China. In the paper, based on the simulation and real data of BDS, we estimate and analyze the performance of GBAS Approach Service Type C (GAST C) and GBAS Approach Service Type D (GAST D) for several typical airports in China. The core research contents include four parts as follows. Firstly, the thresholds of B value and D value are developed based on the BDS real data. B(i,j) value is the difference of the average of the pseudorange corrections for the Mi references and the average that excludes reference receiver j. D values are the magnitude of the vertical or lateral projection of the difference between the 30 second and 100 second smoothed position solutions. The existing models of B value and D value are obtained based on GPS data. Since the constellation, frequency, coordinate system, time system are all different between GPS and BDS, the parameters of B value and D value threshold are different. In the paper, based on the data collected by GBAS prototype system developed by Beihang University, the above parameters based on the BDS data will be shown. As follows, the monitor method of ionospheric storm is developed based on BDS data. Code-Carrier Difference (CCD) monitor is used in GBAS to detect abnormally large gradients in the ionospheric delay that could cause unacceptable errors in the differential position solution. The impact of the CCD monitor that could raise a detection flag should be taken into account. The existing CCD monitor threshold is also obtained based on GPS data. Similarly, the new CCD monitor threshold including the parameters of mean and standard deviation will be shown. Since CCD monitor is not available for the ionospheric storm with slow front speed (10 m/s-40m/s), Long Baseline Monitor (LBM) monitor is introduced, which monitors the ionospheric delay at a remote location, i.e. 10-40km form the GBAS ground facility (GGF). Ionospheric spatial gradient can be computed by differencing the two ionosphere delay measurements from the GGF and the LBM. Therefore, CCD monitor method and LAM monitor method are combined to monitor different GBAS ionospheric storm. Then, a software tool for analyzing and estimating performance of GAST C and GAST D based on BDS is developed. The software has two functions: the single point prediction and the flight path prediction. The single point prediction models the temporal domain while the flight path prediction models the spatial domain. Both the single point prediction and the flight path prediction have three sub-functions, and they are Approach Service, including GAST C and GAST D, Terminal Area Path (TAP) Service and Positioning Service respectively. For the single point prediction, there are two kinds results: the percent of the long-term service availability and the relationship figures between Protection Levels (PLs) and Alarm Levels (ALs). For the flight path prediction, there are also two results, the availability (Yes or No) and the relationship figures between PLs and ALs. Finally, several typical airports are selected, and simulations are carried out to analyze and compare GAST C and GAST D performance of GPS and BDS. In simulations, the effects of constellation (such as the number of GEO, MEO and IGSO), the frequency of the navigation signal, and airport latitude are all discussed. Besides, the effect of ionosphere storm is analyzed. And the different temporal gradient (2mm/km-425mm/km) and spatial gradient (1mm/s-10mm/s) are considered. It should be emphasized that, in the existing long-term service availability computation method, a critical parameter, constellation state probability, is computed based on the GPS baseline 24-slot constellation. Hence, it could not be applied to BDS directly. According to the Mean Time Between Failure (MTBF) and Mean Time To Repair (MTTR) of BDS satellite, we computed the constellation state probability of BDS to estimate the availability of GAST C and GAST D.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	1395 - 1402
Cite this article:	Wang, Z., Zhang, J., Zhu, Y., Xue, R., "GAST C and GAST D Performance Analysis Based on BeiDou Navigation Satellite System," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 1395-1402.
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