1. T. Sakai, K. Hoshinoo, and K. Ito Electronic Navigation Research Institute, Japan T. Sakai, K. Hoshinoo, and K. Ito Electronic Navigation Research Institute, Japan ION ITM 2014 San Diego, CA Jan. 27-29, 2014 Ionospheric Correction at the Southwestern Islands for the QZSS L1-SAIF Ionospheric Correction at the Southwestern Islands for the QZSS L1-SAIF

2. ION ITM Jan. 2014 - Slide 1 Introduction QZSS (Quasi-Zenith Satellite System) program:QZSS (Quasi-Zenith Satellite System) program: –Regional navigation service broadcast from high-elevation angle by a combination of three or more satellites on the inclined geosynchronous (quasi-zenith) orbit; –Broadcast GPS-like supplemental signals on three frequencies and two augmentation signals, L1-SAIF and LEX. L1-SAIF (Submeter-class Augmentation with Integrity Function) signal offers:L1-SAIF (Submeter-class Augmentation with Integrity Function) signal offers: –Submeter accuracy wide-area differential correction service; –Integrity function for safety of mobile users; and –Ranging function for improving position availability; all on L1 single frequency. ENRI has been developing L1-SAIF signal and experimental facility:ENRI has been developing L1-SAIF signal and experimental facility: –L1-SAIF signal achieves good accuracy less than 1 meter in an RMS manner at the mainland of Japan; –Ionosphere disturbance sometimes degrades the position accuracy, especially at the Southwestern Islands of Japanese territory; –In order to improve the accuracy at the southwestern islands during ionospheric storm, we have designed some new L1-SAIF messages and tested them.

3. ION ITM Jan. 2014 - Slide 2 QZSS Concept Broadcast signal from high elevation angle;Broadcast signal from high elevation angle; Applicable to navigation services for mountain area and urban canyon;Applicable to navigation services for mountain area and urban canyon; Augmentation signal from the zenith could help users to acquire other GPS satellites at any time.Augmentation signal from the zenith could help users to acquire other GPS satellites at any time. Footprint of QZSS orbit;Footprint of QZSS orbit; Centered at 135E;Centered at 135E; Eccentricity 0.075, Inclination 43deg.Eccentricity 0.075, Inclination 43deg. QZS GPS/GEO

4. ION ITM Jan. 2014 - Slide 3 L1-SAIF Signal User GPS Receivers Three functions by a single signal: ranging, error correction (Target accuracy: 1m), and integrity;Three functions by a single signal: ranging, error correction (Target accuracy: 1m), and integrity; User receivers can receive both GPS and L1-SAIF signals with a single antenna and RF front-end;User receivers can receive both GPS and L1-SAIF signals with a single antenna and RF front-end; Message-oriented information transmission: flexible contents;Message-oriented information transmission: flexible contents; See IS-QZSS for detail (Available at JAXA HP).See IS-QZSS for detail (Available at JAXA HP). SAIF ： Submeter-class Augmentation with Integrity Function RangingFunction ErrorCorrection IntegrityFunction QZS satellites GPS Constellation Ranging Signal

5. ION ITM Jan. 2014 - Slide 4 L1-SAIF Corrections Example of user position error at Site 940058 (Takayama: near center of monitor station network);Example of user position error at Site 940058 (Takayama: near center of monitor station network); Realtime operation with MSAS-like 6 monitor stations;Realtime operation with MSAS-like 6 monitor stations; Period: 19-23 Jan. 2008 (5 days);Period: 19-23 Jan. 2008 (5 days); L1-SAIF provides corrections only;L1-SAIF provides corrections only; No L1-SAIF ranging. HorizontalErrorVerticalError 1.45 m 2.92 m 6.02 m 8.45 m System StandaloneGPS 0.29 m 0.39 m 1.56 m 2.57 m L1-SAIF RMS Max RMS Max Note: Results shown here were obtained with survey- grade antenna and receiver in open sky condition. Standalone GPS Augmented by L1-SAIF Augmentation to GPS Only

