Initial Considerations About Alternative Approaches for Sigma Ground Processing in GAST E

In the future dual-frequency, multi-constellation Ground-Based Augmentation System (GBAS) Approach Service Type (GAST) E, raw pseudorange and carrier phase measurements are intended to be broadcast by the GBAS ground subsystem. The airborne user mitigates noise and multipath in the ground measurements through carrier smoothing with smoothing time constants of up to 600 s. Longer smoothing time constants generally improve the effectiveness of the filter but require an appropriate sigma ground that characterizes residual errors after smoothing depending on the elapsed smoothing time. This paper addresses initial considerations about alternative approaches to process sigma ground which also account for time-correlated multipath in the GAST E airborne subsystem. First, we determined sigma ground for the L1/E1 signals as a function of satellite elevation and elapsed smoothing time experimentally from GPS and Galileo dual-frequency measurements. Sigma ground in the transient state was estimated by computing sigma ground for various smoothing time constants between 50 s and 600 s. Second, we modeled this sigma ground with exponential models that require one and two parameters per elevation bin. As a result, the experimentally determined sigma ground was represented as a three-dimensional surface. It allows to obtain an estimate of sigma ground for each moment of smoothing since smoothing filter initialization and satellite elevation. The results also showed that increasing the smoothing time constant to more than 300 s provides only marginal additional reductions in sigma ground. Furthermore, the one-parametric model generated a closer depiction of the experimentally determined sigma ground for shorter smoothing times, whereas the two-parametric model fitted sigma ground for longer smoothing times more accurately. Given the marginal improvements from applying smoothing time constants of 300 s or more, the one-parametric model may be an alternative approach to GAST E airborne processing of sigma ground. Nevertheless, the question whether additional capacity in the very-high frequency data broadcast is available to uplink the model parameters remains open.