SBAS IWG/26 New Delhi, India Feb. 5-7, 2014 Dual Dual Frequency Frequency SBAS SBAS Trial Trial and and Preliminary Preliminary Results Results (Work (WorkPlan: Plan:Identify IdentifyBenefits) Benefits)
Takeyasu Takeyasu Sakai Sakai Electronic Electronic Navigation Navigation Research Research Institute, Institute, Japan Japan SBAS IWG/26 - Slide 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. SBAS IWG/26 - Slide 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 SBAS IWG/26 - Slide 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. SBAS IWG/26 - Slide 5 VPL Component VPL Ionosphere (5.33 UIRE) Clock & Orbit (5.33 flt) 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. SBAS IWG/26 - Slide 6 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. MSAS GMS 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. SBAS IWG/26 - Slide 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 Ionosphere-Free Combination 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? Investigate These Concerns using DF SBAS Simulator. SBAS IWG/26 - Slide 8 DF SBAS Experiment User Receiver SBAS MCS (Simulator) SBAS Message L1 Data Clock/Orbit
Correction New DF L1 Data MT 2 to 6, 24, and 25 L2 Data SF MT 26 L2 Data Ionosphere Correction
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. New SF DF Position Computation 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). SBAS IWG/26 - Slide 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). SBAS IWG/26 - Slide 10 Result: Quiet Ionosphere GPS SF DF SF augmentation achieves the best accuracy (0.49m HRMS); DF users suffer noisy measurement; Will be reduced using L5. 12/7/22 to 12/7/25 96 Hours Max Kp=3 @GEONET 940058 (Takayama) # GMS: 6 Mask Angle: 5 deg
SBAS IWG/26 - Slide 11 Result: Stormy Ionosphere GPS SF DF SF and GPS are largely affected by the ionospheric activity; DF accuracy is not degraded. 11/10/23 to 11/10/26 96 Hours Max Kp=7 @GEONET 940058 (Takayama) # GMS: 6 Mask Angle: 5 deg
SBAS IWG/26 - Slide 12 Accuracy vs. Location: Quiet 12/7/22 to 12/7/25 96 Hours Max Kp=3 # GMS: 6 Mask Angle: 5 deg RMS Accuracy Large error at the south Max Error 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. SBAS IWG/26 - Slide 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. SBAS IWG/26 - Slide 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. SBAS IWG/26 - Slide 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. SBAS IWG/26 - Slide 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. SBAS IWG/26 - Slide 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.
SBAS IWG/26 - Slide 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. SBAS IWG/26 - Slide 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. SBAS IWG/26 - Slide 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.
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