Analysis on the Impact of Different Time Constants of Smoothing Filter on Compliance of Ranging Source Monitors in Dual-Frequency GBAS

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In Ground Based Augmentation System (GBAS), the ground and the airborne uses the same magnitude of time constants for the carrier smoothing filter. This is to avoid the relative errors induced by the ionospheric divergence according to Minimal Operational Performance Standards (MOPS) airborne requirement [1]. This is especially true for a single-frequency GBAS solutions such as GBAS Approach Service Types -C and -D, which are to support the Category (CAT) I and CAT II/III precision approaches, respectively. However, in a dual-frequency and multi-constellation GBAS solution GAST-F, which is currently under development within the frame of Single European Sky ATM Research (SESAR) project, the ionospheric delay is completely eliminated through the Ionosphere-Free (IF) mode processing. Therefore, this might allow the airborne side to have flexible time constants of the carrier smoothing filter. In general, using larger time constant for a carrier smoothing filter is more effective in decreasing the standard deviation of the pseudorange measurements when the measurement error is assumed Gaussian. However, in reality, there exists a time-correlated error, namely a multipath error in the measurement, which limits the reduction of a standard deviation of the filtered measurement. Even with this, having the longer time constants would be beneficial to reduce the noise and multipath errors under the assumption that the mean of the multipath error is close to zero. Despite of having this merits of using larger time constant, it should be careful to change the time constant at the airborne, because the impact of the range error is related to the compliance of the monitor performance to the requirement, especially for the GBAS solutions that support CAT II/III [2]. In this paper, we analyze the impact of using nonhomogeneous time constants at the ground and the airborne, especially on the compliance of the ranging source monitor for the Code-Carrier Divergence (CCD) and for the signal deformation faults.