Absolute Gradient Monitoring for GAST-D with a Single-frequency Carrier-phase Based and Code-aided Technique

Susumu Saito, Takayuki Yoshihara, and Seigo Fujita

Abstract:	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 range error associated with the ionospheric gradient shall be less than 1.5 m (or equivalently 300 mm/km in gradient) with a miss-detection probability less than 10??9. [Khanafesh et al., 2010] proposed a carrier-phase based 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 siting criteria of GBAS reference stations. Furthermore, antennas of the reference stations need careful calibration. A new technique to estimate the ionospheric delay difference, 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 short baseline ionospheric delay observations at Ishigaki (24.3N, 124.2), Japan. For nominal ionospheric conditions, the method worked perfectly. The monitor performance is shown to be determined predominantly by the fluctuation of the nomi- nal background ionosphere. The technique is then applied to disturbed conditions by plasma bubbles. The technique worked 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 by two SD ionospheric delays estimated and validated by our technique. Obtained results are smoothly changing and the directions of gradients are consistent with a typical plasma bubble 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 ionospheric delay gradients in arbitrary runway directions with three receivers. This would be great advantage to realize more flexible siting of GBAS ground reference receivers.
Published in:	Proceedings of the 25th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2012) September 17 - 21, 2012 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	2184 - 2190
Cite this article:	Saito, Susumu, Yoshihara, Takayuki, Fujita, Seigo, "Absolute Gradient Monitoring for GAST-D with a Single-frequency Carrier-phase Based and Code-aided Technique," Proceedings of the 25th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2012), Nashville, TN, September 2012, pp. 2184-2190.
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