6. ION ITM Jan. 2014 - Slide 5 Problem: Ionosphere Ionosphere Density (NASA/JPL) The largest error source: Ionospheric propagation delay;The largest error source: Ionospheric propagation delay; Varies on the local time, solar activity, earth magnetic field, and so on;Varies on the local time, solar activity, earth magnetic field, and so on; Cannot be predicted; Causes large effect in the low magnetic latitude region.Cannot be predicted; Causes large effect in the low magnetic latitude region.

7. ION ITM Jan. 2014 - Slide 6 Accuracy at Southwestern Island At Southwestern Island (960735 Wadomari) At Northernmost City (950114 Kitami) During severe ionospheric storm condition (Kp~7+), position accuracy with differential correction largely degrades at the Southwestern Islands;During severe ionospheric storm condition (Kp~7+), position accuracy with differential correction largely degrades at the Southwestern Islands; The effect is not so large at the mainland of Japan;The effect is not so large at the mainland of Japan; It is confirmed that increase of the number of GMS shows a little improvement.It is confirmed that increase of the number of GMS shows a little improvement. LT 14:00

8. ION ITM Jan. 2014 - Slide 7 Actual Ionosphere Corrections PRN20 PRN28 At Southwestern Island At Northernmost City Ionospheric correction continuously differs from the true delay by 5m or more;Ionospheric correction continuously differs from the true delay by 5m or more; Degradation of position accuracy during storm is due to inaccurate ionospheric correction.Degradation of position accuracy during storm is due to inaccurate ionospheric correction. 5m

9. ION ITM Jan. 2014 - Slide 8 L1-SAIF Ionospheric Correction IGP IGP IPP Vertical ionospheric delay information at IGPs ( ) located at 5-degree grid points will be broadcast to users.Vertical ionospheric delay information at IGPs ( ) located at 5-degree grid points will be broadcast to users. User receiver computes vertical ionospheric delays at IPPs with bilinear interpolation of delays at the surrounding IGPs.User receiver computes vertical ionospheric delays at IPPs with bilinear interpolation of delays at the surrounding IGPs. Vertical delay is converted to slant delay by multiplying a factor so- called obliquity factor.Vertical delay is converted to slant delay by multiplying a factor so- called obliquity factor. 120150180 0 30 60 Longitude, E Latitude, N 15 30 45 0 60 IGP

10. ION ITM Jan. 2014 - Slide 9 Thin-Shell Ionosphere The ionosphere model used by the L1-SAIF;The ionosphere model used by the L1-SAIF; Ionospheric propagation delay caused at a single point on the thin shell;Ionospheric propagation delay caused at a single point on the thin shell; The vertical delay is converted into the slant direction via the slant-vertical conversion factor so-called obliquity factor, F(EL).The vertical delay is converted into the slant direction via the slant-vertical conversion factor so-called obliquity factor, F(EL). Earth Ionosphere EL Vertical Delay I v Slant Delay F(EL) I v Shell Height (350km) IPP

11. ION ITM Jan. 2014 - Slide 10 Obliquity Factor, F(EL) Slant-vertical conversion factor as a function of the elevation angle;Slant-vertical conversion factor as a function of the elevation angle; Also a function of the shell height; The current L1-SAIF specifies the shell height of 350 km.Also a function of the shell height; The current L1-SAIF specifies the shell height of 350 km. 0153045 2 46 Obliquity Factor Satellite Elevation, deg H=100km H=1000km H=350km Verticaldelay Slant delay ElevationAngle IonosphereHeight Obliquity Factor = Slant / Vertical

12. ION ITM Jan. 2014 - Slide 11 Limitation due to Iono-Model Observe here if H=350km Observedifferent points if H=600km Shell Height H=350km, EL=25deg Shell Height H=600km, EL=25deg MCS assumes 2 GMS are observing same location of ionosphere;MCS assumes 2 GMS are observing same location of ionosphere; However, if true height is not 350km, they are looking at different locations.However, if true height is not 350km, they are looking at different locations.

