A Study on Integrity Improvement of GBAS Ground Subsystem Using CSAC (Chip Scale Atomic Clock)

Takayuki Yoshihara, Francois Murashi, Susumu Saito, Kazuaki Hoshinoo

Peer Reviewed

Abstract:	GBAS (Ground-Based Augmentation System) is a system based on local differential correction technique to support aircraft precision approach. Its approach service of Category III (CAT-III) requires extremely high safety. In May 2010, ICAO (International Civil Aviation Organization) NSP (Navigation Systems Panel) working group of category II/III subgroup developed “development baseline SARPs (Standards and Recommended Practices)” (DBS) for CAT-III GBAS subsystem, which used a single frequency signal of GNSS L1-C/A as GAST-D (GBAS Approach Service Type D). Based on the DBS, Electronic Navigation Research Institute (ENRI), Japan has developed a research prototype for GAST-D ground subsystem and has operated it in New Ishigaki airport (24.4N / 124.2E) since February 2014 to collect continuous measurement data. We are especially going to validate GAST-D safety requirements against impacts of ionospheric disturbance due to “plasma bubble”, which is frequently occurred after sunset to midnight in the low magnetic latitude around spring and autumn. It produces a rapid temporal and spatial delay change. The prototype has been designed to optionally accept external clock signal from CSAC (Chip Scale Atomic Clock) of 10MHz as a stable reference for each GBAS reference station. CSAC is an atomic clock which provides a reference signal with better frequency stability more than Temperature Compensated Crystal Oscillator (TCXO) and worse less than rubidium atomic clock. The previous study discussed its advantages including our expectations. Namely, one of the most important issues is performance improvement of an ionospheric spatial gradient monitor to detect ionospheric anomalies by using simpler clock drift model for Kalman filter in its algorithm. We also expect enhancement of reliability in ranging source measurement and improvement of monitoring performance for multiple receiver faults including simultaneous and correlated range errors due to common obstacles and/or multipath at two reference stations. To evaluate the expectations, this study performs statistical analysis to investigate fundamental characteristics of GPS receiver clock with CSAC reference signal. At first, we use GPS measurement data which is collected on the roof of building by GPS receivers with and without an input signal of CSAC. Using the dataset with duration of about a month, we investigate characteristics of receiver clock component. Consequently, GPS receivers with CSAC free run signal at two stations show clock drifts of about 0-0.08 and 0.07-0.15 m/s in one-hour average at two stations. Although the average values are relatively large in comparison with the specifications, their standard deviations of clock drift from one-hour average are 0.03 m/s. The same analysis is expanded to five GBAS reference stations installed in New Ishigaki airport. As results, a wider range of averaged clock drifts is obtained roughly from -0.125 to 0.3 m/s, however, their standard deviations are almost the same as the first evaluation. Additionally, we examine another operational mode of CSAC, which uses its GPS module to control stability of reference signal output by synchronization to GPS time. This operational mode also brought the same standard deviations although averaged clock drifts are improved as several mm/s. In presentation, we will additionally report on results of quantitative evaluations of integrity monitor performances by integrating CSAC into the prototype using the real prototype data, especially for the ionospheric spatial gradient and multiple receiver fault monitors.
Published in:	Proceedings of the 2015 International Technical Meeting of The Institute of Navigation January 26 - 28, 2015 Laguna Cliffs Marriott Dana Point, California
Pages:	593 - 599
Cite this article:	Yoshihara, Takayuki, Murashi, Francois, Saito, Susumu, Hoshinoo, Kazuaki, "A Study on Integrity Improvement of GBAS Ground Subsystem Using CSAC (Chip Scale Atomic Clock)," Proceedings of the 2015 International Technical Meeting of The Institute of Navigation, Dana Point, California, January 2015, pp. 593-599.
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