SBAS IWG/26 New Delhi, India Feb. 5-7, 2014

1. SBAS IWG/26 New Delhi, India Feb. 5-7, 2014 Dual Frequency SBAS Trial and Preliminary Results Takeyasu Sakai Electronic Navigation Research Institute

2. Introduction • Dual Frequency SBAS = The solution for Ionosphere: • The dominant factor which lowers the performance of single frequency SBAS is the uncertainty of ionosphere, especially at the low magnetic latitude region; • Employing dual frequency system is an essential solution against ionosphere; It becomes no longer necessary to have a large margin for ionosphere threat; • The signal specification of dual frequency SBAS is now being discussed at SBAS IWG (interoperability working group) meeting as a preparation for standardization at the ICAO. • Simulation of Dual Frequency (DF) SBAS: • It is necessary to characterize the performance of dual frequency SBAS to assist making the standard properly; • We have implemented DF-SBAS simulator and evaluated the performance; • It is confirmed that employing DF system eliminates ionosphere threat and improves availability of the system especially for the ionospheric storm condition.

3. Motivation: Situation of MSAS • MSAS = Japanese SBAS: • Has been operational since Sept. 2007; • Configuration: 2 GEO (MTSAT-1R and MTSAT-2) + 2 MCS; • Single Frequency and Single Constellation (GPS only); • Achieves 100% availability for Enroute (RNP 0.3) to NPA flight modes within Fukuoka FIR. • Currently Horizontal Navigation Only: • MSAS is built on the IOC WAAS; • The major concern for vertical guidance is ionosphere; Users must be protected during ionospheric storm as well as normal condition; • Need to reduce ionospheric uncertainty to provide vertical guidance. MTSAT-1R GEO

4. APV-I Availability of MSAS MSAS Broadcast 06/10/17 00:00-24:00 PRN129 (MTSAT-1R) Test Signal Contour plot for: APV-I Availability HAL = 40m VAL = 50m Note: 100% availability of Enroute through NPA flight modes.

5. VPL Component VPL Ionosphere (5.33 sUIRE) Clock & Orbit (5.33 sflt) MSAS Broadcast 06/10/17 00:00-12:00 @93011 Tokyo PRN129 (MTSAT-1R) Test Signal • The ionospheric term is dominant component of Vertical Protection Level.

6. MSAS GMS Solution: Dual Frequency • Problem of MSAS: • The distribution of monitor stations is almost linear; Difficult to observe ionosphere enough; • The service area of MSAS contains a low magnetic latitude region where ionospheric disturbance is severe. • Dual Frequency Operation: • An essential solution against ionosphere; No longer necessary to have a large margin against ionosphere threat; • We need L5 signal for aviation use; Now we have 4 Block IIF satellites transmitting L5 signal; 24 satellites by 2020? • Japanese QZSS will also broadcast L5 signal; Planned 4 satellites by 2018.

7. Concerns • Amplified Measurement Noise: • Measurement for DF receivers, so-called Ionosphere-Free combination, is noisy due to differential computation between two frequencies; • 2.6 times of SF mode (L1 and L5); • 3.0 times of SF mode (L1 and L2). • This noise cannot be corrected by DGPS correction information. • No correlation between DGPS station and users. • Compatibility with Single Frequency (SF) Users: • Could two sets of SBAS messages generated for SF users and for DF users, respectively, be same? • In other words, is it possible to apply a set of SBAS messages to both DF users and SF users? • Need to Investigate These Concerns by Simulation. Ionosphere-Free Combination

8. User Receiver SBAS MCS (Simulator) SBAS Message Clock/Orbit Correction L1 Data L2 Data L1 Data MT 2 to 6, 24, and 25 New! DF SF New! MT 26 SF DF L2 Data Ionosphere Correction Position Computation DF SBAS Experiment • The software SBAS simulator is upgraded to be able to generate DF mode corrections; • Internal Ionosphere Correction is: • Based on broadcast MT26 (SF mode); • Linear combination of L1 and L2 pseudoranges (DF mode). • Message is based on the current standard. • The user receiver software is also upgraded for DF mode processing; • Ionosphere Correction is: • Based on received MT26 (SF mode); • Linear combination of L1 and L2 pseudoranges (DF mode).

