0. SBAS Satellite Selection and Performance Monitoring at the Region
ION GNSS+ 2014
Tampa, FL
Sept. 8-12, 2014
SBAS Satellite Selection
and Performance Monitoring at the Region
Where Multiple SBAS are Available
Takeyasu Sakai, Kazuaki Hoshinoo, and Ken Ito
Electronic Navigation Research Institute

1. Introduction SBAS Selection Problem: Necessity of Monitoring:
ION GNSS+ Sept Slide 1
Introduction
SBAS Selection Problem:
We already have four operational SBAS served by the US, Japan, Europe, and India;
A possible issue for users in Enroute to NPA flight modes: An SBAS does not broadcast any information on the service area of SBAS nor information on which SBAS should be used; The current SBAS specifications do not define any procedure, or satellite selection criteria, to be used by receivers to choose SBAS satellites for use;
In order to investigate the situation, the authors considered the expected NPA (or RNP0.3) availability for some SBAS selection criteria based on the actual broadcast message of MSAS and GAGAN.
Necessity of Monitoring:
Monitoring the performance of the system operated by other States in a specific airspace as a responsibility of ANSP for the airspace;
As an example, a result of evaluation of the performance of GAGAN in Japanese territory is shown.

2. Motivation Geostationary Satellites: MSAS MTSAT-1R @140E MTSAT-2 @145E
ION GNSS+ Sept Slide 2
Motivation
GSAT-8
GSAT-10
MTSAT-1R
MTSAT-2
Geostationary Satellites:
MSAS
GAGAN
India began the operation of GAGAN in Feb by removing MT0;
In Japan, we can receive GAGAN signals in addition to MSAS signals;
Which SBAS does the SBAS-receiver use for augmentation?
Likely MSAS, but using GAGAN is allowed and possible;
There is some reports on observation of some receivers using GAGAN.

3. MSAS NPA-Mode Availability
ION GNSS+ Sept Slide 3
MSAS NPA-Mode Availability
Available
Without GPS Outage
1 Critical GPS Out
NPA mode (RNP0.3 for Enroute) availability for 3 days of May 25 to 27, 2014;
MSAS has been serving NPA mode horizontal navigation with 100% availability over almost whole of Fukuoka (Japanese) FIR;
In condition of 1 critical GPS out, the area with NPA service is largely reduced and roughly half of the FIR is supported with 100% availability.

4. GAGAN NPA-Mode Availability
ION GNSS+ Sept Slide 4
GAGAN NPA-Mode Availability
Available
Without GPS Outage
1 Critical GPS Out
For GAGAN, the area with NPA service is larger than MSAS;
Without any GPS satellite outage, NPA mode availability is 100% over almost whole of India FIR, while the area with NPA service is largely reduced if 1 critical GPS is out;
In Japan, GAGAN can provide NPA service with 95 to 100% availability.

5. ION GNSS+ Sept Slide 5
Other Systems
Russian SDCM (System for Differential Corrections and Monitoring):
Already broadcasting test signal from their geostationary satellites.
Consists of 3 geostationary satellites:
Luch-5A launched in 2011 at 167E
Luch-5B launched in 2012 at 16W
Luch-5V to be launched in 2014 and placed at 95E
In Japan, signals from Luch-5A and Luch-5V are possibly received; We have already received PRN140 signal from Luch-5A.
Chinese SNAS:
Served as a part of BeiDou Satellite Navigation System;
Consists 3 Geostationary satellites at 80E, 110E, and 140E; All signals can be received in Japan.
Korean K-SBAS:
Potentially 1 or 2 more SBAS geostationary satellites by around 2020.

