Revaluation Of Spatial Decorrelation Parameters Of Atmospheric Delay For Gbas (Ground-Based Augmentation System) Safety Design

GBAS (Ground-Based Augmentation System) is a system based on local GNSS differential correction technique to support aircraft precision approaches. It is one of the most important issues for GBAS safety design to evaluate spatial gradients of ionospheric delay especially with a horizontal scale of about 10km. In our previous study, we examined spatial gradients of ionospheric delay using experimental equipment associated with a prototype of CAT-III GBAS at New Ishigaki airport, which was located in southern part of Japan. As a result, large spatial gradients of ionospheric delay up to +/- 54 mm/km in slant direction were found under an event of nighttime medium-scale traveling ionospheric disturbances (MSTIDs). In addition, our latest result derived from the same equipment shows that a large tropospheric gradient of about 87mm/km in slant direction is observed. Because the facts related to the MSTIDs and the tropospheric disturbances are recently recognized, it is important to reevaluate effects of them on GBAS safety design. There are three key points for the revaluation, which are broadcast parameters of sigma_pr_ground and sigma_vertical_iono_gradient, and system parameters related to integrity monitors. The effects on the broadcast parameters might be potentially reflected in them because they are determined through safety assessment based on the actual field data. However, these effects have not been explicitly evaluated in view point of such large atmospheric "nominal" spatial gradients. As the first step of the revaluation, we used a data set of the prototype with 12-day duration and investigated the two broadcast parameters, which are sigma_ pr_ground and sigma_vertical_iono_gradient. Regarding the former parameter, we analyzed probability density functions of B values, which are also GBAS broadcast parameters related to sigma_pr_ground, on each atmospheric disturbed and quiet days. For evaluating the latter parameter, RMS (root mean square) of vertical ionospheric gradients derived from an ionospheric spatial gradient monitor of the prototype are also investigated as the same manner. As results, we confirmed that the RMS of vertical ionospheric gradients tend to increase on high MSTIDs/troposphere delay activity day than quiet day. For future work, it is essential to examine the three key points of our purpose with long term data, distinguishing between ionospheric and tropospheric delay components.