Avoiding Improper Modeling in SBAS Ionospheric Correction ION ITM 2018 Reston, VA Jan. 29-Feb. 1, 2018 Avoiding Improper Modeling in SBAS Ionospheric Correction with Shrunk Observations Takeyasu Sakai Electronic Navigation Research Institute National Institute of Maritime, Port and Aviation Technology, Japan

Introduction Japanese SBAS: The MSAS program ION ITM Jan. 2018 - Slide 1 Introduction Japanese SBAS: The MSAS program MSAS, Japanese version of WAAS, has been operational since Sept. 2007. Still serving horizontal navigation only. Transition to the new system in 2020, as a part of QZSS program; Planning update for vertical guidance in 2023 with two GEOs. The ionospheric effect is a major error source for Single-Frequency SBAS: The ionospheric term is the dominant factor of degradation of availability. Necessary to develop improved ionosphere algorithms to introduce vertical guidance capability to MSAS. Proposal: Reduction of false alerts of the current ionosphere algorithm. The storm detector makes a lot of false trips for the low-latitude ionosphere. Set the maximum uncertainty to the estimation even there is no storm. It is possible to improve system availability by reducing such false trips. Basic idea: Reduce observations for fit to keep the storm detector quiet.

Current Status MSAS: Japanese SBAS in operation. ION ITM Jan. 2018 - Slide 2 Current Status MSAS: Japanese SBAS in operation. MTSAT Satellite-based Augmentation System. Operational since Sept. 27, 2007. Continues operation with 2 signals via 1 GEO. MTSAT-1R decommissioned in Dec. 2015 (MTSAT-2 also in 2020). Hawaii and Australia MRS sites decommissioned in Feb. 2015. Service for Air Navigation Providing GPS augmentation information. Available for RNAV, from En-route through NPA (Non Precision Approach), i.e. RNP 0.3. Within Fukuoka FIR. Only horizontal navigation due to ionosphere activities. NOTAM is available to MSAS users. Alert for service interruption. Alert for predicted service outage.

Current Configuration ION ITM Jan. 2018 - Slide 3 Current Configuration Ranging Signals GPS Satellites Ground Network Naha GMS Fukuoka GMS Tokyo GMS Sapporo Hitachi-Ota MCS (and GMS) Kobe MCS Users MTSAT-2 Augmentation Signals 6 GMS in Japan PRN137 PRN129 MSAS Monitor Stations 1 GEO, 2 MCS (Master Control Station), and 4 GMS (Ground Monitor Station); MCS also has GMS function; MSAS has 6 domestic monitor stations. MTSAT-2 is broadcasting 2 signals from 2 MCS (PRN129 and PRN137).

Performance of MSAS GPS only GPS only MSAS PRN129 MSAS PRN137 ION ITM Jan. 2018 - Slide 4 Performance of MSAS GPS only GPS only Horizontal 0.722m RMS Horizontal 0.717m RMS MSAS PRN129 MSAS PRN137 GEONET 940058 (Takayama) 2016/8/8-12 (5 days) PRN129 and PRN137 Broadcast Signal

Continuous Operation Replacement in 2020: MSAS V2 ION ITM Jan. 2018 - Slide 5 Continuous Operation Replacement in 2020: MSAS V2 MSAS continues operation with 1 GEO (MTSAT-2) and 6 GMS until 2020. In 2020, MSAS will continue operation with a new GEO of the QZSS. QZSS (Quasi-Zenith Satellite System): Japanese regional satellite navigation system with IGSO and GEO satellites. The L1Sb signal of QZS-3 (GEO) will be used for MSAS service. MCS equipment will also be fully replaced at the same time. 7 GMS will be added: Totally 13 GMS domestic. The performance will be similar with the current MSAS: Horizontal only. Supporting vertical guidance in 2023: MSAS V3 Vertical guidance: LPV and LPV-200 operation. Needs software upgrade: Adding GMS cannot overcome ionospheric effects. Needs development of the improved algorithms for ionospheric correction. Will be supported in accordance with introduction of the 2nd GEO in 2023.

Toward Vertical Guidance ION ITM Jan. 2018 - Slide 6 Toward Vertical Guidance The current MSAS is built on the IOC WAAS: Achieves 100% availability for Enroute to NPA flight modes. The primary purpose is providing horizontal navigation means to aviation users. No vertical guidance; Ionospheric corrections may not be used. As the first satellite navigation system developed in Japan, the design tends to be conservative, especially for ionospheric corrections.. The major concern for vertical guidance is ionosphere: The ionospheric term (uncertainty of estimation) is dominant factor of degradation of system availability. Necessary to reduce ionospheric terms to provide vertical guidance with reasonable availability.