6. SBAS in the World SDCM SNAS SACCSA Operational Since 2003 Since 2011
ION GNSS+ Sept Slide 6
SBAS in the World
SDCM
SNAS
SACCSA
Operational
Since 2003
Since 2011
Since 2007
Since 2014

7. ION GNSS+ Sept Slide 7
SBAS Selection Issue
SBAS does not broadcast any information for selection:
No information on the service area of SBAS and no information on which SBAS should be used by a user in NPA mode;
For PA mode, FAS (Final Approach Segment) data specifies SBAS to be used.
SBAS specifications:
The ICAO SBAS SARPS and RTCA WAAS MOPS do not define the specific procedure, or satellite selection criteria, to be used by receivers.
The RTCA MOPS states an option for SBAS selection as a note:
“Selecting the SBAS satellite with the highest elevation angle that is broadcasting applicable ionospheric corrections is normally sufficient.”
Receiver manufacturers have no restriction on the algorithm for the selection; Each user receiver has its own satellite selection scheme if it is tracking multiple SBAS signals; The situation depends on the implementation.
What are possible criteria for SBAS selection?

8. Possible Criteria SBAS Satellite Selection Criteria:
ION GNSS+ Sept Slide 8
Possible Criteria
SBAS Satellite Selection Criteria:
In the literature by Jed Dennis at the ION ITM 2014, some possible criteria are listed and evaluated;
Here we consider possible criteria which do not need human interaction and/or database information.
Elevation Angle [EL]:
Selects SBAS signal coming from the highest elevation angle;
SBAS selection becomes a function of longitude of receiver location only;
No direct relationship with distribution of SBAS ground network which dominates the spatial extent to which the augmentation information can be applied.

9. Possible Criteria Elevation Angle and Ionosphere [ELi]:
ION GNSS+ Sept Slide 9
Possible Criteria
Elevation Angle and Ionosphere [ELi]:
Selects SBAS signal coming from the highest elevation angle and providing the applicable ionospheric correction information;
Straightforward implementation along with the note of MOPS;
Here we assume the determination if the applicable ionospheric information is or not is done by testing the availability of IGPs at the user location;
This option does not directly reflect the distribution of ground network, but ionosphere information cannot be obtained at the location far away from the service area.
Protection Levels [HPL] [VPL]:
Selects SBAS signal giving lower protection level (HPL or VPL) which means better performance;
In the NPA mode, it is natural to employ HPL for the selection because SBAS gives just a horizontal navigation; Selection based on VPL is also possible;
Advantage: Reflects the performance of each SBAS in real time.
Computational load inside the receiver is relatively high.

10. Possible Criteria Number of Satellites [NSV]:
ION GNSS+ Sept Slide 10
Possible Criteria
Number of Satellites [NSV]:
Selects SBAS signal which gives the largest number of GPS satellites available;
Reflects the spatial relationship between each GPS satellite and the ground network in real time.
Degradation of Correction [dUDRE]:
Selects SBAS signal giving less dUDRE;
The dUDRE indicates spatial degradation of the accuracy of correction information for each GPS satellite; Basically dUDRE grows larger if the GPS satellite goes away from the ground monitoring network;
This criterion is computed for each individual GPS satellite, not for the SBAS itself; Here we assume that the receiver compares the number of GPS satellites which gives lower dUDRE than the other SBAS, and selects the SBAS which have the largest number of such GPS satellites.
The computational load would be medium.

11. Possible Criteria Criteria Designator Reflecting Performance
ION GNSS+ Sept Slide 11
Possible Criteria
Criteria
Designator
Reflecting Performance
Reflecting Spatial Relationship
Computational Load
Elevation Angle
[EL] No
Low
Elevation Angle and Ionosphere
[ELi]
A Little
Medium
Horizontal Protection Level
[HPL]
Yes
High
Vertical Protection Level
[VPL]
Number of Satellites
[NSV]
Degradation of Correction
[dUDRE]
Here we assume GAGAN-preferred receiver: It is assumed that GAGAN is selected in case of the equal evaluation between two SBAS for [NSV] and [dUDRE].

12. SBAS Selection [EL] [ELi]
ION GNSS+ Sept Slide 12
SBAS Selection [EL] [ELi]
Selection by [EL]
Selection by [ELi]
GAGAN selection rate for [EL] and [ELi];
SBAS selection is a function of longitude of user location;
The border is 111.5E = mean of 83E (GSAT-10) and 140E (MTSAT-1R);
This option has the temporal invariance: The result is simply 0% or 100%;
With ionosphere option, the spatial relationship is reflected a little.

