Absolute gradient monitoring for GAST-D with a single-frequency carrier-phase based and code- aided technique

摘要

GAST-D (GBAS Approach Service Type D) is a single-frequency based GNSS ground-based augmentation system(GBAS) that supports Category II and III landing. The de-velopment baseline SARPs [ICAO GBAS CAT II/III De-velopment Baseline SARPs, 2010] requires that the rangeerror associated with the ionospheric gradient shall be lessthan 1.5 m (or equivalently 300 mm/km in gradient) with amiss-detection probability less than 109.[Khanafesh et al., 2010] proposed a carrier-phasebased absolute ionospheric gradient detection. The tech-nique is simple but there are insensitive ranges of iono-spheric gradient due to ambiguity in carrier-phase measure-ments.This would require additional restrictions to sitingcriteria of GBAS reference stations. Furthermore, antennasof the reference stations need careful calibration.A new technique to estimate the ionospheric delaydifference, base on [Fujita et al., 2010], by using single-frequency carrier-phase measurements aided by code mea-surements is introdued. Validation process of obtained so-lutions by using the third receiver is proposed.The technique is applied to data obtained from shortbaseline ionospheric delay observations at Ishigaki (24.3N,124.2), Japan. For nominal ionospheric conditions, the methodworked perfectly. The monitor performance is shown to bedetermined predominantly by the fluctuation of the nomi-nal background ionosphere. The technique is then appliedto disturbed conditions by plasma bubbles. The techniqueworked very well even for disturbed conditions, and is ca-pable of realizing an ionospheric gradient monitor by satis-fying the requirements.2-D ionospheric delay gradients are reconstructed bytwo SD ionospheric delays estimated and validated by ourtechnique. Obtained results are smoothly changing and thedirections of gradients are consistent with a typical plasmabubble structure.Our method is not only capable of meeting the GAST-D development SARPs requirement on the ionospheric de-lay gradient but also capable of determining the ionosphericdelay gradients in arbitrary runway directions with threereceivers. This would be great advantage to realize moreflexible siting of GBAS ground reference receivers.