GNSS is widely used in aircraft navigation such as RNP0.3 or RNP-AR. On the other hand, APNT (Alternate Position, Navigation and Timing) is required in order to maintain efficient aircraft operations and safety in the case of GNSS losses. Some concepts and systems are suggested for APNT, such as utilization of existing ground facilities (e.g. DME) or transmissions of aircraft positions by surveillance(WAM). The authors focus on DME because they are widely used in various countries. There are various error factors in determination of an aircraft position for APNT by using DME. The error factors of DME are divided to an interrogator, transponder and propagation. One of the factors of propagation is tropospheric propagation delay. The ranging errors caused by tropospheric delays are smaller than that of multipath effects or an interrogator for short distance ranging and can be neglected. However, tropospheric delays can't be neglect for long distance ranging. So, at first, the DME ranging errors caused by tropospheric delays are clarified for long distance ranging. The DME ranging error caused by the tropospheric delays can be corrected mostly by FMS using altitudes, distances between DME stations and aircrafts with a standard tropospheric model. However, the correction of the fluctuations(variation) of ranging errors caused by meteorological phenomena is difficult. Especially, elevation angles become very small for long distance ranging because of the rounded surface shape of the earth. The propagations near the earth surface increase fluctuations of ranging errors. The fluctuations of ranging errors caused by meteorological phenomena can't be corrected easily by FMS. So, this paper also shows the fluctuations of DME ranging errors caused by tropospheric propagation delay using numerical analyses. The ray-tracing method is employed in order to calculate electromagnetic wave propagation for long distance ranging (up to 200nmi). The ray-tracing is a method for calculating a path of an electromagnetic wave. The ranging errors of DME are calculated by the propagation delay amount obtained by the ray-tracing. Temperatures, pressures and humidities change gradually according to the altitude in the troposphere. In the ray-tracing analyses, refraction indexes are calculated by the temperatures, pressures and humidities which are obtained by radiosonde observations. Therefore, rays bend gradually in the troposphere. Temperatures, pressures and humidities are also changed by weather, seasons, atmospheres, air masses and other meteorological phenomena. The fluctuations of DME ranging errors caused by meteorological phenomena are shown clearly in this paper by using the ray-tracing with radiosonde observation data. The calculation results show the fluctuation of DME ranging error become 18.5m at 370km (200nmi) which is the maximum range of DME. It is difficult to correct this fluctuation by FMS without additional weather information. In addition, ray-tracing shows that the round surface of the earth ground emphasizes the DME ranging error for long distance ranging. The amount of the fluctuation of DME ranging error is important to discuss the accuracy of the ranging by DME(WAM) for APNT. The results provide the estimation of the ranging limitation for APNT by DME.
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