Development of a GAST-D ground subsystem prototype and its performance evaluation with a long term-data set T. Yoshihara, S. Saito, A. Kezuka, K. Hoshinoo, S. Fukushima, and S. Saitoh Electronic Navigation Research Institute (ENRI) November 16, 2017 EIWAC 2017, Tokyo 1
Contents Outline of CAT-III GBAS (GAST-D) Extreme high safety system International standardization and validation activities including ENRI s R&D program A Research prototype of GAST-D ground subsystem Purpose Unique points Installation of the prototype and collection of longterm data at New Ishigaki Airport Optimization of parameters in integrity monitors Performance evaluations Summary and future works November 16, 2017 EIWAC 2017, Tokyo 2
Ground-based Augmentation System: GBAS GNSS reference stations Four sets of ANT & Rx Data processing system Range corrections Integrity messages Anomaly detection and user protection GNSS satellite faults Propagation anomaly Ground system faults Augmentation information transmission VHF Data Broadcast ICAO SARPs for Category I (CAT-I) GBAS Published in 2001 GBAS ground subsystem GNSS Satellite Augmentation information GBAS airborne subsystem November 16, 2017 EIWAC 2017, Tokyo 3
Outline of CAT-III GBAS GAST-D (GBAS Approach Service Type-D) CAT-III GBAS based on ranging sources with single frequency of L1 signal To support precision approach bellow 200ft (60m) including rollout GAST-D should cover all requirements of CAT-I GBAS Decision Height 200ft (60m) CAT-I CAT-III Integrity Integrity Higher safety requirement 1-2 x 10-7 1-1 x 10-9 November 16, 2017 EIWAC 2017, Tokyo 4
Outline of GAST-D Ionospheric anomaly: GBAS integrity risk A large spatial gradient in iono. delay: Significant positioning error CAT-I Assumption of large potential errors without detection of ionospheric anomaly by ground subsystem IFM could reduce a part of the potential error GNSS satellite CAT-III Detection of ionospheric anomaly by integrity monitors at both of ground and airborne subsystems GNSS satellite Iono. anomaly GBAS reference stations GBAS reference stations IFM (Ionosphere Field Monitor) November 16, 2017 EIWAC 2017, Tokyo 5
Outline of GAST-D Standardization and validation activities Development baseline SARPs International Civil Aviation Organization (ICAO) Navigation Systems Panel (NSP) working group has developed a baseline Standards and Recommended Practices (SARPs) May 2010 November 2016 Development of baseline SARPs with technical validation Operational validation with prototypes of GAST-D ground subsystem Atlantic City (US) Frankfurt (Germany) Toulouse (France) New Ishigaki (Japan) Operational validation Development of prototypes for ground / airborne subsystems The magnetic middle latitude The magnetic low latitude November 16, 2017 EIWAC 2017, Tokyo 6
Outline of GAST-D ENRI s program (Apr. 2011 Mar. 2015) Purposes Validation of the baseline GAST-D SARPs Identification of major technical subjects and development of fundamental solutions for GAST-D ground subsystem Major subjects Development of a prototype of GAST-D ground subsystem Development of a flight experimental system including major airborne integrity monitors Validation of the GAST-D ionospheric threat model for low magnetic latitude Ionospheric threat model defines a range of each parameter such as gradient, speed, and width of ionospheric disturbances to be assumed by the system November 16, 2017 EIWAC 2017, Tokyo 7
A prototype of GAST-D ground subsystem Development ENRI contracted with NEC corp. From Mar. 2012 to Sept. 2013 Total 23 meetings were held for safety design review almost every three weeks System design and its validation In compliance with the GAST-D requirements of the ICAO baseline SARPs, RTCA Do-246D and Do-253C A fundamental process of safety design and validation was done according to a guidance of SAE ARP4754 and ARP4761 Not includes software integrity assurances and redundant hardware November 16, 2017 EIWAC 2017, Tokyo 8
A prototype of GAST-D ground subsystem System safety design & validation System design Hardware Software Documentation FY 2011 FY2012 FY2013 4 / 4 1 / 4 2 / 4 3 / 4 4 / 4 1 / 4 2 / 4 Design Manufacturing Algorithm descript Software design & coding document (ADD) Preliminary System Hardware Integration & tests Safety Assessment (PSSA) Safety design & validation System safety design FY: April to March in Japan Extraction of risks and FTA (Fault tree analysis) Quantitative risk analysis and identify key risks Risk allocations Development of mitigation methods such as integrity monitors Evaluation of remained risks November 16, 2017 EIWAC 2017, Tokyo 9
A prototype of GAST-D ground subsystem Integrity monitors for GAST-D Integrity monitor for GAST-D Integrity risks for the ground Iono. anomaly Satellite faults RFI Receiver faults Baseline SARPs Ionospheric spatial X Required gradient Signal deformation X Required Code Carrier Divergence (CCD) X X Required Excessive acceleration X Required Ephemeris X Required Radio frequency X - interference (RFI) Multiple receiver faults X - November 16, 2017 EIWAC 2017, Tokyo 10