9. Monitor and User Locations • Observation Data from GEONET: • Operated by Geospatial Information Authority of Japan; • Survey-grade receivers over 1,200 stations within Japanese territory; • RINEX archive open to public: Dual frequency (L1C/A and L2P/Y) measurement of 30s interval. • Monitor Stations: • Selected MSAS-like 6 stations from GEONET: (a) to (f). • User Stations: • Selected 15 stations from North to South: (1) to (15).

10. Result: Quiet Ionosphere GPS SF DF 12/7/22 to 12/7/25 96 Hours Max Kp=3 @GEONET 940058 (Takayama) # GMS: 6 Mask Angle: 5 deg • SF augmentation achieves the best accuracy (0.49m HRMS); • DF users suffer noisy measurement; Will be reduced using L5.

11. Result: Stormy Ionosphere GPS SF DF 11/10/23 to 11/10/26 96 Hours Max Kp=7 @GEONET 940058 (Takayama) # GMS: 6 Mask Angle: 5 deg • SF and GPS are largely affected by the ionospheric activity; • DF accuracy is not degraded.

12. Max Error RMS Accuracy Accuracy vs. Location: Quiet 12/7/22 to 12/7/25 96 Hours Max Kp=3 # GMS: 6 Mask Angle: 5 deg Large error at the south • SF augmentation achieves the best accuracy; • RMS accuracy has no relationship with the latitude of user; • The maximum error becomes large at the south for SF and standalone GPS.

13. Accuracy vs. Location: Quiet 12/7/22 to 12/7/25 96 Hours Max Kp=3 # GMS: 6 Mask Angle: 5 deg • Using DF, the maximum error tends to be large at the north.

14. Accuracy vs. Location: Storm 11/10/23 to 11/10/26 96 Hours Max Kp=7 # GMS: 6 Mask Angle: 5 deg • SF and DF augmentations expect similar accuracy at the mid-latitude region; • The accuracy of SF mode degrades at the southwestern islands; • DF augmentation maintains a constant accuracy regardless of the user location.

15. Accuracy vs. Location: Storm 11/10/23 to 11/10/26 96 Hours Max Kp=7 # GMS: 6 Mask Angle: 5 deg • The maximum error of SF mode becomes large at the southwestern islands; • In case of DF, the maximum error is not affected by the user location.

16. Integrity: Single Frequency User (1): Northenmost Station User (13): Near Naha (Southwestern Island) • Vertical Protection Level with regard to the actual error during ionospheric storm; • Unsafe condition does not exist at both user location; • The system is available if PL is less than AL; The availability of APV-I flight mode (VAL=50m) is 98% at User (1) and 50% at User (13) for SF mode.

17. Integrity: Dual Frequency User (1): Northenmost Station User (13): Near Naha (Southwestern Island) • Using DF, the availability of APV-I flight mode is 100% at both user location; • LPV-200 mode (CAT-I equivalent, VAL=35m) is also supported with 100% availability.

18. Compatibility: Quiet 12/7/22 to 12/7/25 96 Hours Max Kp=3 # GMS: 6 Mask Angle: 5 deg • Compatibility issue: Is it possible that DF users apply the set of messages generated by SF MCS? • The combination of SF MCS and DF users works not so bad.

19. Compatibility: Storm 11/10/23 to 11/10/26 96 Hours Max Kp=7 # GMS: 6 Mask Angle: 5 deg • DF users at the south reduce error regardless of MCS mode; • The set of messages generated by SF MCS could be applied to both SF and DF users; Further consideration needed in terms of integrity assurance.

20. Conclusion • Dual Frequency SBAS: • Dual Frequency SBAS simulator is implemented and tested successfully; • Generated message is based on the current standard for Single Frequency; This trial intends to characterize the performance of dual frequency SBAS to assist making the standard properly; • It is confirmed that employing DF system eliminates ionosphere threat and improves availability of the system especially for the ionospheric storm condition; • It might be possible that the set of messages generated by SF MCS could be applied to both SF and DF users; Need further study for this issue. • Ongoing and future works: • Improvement of DF mode accuracy; • Consideration of the message structure for DF operation; • Further investigation on the compatibility issue in terms of integrity assurance.