Ionosphere Estimation: Planar Fit ION ITM Jan. 2018 - Slide 7 Ionosphere Estimation: Planar Fit Cutoff Radius Vertical Delay Fit Plane IPP IGP Fitting Radius, Rfit Developed for WAAS; MSAS employs the same algorithm. Assumes ionospheric vertical delay can be modeled as a plane. Model parameters are estimated by the least square fit. Output: Vertical delay estimation at the IGP. GIVE (Grid Ionosphere Vertical Error): Uncertainty of the estimation. GIVE Equation Formal Sigma Spatial Threat Temporal Threat

Storm Detector ^ c2 = ( Iv,IPP -Iv,IPP )T W ( Iv,IPP -Iv,IPP ) ION ITM Jan. 2018 - Slide 8 Storm Detector The current storm detector algorithm is ‘Chi-Square Test’: Chi-square statistics is the sum of squared residuals: If Chi-square < Threshold (Storm Detector does not trip), The ionosphere model is valid to use. GIVE is computed from the GIVE equation. If Chi-square > Threshold (Storm Detector trips), The ionosphere model is judged not valid. The assumption that the distribution of residual errors is the normal (Gaussian) may not be valid. In such a case, the associate IGP is determined as storm condition. Set the maximum GIVE (Storm Detector ‘trips’). This action leads larger protection levels degrading availability of the system. False trips should be reduced. c2 = ( Iv,IPP -Iv,IPP )T W ( Iv,IPP -Iv,IPP ) ^

Example Detector Response ION ITM Jan. 2018 - Slide 9 Example Detector Response Severe storm condition with the largest Kp index of 9-. Chi-square metric is chi-square statistics divided by the associate threshold. Inflation factor Rirreg applied. Normalized residual means residual error divided by sGIVE; It must be within 5.33 for integrity. We can observe many false trips; These trips lower the availability of MSAS. True Trips 5.33 False Trips 1 No Trips Storm Detector Trips

Possible Situation Fit with N IPPs Storm Detector trips due to anomaly ION ITM Jan. 2018 - Slide 10 Possible Situation Fit with N IPPs Storm Detector trips due to anomaly Rfit Anomaly IPP for fit not for fit Fit with N-DN IPPs Storm Detector does not trip Anomaly IPP for fit not for fit Rfit Reduce IPPs for Fit Normally, larger Rfit is preferable to have enough N, the number of IPPs. Due to curvature of the real ionosphere, large Rfit (means large N) sometimes reaches an anomalous region where ionosphere is not on the planar model. In such a condition, storm detector trips and sets GIVE to the maximum. Basic Idea: How about decreasing N to avoid observations within anomaly ?

ION ITM Jan. 2018 - Slide 11 Shrink Observations Storm Avoidance Algorithm: If Storm Detector trips, shrink observations by: Radius Method: Decrease Rfit until Storm Detector does not trip. Southernmost Method: Invalidate some IPPs at the southernmost. Note: Keeps spatial threat not reduced. For conservatism, employs the spatial threat model once computed before these adjustment of Rfit. This means the adjustment of Rfit affects to fitting and Storm Detector only and does not reduce the spatial threat model. (1) Decrease Rfit while Storm Detector trips (2) Stop if DRfit = DRmax, DN=DNmax, or N = Nmin Invalidate the southernmost IPP while Storm Detector trips (2) Stop if DRfit = DRmax, DN=DNmax, or N = Nmin

Simulation for MSAS Simulates MSAS V3 ION ITM Jan. 2018 - Slide 12 Simulation for MSAS Simulates MSAS V3 SBAS simulator developed by the ENRI, with some modifications for proposed algorithm. 13 GMS locations after 2020 (Red Triangles). Test periods For each period, check: GIVEI distribution in the message stream Availability of LPV flight mode Period Max Kp Remark (1) 2011/5/16 4- Moderate Storm (2) 2011/11/19 3 Quiet Note: GIVE Filter and IPP Filter do not apply.

Histogram for GIVEI in the Message Stream ION ITM Jan. 2018 - Slide 13 GIVEI: Radius Method Histogram for GIVEI in the Message Stream 8 9 10 11 12 13 14 15 GIVEI Reduction GIVEI= Applies Storm Avoidance with Radius Method for Period (2). Replaces GIVEI of 14 conditions with lower GIVEI. Improves navigation availability with lower protection levels.

GIVEI: Southernmost Method ION ITM Jan. 2018 - Slide 14 GIVEI: Southernmost Method Histogram for GIVEI in the Message Stream 8 9 10 11 12 13 14 15 GIVEI Reduction GIVEI= Applies Storm Avoidance with Southernmost Method for Period (2). Replaces GIVEI of 14 conditions with lower GIVEI for more IGPs. Improves navigation availability with lower protection levels.

LPV Availability: Period (1) ION ITM Jan. 2018 - Slide 15 LPV Availability: Period (1) 95% 99.9% Baseline MSAS Southernmost Method Availability of LPV flight mode with HAL=40m and VAL=50m. Proposed method improves availability to reasonable level. Other additional algorithms can be applied for more availability improvement.

LPV Availability: Period (2) ION ITM Jan. 2018 - Slide 16 LPV Availability: Period (2) 80% 99% Baseline MSAS Southernmost Method Availability of LPV flight mode with HAL=40m and VAL=50m. Proposed method improves availability to reasonable level. Other additional algorithms can be applied for more availability improvement.

Conclusion MSAS will have a new GEO and new ground facilities in 2020. ION ITM Jan. 2018 - Slide 17 Conclusion MSAS will have a new GEO and new ground facilities in 2020. Alters the current GEO and ground facilities: MSAS V2. Update for MSAS V3 with vertical guidance capability is also planned in 2023 with the 2nd GEO. Development of ionospheric algorithms for MSAS vertical guidance: Storm Avoidance Algorithm: Decrease Rfit or N to keep the storm detector quiet. With this algorithm, Invalidating southernmost IPPs is much better method. The simulation confirmed improvement of system availability. Further activities: Verification with more historical storm data archives. Testing combined use with other algorithms. Contact for more information: Dr. Takeyasu Sakai <sakai@mpat.go.jp> Electronic Navigation Research Institute National Institute of Maritime, Port and Aviation Technology, Japan