Assessment of Nominal Ionosphere Spatial Decorrelation for LAAS

Abstract

The Local Area Augmentation System (LAAS) is a ground-based differential GPS system being developed to support aircraft precision approach and landing navigation with guaranteed integrity. To quantitatively evaluate navigation integrity, an aircraft computes vertical and lateral protection levels as position-error bounds using integrity parameters broadcast by a nearby LAAS Ground Facility (LGF). These parameters include a standard deviation of ionosphere spatial decorrelation because the range errors introduced by the ionosphere vary between LGF receivers and LAAS users. Thus, it is necessary to estimate typical ionosphere gradients for nominal days and to determine an appropriate upper bound to sufficiently cover the differential error due to the ionosphere spatial decorrelation. In this paper, both Station-Pair and Time-Step methods are used to assess the standard deviation of vertical (or zenith) ionosphere gradients ( vig ó ). The Station-Pair method compares the simultaneous zenith delays from two different reference stations to a single satellite and observes the difference in delay across the known ionosphere pierce point (IPP) separation. Because most of these IPP separations are larger than 100 km, the Time-Step method is also used to better understand ionosphere gradients at LAAS-applicable distance scales (10 – 40 km). The Time-Step method compares the ionospheric delay of a single line-of-sight (LOS) at one epoch with the delay for the same LOS at the other epoch a short time (seconds or minutes) later. This method has the advantage of removing inter-frequency bias (IFB) calibration errors on different satellites and receivers while possibly introducing an estimation error due to temporal ionosphere gradients. This paper shows results from analyzing the post-processed ionosphere database for the Wide Area Augmentation System (WAAS), known as “supertruth”, as well as JPL post-processed data from the Continuously Operating Reference Stations (CORS) database. CORS data is adequate for the Station-Pair method because of the relatively dense CORS receiver network. However, WAAS data is of higher quality since each reference station has three high-quality receivers that aid in removing measurement outliers and reducing noise. The results of this study demonstrate that typical values of vig ó are on the order of 1 – 3 mm/km for non-stormy ionosphere conditions. As a result, a broadcast vig ó of 4 mm/km is conservative enough to bound ionosphere spatial decorrelation for nominal days with margin for more active days and for non-Gaussian tail behavior. Future work will attempt to better resolve the details of nominal ionosphere behavior over short distances as well as determine if the broadcast “bounding value” of vig ó can be reduced prior to LAAS commissioning.