0. Dual Frequency SBAS Trial and Preliminary Results
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

1. Introduction Dual Frequency SBAS = The solution for Ionosphere:
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.

2. Motivation: Situation of MSAS
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

3. APV-I Availability of MSAS
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.

4. VPL Component VPL Ionosphere (5.33 sUIRE) Clock & Orbit (5.33 sflt)
SBAS IWG/26 - Slide 4
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.

5. Solution: Dual Frequency
SBAS IWG/26 - Slide 5
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.
MSAS GMS

6. Concerns Amplified Measurement Noise:
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.
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.
Ionosphere-Free Combination

7. SBAS IWG/26 - Slide 7
DF SBAS Experiment
L1 Data
L2 Data
Ionosphere
Correction
Clock/Orbit
Position
Computation
SBAS MCS (Simulator)
User Receiver
SBAS Message
MT 2 to 6,
24, and 25
MT 26
New SF DF
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).

8. Monitor and User Locations
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).

9. Result: Quiet Ionosphere
SBAS IWG/26 - Slide 9
Result: Quiet Ionosphere
GPS SF DF
12/7/22 to 12/7/25
96 Hours Max Kp=3
@GEONET
(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.

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

11. Accuracy vs. Location: Quiet
SBAS IWG/26 - Slide 11
Accuracy vs. Location: Quiet
Max
Error
RMS
Accuracy
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.

12. Accuracy vs. Location: Quiet
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.

13. Accuracy vs. Location: Storm
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.

14. Accuracy vs. Location: Storm
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.

15. Integrity: Single Frequency
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.

16. Integrity: Dual Frequency
SBAS IWG/26 - Slide 16
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.

17. SBAS IWG/26 - Slide 17
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.

18. SBAS IWG/26 - Slide 18
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.

19. Conclusion Dual Frequency SBAS: Ongoing and future works:
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.