13. ION ITM Jan. 2014 - Slide 12 Limitation due to Iono-Model Too Simple Vertical Structure:Too Simple Vertical Structure: –Assuming the thin-shell ionosphere at the fixed height of 350km; –IPP location may differ from the actual point with the peak density; Essentially, the ionospheric delay is caused over a certain distance within ionosphere;  The model may not represent the horizontal structure as well as vertical. –Obliquity factor may not reflect the true vertical structure of the ionosphere. Linear Interpolation of Vertical Delays at IGP:Linear Interpolation of Vertical Delays at IGP: –Assumption that the spatial scale of the ionosphere variation is roughly 500km or more; –Small structure cannot, even if observed, be reflected to the delay information. Need Alternative Ionospheric Correction Methods:Need Alternative Ionospheric Correction Methods: –Change assumptions on the ionosphere or avoid error by some way; –Allow definition of new L1-SAIF messages; –Minimize modifications from the current message and correction procedure.

14. ION ITM Jan. 2014 - Slide 13 Candidate Methods Maintain Single-Layer Thin-Shell Ionosphere Model:Maintain Single-Layer Thin-Shell Ionosphere Model: –Employ widely-used simple model to minimize modifications and to avoid complexity of user receivers; –MT26-like message structure: Share IGP information given by MT18;  Note: MT26 has 7 spare (unused) bits. –Define new message as MT55 (Message Type 55) for this purpose. Method 1: Variable Ionosphere Height:Method 1: Variable Ionosphere Height: –Broadcast the peak height of ionosphere in addition to grid delay information. Method 2: Ionospheric Correction per Satellite:Method 2: Ionospheric Correction per Satellite: –Generate vertical delay information at the grid points per each GPS satellite. Method 3: Ionospheric Correction per Direction:Method 3: Ionospheric Correction per Direction: –Generate vertical delay information at the grid points per each line-of-sight direction from receiver to satellite.

15. ION ITM Jan. 2014 - Slide 14 Existing Message Type 26 MT26: Broadcast Ionospheric Vertical DelayMT26: Broadcast Ionospheric Vertical Delay –Contains vertical delay information at IGP; –A MT26 message contains information at 15 IGPs; RepeatContentBitsRangeResolution 1IGP Band ID40 to 101 1IGP Block ID40 to 131 15 IGP Vertical Delay 90 to 63.875 m0.125 m GIVEI4(Table)— 1IODI20 to 31 1Spare7—— Message Type 26: Ionospheric Delay Information

16. ION ITM Jan. 2014 - Slide 15 New Message Design (1) Method 1: Variable Ionosphere Height:Method 1: Variable Ionosphere Height: –Broadcast the peak height of ionosphere in addition to grid delay information; –Both MCS and user receivers need to compute the ionospheric pierce point and the obliquity factor appropriately for given peak height; –MT55 contains the information of the peak height of the ionosphere. RepeatContentBitsRangeResolution 1IGP Band ID40 to 101 1IGP Block ID40 to 131 15 IGP Vertical Delay 90 to 63.875 m0.125 m GIVEI4(Table)— 1IODI20 to 31 1Peak Height2(Table)— 1Spare5—— 00: 350 km 01: 250 km 10: 600 km 11: 1,000 km Peak Height of Ionosphere Identical to MT26 Message Type 55 (1): Advanced Ionospheric Correction

