空港面受動測位におけるマルチパス誤差低減時間検出手法

Abstract

This paper describes experiments and simulations to consider the reduction of multipath error, with the aim of realizing a precise passive positioning system using Mode S signal on airport surfaces. There are many reflective objects on airport surfaces, and broad pattern fixed antennas used for passive positioning systems, must be influenced by the multipath. The simulation of the multipath error characteristic of various pulse time of arrival detection methods was performed in order to study the influence of the multipath. As the simulation results showed, influence of the multipath remains even if delay time is long by the half-amplitude point method, which is widely used for the detection of pulse time of arrival (TOA). The correlation method, which is used for GPS, the DAC (Delay Attenuate and Compare) method, and the least square matching method, which extends DAC, can reduce the error created by an obstacle exceeding the delay time of the multipath by about 40ns, so the obstacles that influence an airport surface could be limited. The peak multipath errors of the DAC method and the least square matching method are less than half that of the correlation method. Compared with the correlation method, the multipath mitigation effect of the DAC method and the least square matching method, which carry out time detection in the early portion of a pulse waveform with low possibility of delayed multipath overlapping, are clear. However, since delay time is small, the influence of ground reflection can not be avoided. Although the error of the least square matching method is the smallest of these methods, the DAC method is practical at present, taking into account data processing load. An experimental system was developed using optical fiber signal transmission. Termed OCTPASS （Optically Connected PAssive Surveillance System）, it enables precise synchronization between the receiving stations of an airport surface passive positioning system. An experiment conducted at Sendai Airport showed that the error using the DAC method is less than half that compared with the correlation method. The experimental results of positioning error using the DAC method, without using smoothing filter obtained on the runway and taxiway were less than the 7.5 m (2RMS) required for airport surface surveillance. It is considered necessary that M/D (Multipath to Direct signal level ratio) be reduced to less than one, taking into account the height and position of the installed receiving antennas.