Ground Station Pseudorange Pre-Smoothing and Error Model Development for DFMC GBAS

Abstract

In the Ground-Based Augmentation System (GBAS), carrier smoothing is applied to reduce noise and multipath in pseudorange measurements. A significant difference in the future dual-frequency, multi-constellation GBAS Approach Service Type (GAST) E compared to previous GASTs is that smoothing of the ground measurements happens in the airborne subsystem. For this, the raw ground measurements are uplinked in the Very-high frequency Data Broadcast (VDB) at 1/3 Hz. At this rate, the smoothing performance degrades compared to previous GASTs. To avoid such degradations, we suggest applying smoothing with a very short time constant in the ground station before transmitting the measurements. This process is referred to as pre-smoothing. Furthermore, the GAST E airborne subsystem is responsible to compute a bound on residual noise and multipath after smoothing in the ground measurements. As the VDB capacity is limited, it must thereby rely on as few parameters from the ground station as possible. In this paper, we address two objectives. First, we determine an ideal pre-smoothing time constant considering various tradeoffs. Second, we investigate the feasibility of applying first-order Gauss-Markov models to bound residual noise and multipath after smoothing. The results showed that pre-smoothing with 1.5 s mitigates smoothing performance degradations due to the lower VDB transmission rate. Thus, we recommend pre-smoothing with this time constant as an optional feature for GAST E. Nevertheless, potential impacts for the GAST E validation or on different monitors remain to be considered. Furthermore, we showed that the Gauss-Markov models bound the transient and steady states of the smoothing filter. However, the used data contained signal reflections from nearby structures. This made the modeling process challenging, producing conservative models. Therefore, it was shown that this modeling approach may work even under challenging conditions and we suggest pursuing it.