17. ION ITM Jan. 2014 - Slide 16 New Message Design (2) Method 2: Ionospheric Correction per Satellite:Method 2: Ionospheric Correction per Satellite: –Generate every grid delay information for each ranging source satellite in view; –MT55 contains an identification of satellite;  Satellite ID requires at least 8 bits, however, we have only 7 spare bits in MT26;  Here we use only GPS satellites for the experimental purpose. –May need more measurements (ground stations) for this correction. RepeatContentBitsRangeResolution 1IGP Band ID40 to 101 1IGP Block ID40 to 131 15 IGP Vertical Delay 90 to 63.875 m0.125 m GIVEI4(Table)— 1IODI20 to 31 1SV ID51 to 321 1Spare2—— SV ID (PRN - 1) Identical to MT26 Message Type 55 (2): Advanced Ionospheric Correction

18. ION ITM Jan. 2014 - Slide 17 Example Definition of LOS Direction New Message Design (3) Method 3: Ionospheric Correction per Direction:Method 3: Ionospheric Correction per Direction: –Generate every grid delay information for each line-of-sight direction from receiver to satellite (azimuth and elevation angle); –Divide the sky into, for example, 5 directions;  MT55 contains the information of the direction. –Also may need more measurements (ground stations) for this correction. RepeatContentBitsRangeResolution 1IGP Band ID40 to 101 1IGP Block ID40 to 131 15 IGP Vertical Delay 90 to 63.875 m0.125 m GIVEI4(Table)— 1IODI20 to 31 1Direction3(Table)— 1Spare4—— 000: All 001: Zenith 010: North 011: East 100: South 101: West LOS Direction Identical to MT26 Message Type 55 (3): Advanced Ionospheric Correction 010 100 101011 001

19. Experiment: Configuration ION ITM Jan. 2014 - Slide 18 L1SMSGEONET QZS QZSS MCS GPSSatellites Measure-mentsL1-SAIFMessage GSI Server (Tokyo)ENRI(Tokyo) JAXA TKSC (Tsukuba) L1-SAIF Signal Nav Message Ranging Signal K-band Uplink Operates in Off-Line Mode Evaluation by User Receiver Software Experiment Using L1-SAIF Master Station (L1SMS):Experiment Using L1-SAIF Master Station (L1SMS): –Upgrade to support new messages (MT55) for Methods (1) to (3); –For this experiment, L1SMS operates in off-line mode; No realtime connection to GEONET and QZSS MCS; RINEX files from GEONET; –Evaluate augmentation performance of new messages by receiver software also upgraded to support MT55.

20. Experiment: Configuration ION ITM Jan. 2014 - Slide 19 Upgrade of L1-SAIF Master Station (L1SMS):Upgrade of L1-SAIF Master Station (L1SMS): –Support new messages (MT55) for Methods (1) to (3); –Accept additional measurements from IMS (Ionosphere Monitor Station) sites to increase the number of measurements (IPPs) for Method (2) and (3); –User receiver software is also upgraded to decode and apply MT55. L1SMS Receiver ReceiverSoftware L1-SAIFMessage Upgraded for MT55 UserAlgorithms PerformanceEvaluationGPSSatellitesGMS/IMSMeasurements RINEXFiles GEONET Ranging Signal GMSData GMS+IMSData UserMeasurements MT26/55 Clock/OrbitCorrections IonosphereCorrections

21. ION ITM Jan. 2014 - Slide 20 Experiment: Monitor Stations Observation Data from GEONET:Observation Data from GEONET: –GPS network operated by Geospatial Information Authority of Japan; –Survey-grade receivers over 1,200 stations within Japanese territory. Monitor Stations for Experiment:Monitor Stations for Experiment: –6 GMS (Ground Monitor Station) near MSAS GMS locations for clock/orbit and ionospheric corrections; –8 IMS (Ionosphere Monitor Station) for Method (2) and (3) ionospheric corrections. User Stations:User Stations: –Selected 5 stations from North to South: (1) to (5) for performance evaluation.