13. SBAS Selection [HPL] [VPL]
ION GNSS+ Sept Slide 13
SBAS Selection [HPL] [VPL]
Selection by [HPL]
Selection by [VPL]
GAGAN selection rate for [HPL] and [VPL];
Show which SBAS gives the lower HPL/VPL between MSAS and GAGAN as a function of user location.
The performances of MSAS and GAGAN are different; At the transition region, GAGAN likely provides the better performance than MSAS.

14. SBAS Selection [NSV] [dUDRE]
ION GNSS+ Sept Slide 14
SBAS Selection [NSV] [dUDRE]
Selection by [NSV]
Selection by [dUDRE]
GAGAN selection rate for [NSV] and [dUDRE];
[NSV] Smooth and linear characteristics along with a line connecting the centroids of ground networks of both SBAS.
[dUDRE] Possibly remote monitor stations of MSAS in Hawaii and Australia contribute to the better orbit determination and thus to provide the lower dUDRE.

15. NPA Availability: GAGAN Only
ION GNSS+ Sept Slide 15
NPA Availability: GAGAN Only
GAGAN Only
GAGAN Only (1 GPS OUT)
Baseline GAGAN NPA availability with no SBAS selection: A case that MSAS is not available.

16. Selection by [EL] (1 GPS OUT)
ION GNSS+ Sept Slide 16
NPA Availability [EL]
Selection by [EL]
Selection by [EL] (1 GPS OUT)
NPA availability with SBAS selection of the option [EL];
Switching two SBAS improves availability largely and expand the area where NPA mode is available; Navigation is available if either two SBAS is available;
But this option is not the optimum solution.

17. NPA Availability [ELi]
ION GNSS+ Sept Slide 17
NPA Availability [ELi]
Selection by [ELi]
Selection by [ELi]
Selection by [ELi] (1 GPS OUT)
Selection by [ELi] (1 GPS OUT)
NPA availability with SBAS selection of the option [ELi];
Ionosphere restriction reduces availability at the edge of the service area of either SBAS.

18. NPA Availability [HPL]
ION GNSS+ Sept Slide 18
NPA Availability [HPL]
Selection by [HPL]
Selection by [HPL] (1 GPS OUT)
NPA availability with SBAS selection of the option [HPL];
This option gives the best availability characteristics among candidate criteria; This option is the optimum solution giving the largest area of 100% NPA availability.

19. NPA Availability [VPL]
ION GNSS+ Sept Slide 19
NPA Availability [VPL]
Selection by [VPL]
Selection by [VPL] (1 GPS OUT)
NPA availability with SBAS selection of the option [VPL];
The characteristics is similar to [HPL] but sub-optimum because availability of NPA mode is determined based on HPL.

20. NPA Availability [NSV]
ION GNSS+ Sept Slide 20
NPA Availability [NSV]
Selection by [NSV]
Selection by [NSV] (1 GPS OUT)
NPA availability with SBAS selection of the option [NSV];
Availability is similar to [HPL] and [VPL] in case of no GPS outage;
In case of 1 critical GPS out, availability is better than [EL] and [ELi], but lower than [HPL] and [VPL], somewhere for example Philippine Sea.

21. NPA Availability [dUDRE]
ION GNSS+ Sept Slide 21
NPA Availability [dUDRE]
Selection by [dUDRE]
Selection by [dUDRE] (1 GPS OUT)
NPA availability with SBAS selection of the option [dUDRE];
Availability degrades from [NSV] although this option requires more computational load.

