A Study on Practical Use of CSAC (Chip Scale Atomic Clock) for GBAS Ground Subsystem

T. Yoshihara, S. Saito, K. Hoshinoo

Abstract:	GBAS (Ground-Based Augmentation System) is a system which provides correction and integrity information on GNSS ranging sources with ground facilities installed in an airport to support aircraft precision approach. GBAS Approach Service Type D (GAST-D) is expected to support category III (CAT-III) precision approach including runway rollout using a single frequency signal of GNSS L1-C/A. Its draft international standard has been developed as a baseline SARPs (Standards and Recommended Practices) by ICAO (International Civil Aviation Organization) NSP (Navigation Systems Panel) working group and it has been under operational validation including development of prototypes for ground and airborne subsystems. Electronic Navigation Research Institute (ENRI), Japan has also participated in GAST-D validation activities and developed a prototype for GAST-D ground subsystem. Primary purposes of the prototype are to validate the baseline SARPs, identify and solve major technical subjects under the environment in Japan. Regarding development of the prototype, monitoring of ionospheric anomaly is one of the most important subjects to meet the GAST-D requirements. It is required for GAST-D ground subsystem to monitor spatial gradient of ionospheric delay using GNSS reference receivers with a separation of several 100 meters. To develop this monitor, there are difficulties which are cycle-slip detection and extraction of ionospheric delay from ranging measurements against receiver clock drifts. As another topic on the prototype, we considered multiple receiver faults monitoring. This monitor is not required in the baseline SARPs, but we think that such faults might become relatively important issues in the future. It is an integrity monitor to detect multiple receiver faults including simultaneous and correlated range errors due to common obstacles and/or multipath at two GNSS reference stations. There is a difficulty to detect two fault receivers from four receiver measurements. To solve these difficulties, we employed a chip scale atomic clock (CSAC) to input stable signal to each GPS reference station as an external clock. CSAC is an atomic clock which provides a reference signal with better frequency stability more than Temperature Compensated Crystal Oscillator (TCXO). Improved performances using stable input from CSAC are expected for extraction of accurate ionospheric delay against clock drifts for ionospheric spatial gradient monitor and enhancement of GNSS receiver reliability for multiple receiver fault monitor. We performed preliminary data collection for evaluation of CSAC effects on GBAS in Ishigaki Island (located in southern part of Japan) during month and a half in the end of 2013. GPS antennas with a separation of about 1.5 km were used for the experiment. They were a part of dense and continuous observational system and set up on the roof of building in each site. Two GPS receivers without and with input signal of CSAC were installed at each site and connected to the same GPS antenna. Initial result for GPS receiver with CSAC has a constant drift of 0.014 meters per 0.5 seconds. However, random noise with CSAC is about 5 times better than TCXO in receiver clock stability. Using the preliminary collected data, we have an evaluation plan to validate improvement in performance of inoospheric spatial gradient monitor and we are further going to collect and evaluate validation data with the real GAST-D prototype which are installed at New Ishigaki Airport.
Published in:	Proceedings of the 2014 International Technical Meeting of The Institute of Navigation January 27 - 29, 2014 Catamaran Resort Hotel San Diego, California
Pages:	657 - 661
Cite this article:	Yoshihara, T., Saito, S., Hoshinoo, K., "A Study on Practical Use of CSAC (Chip Scale Atomic Clock) for GBAS Ground Subsystem," Proceedings of the 2014 International Technical Meeting of The Institute of Navigation, San Diego, California, January 2014, pp. 657-661.
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