22. ION ITM Jan. 2014 - Slide 21 Baseline Performance LT 14:00 At Southwestern Island (User #4) At Northernmost City (User #1) During severe ionospheric storm condition (Kp~7+), position accuracy with differential correction largely degrades at the Southwestern Islands;During severe ionospheric storm condition (Kp~7+), position accuracy with differential correction largely degrades at the Southwestern Islands; The effect is not so large at the mainland of Japan;The effect is not so large at the mainland of Japan; All corrections are derived by measurements from 6 GMS.All corrections are derived by measurements from 6 GMS.

23. ION ITM Jan. 2014 - Slide 22 Variable Ionosphere Height LT 14:00 At Southwestern Island (User #4) At Northernmost City (User #1) The ionosphere shell height of 600 km improves position accuracy a little;The ionosphere shell height of 600 km improves position accuracy a little; However, some degradation is observed at the north and during quiet conditions; The effect is limited;However, some degradation is observed at the north and during quiet conditions; The effect is limited; All corrections are derived by measurements from 6 GMS.All corrections are derived by measurements from 6 GMS.

24. ION ITM Jan. 2014 - Slide 23 Variable Ionosphere Height Storm Condition (11/10/23 to 11/10/26) Quiet Condition (12/7/22 to 12/7/24) The ionosphere shell height of 600 km may improve the balance of the accuracy between North and South;The ionosphere shell height of 600 km may improve the balance of the accuracy between North and South; The effect is not so large; Need more investigation.The effect is not so large; Need more investigation.

25. ION ITM Jan. 2014 - Slide 24 Iono-Correction per Satellite LT 14:00 At Southwestern Island (User #4) At Northernmost City (User #1) Reduces position error by roughly 40% at the Southwestern Islands, while maintains the accuracy at other regions;Reduces position error by roughly 40% at the Southwestern Islands, while maintains the accuracy at other regions; Clock/Orbit corrections by 6 GMS; Ionospheric corrections by 6 GMS + 8 IMS.Clock/Orbit corrections by 6 GMS; Ionospheric corrections by 6 GMS + 8 IMS.

26. ION ITM Jan. 2014 - Slide 25 Iono-Correction per Direction LT 14:00 At Southwestern Island (User #4) At Northernmost City (User #1) This method also has a capability to reduce position error at the Southwestern Islands;This method also has a capability to reduce position error at the Southwestern Islands; Desirable behavior at other regions;Desirable behavior at other regions; Clock/Orbit corrections by 6 GMS; Ionospheric corrections by 6 GMS + 8 IMS.Clock/Orbit corrections by 6 GMS; Ionospheric corrections by 6 GMS + 8 IMS.

27. ION ITM Jan. 2014 - Slide 26 Iono-Correction per SV/Direction Storm Condition (11/10/23 to 11/10/26) Quiet Condition (12/7/22 to 12/7/24) These methods have similar performance on ionospheric corrections;These methods have similar performance on ionospheric corrections; In terms of the number of messages to be broadcast, Method (3) correction per direction has the advantage.In terms of the number of messages to be broadcast, Method (3) correction per direction has the advantage.

28. ION ITM Jan. 2014 - Slide 27 Conclusion ENRI has been developing L1-SAIF signal:ENRI has been developing L1-SAIF signal: –Signal design: GPS/SBAS-like L1 C/A code (PRN 183); –Planned as an augmentation to mobile users. Ionosphere disturbance is a concern:Ionosphere disturbance is a concern: –L1-SAIF signal achieves good accuracy less than 1 meter in an RMS manner at the mainland of Japan; –Ionosphere disturbance sometimes degrades the position accuracy, especially at the Southwestern Islands of Japanese territory; –In order to improve the accuracy at the southwestern islands during ionospheric storm, we have designed some new L1-SAIF messages and tested them.  Method (3) corrections per direction has a good property. Further Investigations will include:Further Investigations will include: –Validation of performance against historical storm events at many locations; –Performance at other Asian Countries; –More investigation of other correction methods against ionospheric disturbances.