22. Combined Use of SBAS Simultaneous usage of multiple SBAS:
ION GNSS+ Sept Slide 22
Combined Use of SBAS
Simultaneous usage of multiple SBAS:
The MOPS allows receivers in the NPA mode to use multiple SBAS simultaneously;
The receivers have an option to use a combination of GPS satellites augmented by an SBAS and other GPS satellites augmented by another SBAS which may be operated by a different service provider;
Ionosphere correction is made by GPS navigation message other than SBAS;
In the current situation, receivers in South East Asia receiving MSAS and GAGAN may operate in such a way.
Computation of availability achieved by multiple SBAS:
Assumed procedure in the receiver:
(i) Determine the primary SBAS based on one of the selection criteria;
(ii) Apply augmentation information provided by the primary SBAS to GPS satellites augmented by the SBAS;
(iii) For remaining GPS satellites, apply augmentation information provided by the other SBAS with consideration of time offset by adding 100ns to uncertainties of range measurements of these GPS satellites as described in the MOPS.

23. Baseline: SBAS Selection [HPL]
ION GNSS+ Sept Slide 23
Baseline: SBAS Selection [HPL]
Selection by [HPL]
Selection by [HPL] (1 GPS OUT)
NPA availability with SBAS selection of the option [HPL];
Investigates if combined use of SBAS improves availability from this baseline or not; In other words, identifies advantage of receivers which have the algorithm of combined use of multiple SBAS.

24. Combined Use Combined Use: GAGAN+MSAS
ION GNSS+ Sept Slide 24
Combined Use
Combined Use: GAGAN+MSAS
Combined use: GAGAN+MSAS (1 GPS OUT)
Combined use of SBAS based on [EL] achieves availability better than the result of single SBAS selection by option [HPL];
Results of other options are very similar.

25. Combined use: GAGAN+MSAS
ION GNSS+ Sept Slide 25
Combined Use
Select GAGAN or MSAS
Combined use: GAGAN+MSAS
Comparison of NPA availability between single SBAS and combined use of SBAS in case of 1 critical GPS out;
Combined use of SBAS even based on selection option [EL] achieves availability better than the result of single SBAS selection by option [HPL].

26. Monitoring SBAS Monitoring SBAS: GNSS Monitor System:
ION GNSS+ Sept Slide 26
Monitoring SBAS
Monitoring SBAS:
Another issue might be the necessity of monitoring the performance of the system operated by other States;
SBAS is used as a primary means of navigation; Is it allowable to fly in the airspace of a State dependently upon a primary means of navigation provided by another State?
A possible way to relax legal issues caused by using SBAS operated by another State is to monitor the performance of the SBAS; If necessary, warn to users in the airspace in case that the SBAS should not be used;
Actually, in Spring, MSAS has broadcast messages with illegal preambles.
GNSS Monitor System:
In order to monitor an SBAS, one also needs to monitor GPS satellites augmented by the SBAS to check correctness of augmentation information;
Japan intends to implement monitor systems for GNSS including GPS and SBAS whose signal can be received in Japanese territory.

27. GAGAN Integrity at Tokyo
ION GNSS+ Sept Slide 27
GAGAN Integrity at Tokyo
All-in-View Mode
All Possible Combinations
Triangle chart comparing HPL and the associated position error provided by GAGAN GSAT-10 in Tokyo, Japan;
The availability of NPA mode is 97.8% for all-in-view receivers;
In terms of safety assurance, all possible combinations should be tested like the chart at the right; Unsafe condition that the error exceeds HPL did not occur for this period;
Furthermore, all possible combinations of active SBAS messages should be tested.

28. Conclusion SBAS Selection Problem: Necessity of Monitoring:
ION GNSS+ Sept Slide 28
Conclusion
SBAS Selection Problem:
The current SBAS specifications do not define any procedure, or satellite selection criteria, to be used by receivers to choose SBAS satellites for use;
The expected NPA (RNP0.3 for Enroute) availability for some SBAS selection criteria is computed based on the actual broadcast message of MSAS and GAGAN;
Criteria based on HPL or NSV have advantage to other options like a selection by elevation angle;
Combined use of SBAS also improves availability.
Necessity of Monitoring:
Monitoring the performance of the system operated by other States;
As an example, a result of evaluation of the performance of GAGAN in Japanese territory was shown;
It is necessary to test all possible combinations of ranging sources and augmentation information for integrity assurance.