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

Dual Frequency SBAS Trial and Preliminary Results (Work Plan: Identify Benefits) Takeyasu Sakai

Electronic Navigation Research Institute, Japan

SBAS IWG/26 - Slide 1

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 2

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 3

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 4

VPL Component

VPL Ionosphere (5.33 s UIRE ) Clock & Orbit (5.33 s 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.

Solution: Dual Frequency

SBAS IWG/26 - Slide 5

• 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 6

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.

Ionosphere-Free Combination • 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 7

DF SBAS Experiment

SBAS MCS (Simulator) SBAS Message User Receiver L1 Data New DF SF Clock/Orbit Correction MT 2 to 6, 24, and 25 L1 Data L2 Data New MT 26 SF DF L2 Data Ionosphere Correction Position Computation • 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).

SBAS IWG/26 - Slide 8

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).

Result: Quiet Ionosphere

SBAS IWG/26 - Slide 9

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 10

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 11

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 12

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 13

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 14

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 15

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.

Integrity: Dual Frequency

SBAS IWG/26 - Slide 16

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.

Compatibility: Quiet

SBAS IWG/26 - Slide 17

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.

Compatibility: Storm

SBAS IWG/26 - Slide 18

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 19

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.