Installation of the prototype in New Ishigaki Airport Southern part of Japan Equatorial anomaly: Nominal condition in Spring and Autumn Plasma bubble causes steep spatial gradients in ionospheric delay night time Selection of New Ishigaki airport Validation and demonstration of GAST-D concept with ground/airborne integrity monitors to detect ionospheric anomaly including flight experiments Optimization of parameters of integrity monitors under real airport condition Hazardous Misleading Information (HMI) analysis Ishigaki Island Ionospheric delay map under plasma bubble events November 16, 2017 EIWAC 2017, Tokyo 11
Installation of the prototype in New Ishigaki Airport (2) November 16, 2017 EIWAC 2017, Tokyo 12
Iono. delay diff. (mm) Optimization of parameters in monitors Ionospheric spatial gradient monitor Fundamental algorithm Estimation of gradient and direction A typical value from Consistency check among three baselines requirement Ref1-Ref2 baseline length Requirement of detectability: 119.7mm/km (300mm/km x 0.399km) Over bounded sigma: 9.86 mm/km Missed detection (Pmd): 10-9 Mean: -0.57 [mm] Threshold: False alarm (Pffd): 2 x 10-7 51.3~60.5 (raw): 5.33 [mm] mm/km (Inflated): 9.86 [mm] Feasibility validation Nominal condition [ S. Saito et al., Proc. of ION Pacific PNT 2015, Honolulu, HI, April 2015] November 16, 2017 EIWAC 2017, Tokyo 13
Optimization of parameters in monitors Signal Deformation & CCD Monitors Signal Deformation Monitor (SDM) Data on the roof of building are used for initial design and validation phase during system safety review Validation under a real airport conditions Difficulty to satisfy the requirement with low elevation angles from 5 to 10 degrees due to ground multipath Code Carrier Divergence (CCD) Monitor To detect a rapid temporary change of ionospheric delay due to disturbances Noise component increases for GPS signal with a low satellite elevation angle due to multipath and nominal ionospheric delay gradients Suggestion from the results Multipath Limiting Antenna (MLA) is needed to improve monitoring performances November 16, 2017 EIWAC 2017, Tokyo 14
Performance evaluations Accuracy (95% reliability) IFM data as Pseudo user Different seasons: Each one week in March and August 2014 Results of the bellow table meets the requirement Lateral: 16 meters, Vertical: 4 meters 2014 CAT-I GBAS GAST-D Accuracy of GAST-D is worse than CAT-I GBAS in both horizontal and vertical components Unit: m Horizontal Vertical Horizontal Vertical March 21-27 0.1455 0.3848 0.2010 0.5212 August 07-13 0.1541 0.3705 0.2060 0.4938 In GAST-D, position solution is based on carrier smoothing with a time constant of 30 seconds although it is 100 seconds in CAT-I GBAS GAST-D solution could be affected by observational errors such as multipath effects November 16, 2017 EIWAC 2017, Tokyo 15
Performance evaluations Availability analysis GAST-D availability Depends not only on GAST-D ground subsystem but also on airborne subsystem because of integrity monitors at the both sides to mitigate ionospheric threat GAST-D positioning analysis in offline Using our software originaly for experimental airborne GBAS equipment with airborne integrity monitors November 16, 2017 EIWAC 2017, Tokyo 16
Performance evaluations Availability and HMI analysis Availability result 99.973% from Stanford chart for an sample day A low rate of 86.100% for epochs with final solutions Further optimization of parameters in airborne Integrity monitors could improve the results Investigation of HMI events A larger VPL than actual positioning error Vertical Protection Level / Vertical alert limit A non-disturbed day HMI Vertical error / Vertical alert limit November 16, 2017 EIWAC 2017, Tokyo 17
Summary ENRI developed a prototype of GAST-D ground subsystem It is installed in the low magnetic latitude region to operationally validate the baseline GAST-D SARPs It has been continuously operated to collect a long-term data set to analyze total system performance and HMI events for future GAST-D implementation in Japan The initial and fundamental performance evaluation Integrity monitors such as Ionospheric spatial gradient, signal deformation, and CCD monitors The first performance evaluations of accuracy and availability November 16, 2017 EIWAC 2017, Tokyo 18
Future works Using a long-term data set Validation of the current parameter set, performance of integrity monitors, and total system performances HMI (Hazardous Misleading Information) analysis Issues on ICAO SARPs In Nov. 2016, the operational validation was completed with conclusions that the baseline SARPs covered ionospheric conditions in the middle magnetic latitude region at least It was also recognized that further work may be needed to enhance availability of GAST-D in the low magnetic latitude region, where ionospheric delay and scintillations frequently occur associated with plasma bubbles The data set is useful for investigating impacts of the ionospheric disturbances on GAST-D ground subsystem November 16, 2017 EIWAC 2017, Tokyo 19
Acknowledgement Authors deeply thank Japan Civil Aviation Bureau, Okinawa prefecture, Ishigaki city for their cooperation to installation and operation of ENRI s GAST-D prototype in New Insigaki airport November 16, 2017 EIWAC 2017, Tokyo 20