Using WAAS Ionospheric Data to Estimate LAAS Short Baseline Gradients

The Local Area Augmentation System (LAAS) is intended to provide real-time differential GPS corrections with the high accuracy, availability, and integrity required for Category II/III landings. In order to meet these requirements, errors due to ionospheric effects must be bounded such that integrity is maintained with minimal loss of availability. This paper describes the process by which archived data provided by the Wide Area Augmentation System (WAAS) were used to determine the appropriate upper bound to sufficiently cover this error even during severe ionospheric storms. We seek an answer to the question of how much spatial variation in the ionospheric delay can be expected at distance scales comparable to those between the receivers at the LAAS ground facility (LGF) and an incoming aircraft. WAAS continuously processes ionospheric data, known as “supertruth”, for the Conterminous United States (CONUS) region, obtained from its triply-redundant network of twenty-five dual-frequency ground stations. Comparing the simultaneous zenith delays from two different stations to a single satellite provides information about ionospheric behavior over receiver separation distances only as short as the two nearest WAAS ground stations’ separation (i.e. 255 km). In order to gain an understanding of ionospheric error at LAAS-applicable distance scales, we compare the ionospheric delay of a single line-of-sight (LOS) at one epoch with the delay for the same LOS up to an hour later. We assume a quasistatic ionosphere over this time scale, and choose the ionospheric pierce point (IPP) separation as the distance most analogous to that between the LGF and the aircraft. Using data from days on which the ionosphere behaved nominally and days on which ionospheric storms occurred, we observe the storm effects to exhibit significantly higher spatial gradients than the nominal periods. The bounds implied by these storms indicate that LAAS may need to revise its spatial ionospheric confidence bound to be broadcast.