0. GPS/GLONASS Multi-Constellation SBAS Trial
SBAS IWG/25
St. Petersburg, Russia
June 25-27, 2013
GPS/GLONASS
Multi-Constellation SBAS Trial
Takeyasu Sakai
Electronic Navigation Research Institute

1. Introduction Combined use of GPS and GLONASS with SBAS augmentation:
IWG/25 June Slide 1
Introduction
Combined use of GPS and GLONASS with SBAS augmentation:
GPS/GLONASS-capable receivers are now widely available;
SBAS (satellite-based augmentation system) is an international standard of the augmentation system; US WAAS, Japanese MSAS, and European EGNOS are already operational;
All operational SBAS are augmenting only GPS;
To improve availability of SBAS-augmented position information, a possible way is extending SBAS to support an additional constellation, e.g., GLONASS.
Possibility of Multi-Constellation SBAS (MC SBAS):
SBAS specification already has definitions necessary to augment GLONASS;
Investigating advantages of using GLONASS, we have implemented SBAS simulator capable of augmenting both GPS and GLONASS simultaneously;
It is confirmed that introducing GLONASS improves availability and robustness of position information especially where visibility is limited.

2. Additional Constellation
IWG/25 June Slide 2
Motivation
SBAS
GEO
Augmentation
GPS Constellation
Additional Constellation
= GLONASS
Increase of augmented satellites improves availability of position solution;
Also possibly reduce protection levels; Improve availability of navigation;
Chance of robust position information at mountainous areas and urban canyons.

3. The SBAS standard in the Annex to the Civil Aviation Convention
IWG/25 June Slide 3
Current SBAS Standard
Already has definition of GLONASS:
The SBAS standard is documented as the ICAO SARPS;
GLONASS L1 CSA (channel of standard accuracy) signal has already been described in the SBAS standard based on GLONASS ICD;
SBAS signal is also able to contain information on GLONASS satellites.
Differences from GPS in terms of SBAS augmentation:
(1) FDMA signals;
(2) Reference time and coordination system;
(3) PRN mask numbers;
(4) Missing IOD for ephemeris; and
(5) Satellite position computation.
The SBAS standard in the Annex to the Civil Aviation Convention

4. (1) FDMA Signals FCN (Frequency Channel Number):
IWG/25 June Slide 4
(1) FDMA Signals
FCN (Frequency Channel Number):
GLONASS ICD defines FCN of –7 to +13;
Historically 0 to +13 were used; After 2005 the range of FCN shifts to –7 to +6;
FCN cannot be used for identification of satellites; two satellites share the same FCN.
Difference of carrier frequency affects:
Carrier smoothing:
Wave length per phase cycle is dependent upon carrier frequency.
Ionospheric corrections:
Ionospheric propagation delay is inversely proportional to square of carrier frequency.
(GLONASS ICD v5.0)

5. (2) Time and Coordinate Systems
IWG/25 June Slide 5
(2) Time and Coordinate Systems
GLONASS Time:
GLONASS is operating based on its own time system: GLONASS Time;
The difference between GPS Time and GLONASS Time must be taken into account for combined use of GPS and GLONASS;
The difference is not fixed and slowly changing: about 400ns in July 2012;
SBAS broadcasts the difference by Message Type 12;
GLONASS-M satellites are transmitting the difference as parameter tGPS in almanac (non-immediate) data: tGPS = tGPS − tGLONASS.
PZ-90 Coordinate System:
GLONASS ephemeris is derived based on Russian coordinate system PZ-90;
The relationship between WGS-84
and the current version of PZ-90
(PZ-90.02) is defined in the SBAS
standard as the equation:
No need for PZ ?

6. GLONASS slot number plus 37
IWG/25 June Slide 6
(3) PRN Mask
PRN Mask:
SBAS transmits PRN mask information
indicating satellites which are augmented
by the SBAS;
PRN number has range of 1 to 210;
Up to 51 satellites out of 210 can be
augmented simultaneously by the single
SBAS signal;
But, 32 GPS + 24 GLONASS = 56 !!!
A solution: Dynamic PRN Mask
Actually, PRN mask can change; Controlled by IODP (Issue of Data, PRN Mask);
RTCA MOPS states this occurs “infrequently” while ICAO SARPS does not.
Change PRN mask dynamically (for GLONASS satellites only; semi-dynamic PRN masking) to reflect the actual visibility from the intended service area;
This is a tentative implementation for this MC-SBAS trial by ENRI.
PRN definition for SBAS
PRN
Contents
1 to 37
GPS
38 to 61
GLONASS slot number plus 37
62 to 119
Spare
120 to 138
SBAS
139 to 210

7. (4) IOD (Issue of Data) IOD indicator along with corrections:
IWG/25 June Slide 7
(4) IOD (Issue of Data)
IOD indicator along with corrections:
LTC (Long-Term Correction) in SBAS Message Type 24/25 contains orbit and clock corrections;
Such corrections depend upon ephemeris data used for position computation;
IOD indicates which ephemeris data should be used in receivers.
IOD for GPS satellites:
For GPS, IOD is just identical with IODE of ephemeris data.
Previous Ephemeris
IODE=a
Next Ephemeris
IODE=b
LTC
IOD=a
IOD=b
Time

8. IOD for GLONASS IOD for GLONASS satellites:
IWG/25 June Slide 8
IOD for GLONASS
IOD for GLONASS satellites:
GLONASS ephemeris has no indicator like IODE of GPS ephemeris;
IOD for GLONASS satellites consists of Validity interval (V) and Latency time (L) to identify ephemeris data to be used:
5 MSB of IOD is validity interval, V;
3 LSB of IOD is latency time, L.
User receivers use ephemeris data transmitted at a time within the validity interval specified by L and V.
Ephemeris Validity
Interval L1 V1
Previous Ephemeris
Next Ephemeris
LTC
IOD=V1|L1 V2
IOD=V2|L2 L2
Time

9. Perturbation terms in ephemeris
IWG/25 June Slide 9
(5) Satellite Position
GLONASS ephemeris data:
GLONASS transmits ephemeris information as position, velocity, and acceleration in ECEF;
Navigation-grade ephemeris is provided in 208 bits for a single GLONASS SV;
Broadcast information is valid for 15 minutes or more.
Numerical integration is necessary to compute position of GLONASS satellites;
Note: centripental acceleration is removed from transmitted information.
These terms can be computed for the specific position and velocity of SV;
GLONASS ICD A gives the equations below (with some corrections).
Perturbation terms in ephemeris

10. IWG/25 June Slide 10
MC-SBAS Experiment
ENRI’s software SBAS simulator is upgraded to support GLONASS and Japan’s QZSS constellations.
QZSS currently contains only 1 IGSO broadcasting PRN 193 on L1C/A;
The software generates the complete SBAS message stream based on input measurements given as RINEX files.
GNSS receiver network: GEONET
More than 1,200 stations are GLONASS/ QZSS-capable;
Data format: RINEX 2.12 observation and navigation files.
Monitor stations for this experiment:
8 Reference Stations: (1) to (8).
3 User Stations: (a) to (c); In this presentation, discussion for user (b) only.
Period: 2012/7/18 to 2012/7/20 (3 days).
User Location

11. PRN Mask Transition QZSS GLONASS GPS
IWG/25 June Slide 11
PRN Mask Transition
QZSS
Reflecting our implementation, PRN mask is updated periodically at every 30 minutes;
Semi-dynamic PRN mask: GPS and QZSS satellites are always ON in the masks;
PRN masks are set ON for GLONASS satellites visible from 1 or more stations; Set OFF if not visible.
IODP (issue of Data, PRN Mask) indicates change of PRN mask at every 30 minutes.
GLONASS
GPS

12. Elevation Angle GPS GLONASS QZSS PRN Mask Transition 5 deg @ User (b)
IWG/25 June Slide 12
Elevation Angle
GPS
GLONASS
QZSS
PRN Mask
Transition
5 deg
@ User (b)
Rising satellites appear at 5-12 deg above the horizon; Latency due to periodical update of PRN mask without prediction by almanac;
However, GPS satellites also have similar latency; The latency of GLONASS satellites would not be a major problem.

13. # of Satellites vs. Mask Angle
IWG/25 June Slide 13
# of Satellites vs. Mask Angle
17 SVs
9.8 SVs
7.4 SVs
@ User (b)
Introducing GLONASS satellites increases the number of satellites roughly 75%;
QZSS increases a satellite almost all day by only a satellite on the orbit, QZS-1;
Multi-constellation with QZSS offers 17 satellites for 5 deg mask angle and 9.8 satellites even for 30 deg.

14. Availability vs. Mask Angle
IWG/25 June Slide 14
Availability vs. Mask Angle
100% Availability
@ User (b)
The number of epochs with position solution decreases with regard to increase of mask angle;
Multi-constellation with QZSS achieves 100% availability even for 40 deg mask.

15. User Position Error: Mask 5deg
IWG/25 June Slide 15
User Position Error: Mask 5deg
GPS+GLO+QZS: 0.310m RMS of horizontal error at user location (b);
Looks some improvement by using multi-constellation.

16. User Position Error: Mask 30deg
IWG/25 June Slide 16
User Position Error: Mask 30deg
GPS+GLO+QZS: 0.372m RMS of horizontal error at user location (b);
Multi-constellation offers good accuracy even for 30 deg mask.

17. RMS Error vs. Mask Angle 0.602m @ User (b)
IWG/25 June Slide 17
RMS Error vs. Mask Angle
0.602m
@ User (b)
User location near the centroid of reference station network;
The accuracy degrades but is maintained to 0.6m for horizontal even for 40deg mask angle by using GLONASS and QZSS as well as GPS.

18. Vertical Protection Level
IWG/25 June Slide 18
Vertical Protection Level
Reduce
GPS only
GPS+GLO+QZS
@ User (b)
Protection levels mean the confidence limit at % confidential level;
In these chart, unsafe condition exists if there are plots at the right of the diagonal line;
GLONASS reduces VPL; This means improvement of availability of navigation.

19. Conclusion Combined use of GPS and GLONASS with SBAS:
IWG/25 June Slide 19
Conclusion
Combined use of GPS and GLONASS with SBAS:
Multi-constellation SBAS, capable of augmenting both GPS and GLONASS, and additionally QZSS, is implemented and tested successfully;
Potential problems and solutions on realizing a multi-constellation SBAS based on the current standard were investigated;
It is confirmed that the performance of SBAS-aided navigation is certainly improved by adding GLONASS, especially when satellite visibility is limited;
Adding GLONASS also reduces protection levels and thus improves availability of navigation.
Ongoing and future works:
Realtime operation test to broadcast multi-constellation augmentation information via QZSS L1-SAIF augmentation channel; Preliminary tests have been conducted often in this year successfully;
Using GLONASS observables in generation of ionospheric correction;
Mixed use of different types of receiver for reference/user stations;
Further extension to support Galileo.