0. GPS/GLONASS Multi-Constellation SBAS Trial and Preliminary Results
ION GNSS 2012
Nashville, TN
Sept , 2012
GPS/GLONASS Multi-Constellation
SBAS Trial and Preliminary Results
for East-Asia Region
Takeyasu Sakai, H. Yamada, and K. Hoshinoo,
Electronic Navigation Research Institute

1. Introduction Combined use of GPS and GLONASS with SBAS augmentation:
ION GNSS Sept 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 other 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. Concept of SBAS Geostationary Satellites GPS Augmentation Signal
ION GNSS Sept Slide 2
Concept of SBAS
Geostationary
Satellites
GPS
Ground Monitor
Stations
Users
Uplink
Augmentation
Signal
Ranging
Accuracy
Integrity

3. Additional Constellation
ION GNSS Sept Slide 3
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.

4. in the Current SBAS Standard
ION GNSS Sept Slide 4
Part 1
GLONASS
in the Current SBAS Standard

5. The SBAS standard in the Annex to the Civil Aviation Convention
ION GNSS Sept Slide 5
Current SBAS Standard
Already have definition of GLONASS:
The SBAS standard is documented by ICAO (International Civil Aviation Organization);
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:
FDMA signals;
Reference time and coordination system;
PRN mask numbers;
IOD along with corrections; and
Satellite position computation.
The SBAS standard in the Annex to the Civil Aviation Convention

6. FDMA Signals FCN (Frequency Channel Number):
ION GNSS Sept Slide 6
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)

7. Time and Coordinate Systems
ION GNSS Sept Slide 7
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:

8. GLONASS slot number plus 37
ION GNSS Sept Slide 8
PRN Masks
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 !!!
Solution: Dynamic PRN Mask
Actually, PRN mask can change; Controlled by IODP (Issue of Data, PRN Mask);
RTCA MOPS states this occurs “infrequently” while SBAS SARPS does not.
Change PRN mask dynamically to reflect the actual visibility of satellites from the intended service area.
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

9. IOD (Issue of Data) IOD indicator along with corrections:
ION GNSS Sept Slide 9
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

10. IOD for GLONASS IOD for GLONASS satellites:
ION GNSS Sept Slide 10
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

11. Perturbation terms in ephemeris
ION GNSS Sept Slide 11
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

12. GLONASS Ephemeris Item Bits Range Resolution Contents tb 7 15-1425 min
ION GNSS Sept Slide 12
GLONASS Ephemeris
Item
Bits
Range
Resolution
Contents tb 7
min
15 min
Epoch time tn 22
2-9 s
2-30 s
Clock correction (const) gn 11
2-30 s/s
2-40 s/s
Clock correction (1st order) x 27
27000 km
2-11 km
Position X in ECEF y
Position Y in ECEF z
Position Z in ECEF vx 24
4.3 km/s
2-20 km/s
Velocity X in ECEF vy
Velocity Y in ECEF vz
Velocity Z in ECEF 5
6.2 mm/s2
2-30 km/s2
Acceleration X in ECEF (only perturbation)
Acceleration Y in ECEF (only perturbation)
Acceleration Z in ECEF (only perturbation)
Total
208 x y z ..

13. Implementation and Experiment
ION GNSS Sept Slide 13
Part 2
Implementation and Experiment

14. Software Implementation
ION GNSS Sept Slide 14
Software Implementation
ENRI’s software SBAS simulator:
‘RTWAD’ runs on usual PC and Linux Workstations;
Generates SBAS message stream: one message per second;
Run modes:
Offline operation mode: for preliminary investigation using RINEX files;
Realtime operation mode: verification of actual performance with realtime raw data.
Needs user-domain receiver software to evaluate performance.
Upgrade for GLONASS (and QZSS):
Input module: RINEX observation and navigation files containing GLONASS;
Implemented GLONASS extension of SBAS standard;
User-domain receiver software is also upgraded to be GLONASS-capable;
QZSS is also supported as it is taken into account like GPS.
Software SBAS Simulator (RTWAD)
User-Domain Receiver
Software
Network GPS
observables
Position Error
SBAS Message Stream
Position Output
User-side observations
Reference station observations

15. Dynamic PRN Mask Dynamic PRN mask: Transition of PRN mask:
ION GNSS Sept Slide 15
Dynamic PRN Mask
Dynamic PRN mask:
Changes PRN mask dynamically to reflect the actual visibility of satellites;
Set PRN masks ON for satellites whose pseudorange observations are available; Not based on prediction by almanac information not provided by RINEX;
Semi-dynamic PRN mask: Fix masks ON for GPS and QZSS, and change dynamically only for GLONASS to reduce receiver complexity.
Transition of PRN mask:
Periodical update of PRN mask: updates every 30 minutes;
Transition time 180s is given to users to securely catch the new PRN mask. FC
PRN Mask (IODP=i)
PRN Mask (IODP=i+1)
tcutover
180s
LTC
Corrections before cutover
Corrections after cutover
Transition time
Cutover

16. Time offset broadcast to users
ION GNSS Sept Slide 16
GLONASS Time Offset
Realtime computation:
Computes as the difference between receiver clocks for a group of GPS satellites (and QZSS) and the other group of GLONASS satellites;
Enough accuracy with a filter of long time constant;
Need no almanac information broadcast by GLONASS satellites;
Transmitted to users via Message Type 12 of SBAS. t
tGPS
tGLONASS tR
DtGPS
DtGLONASS
BGLONASS ^
BGPS
True
Time
GLONASS
System Time
GPS
Receiver
-daGLONASS
Receiver clock for
GPS satellites
Time offset broadcast to users

17. Experiment: Monitor Stations
ION GNSS Sept Slide 17
Experiment: Monitor Stations
Recently Japanese GEONET began to provide GLONASS and QZSS observables in addition to GPS;
Currently more than 150 stations are GLONASS/QZSS-capable;
Data format: RINEX 2.12 observation and navigation files.
For our experiment:
8 sites for reference stations;
Reference Station (1) to (8)
3 sites for evaluation.
User Station (a) to (c)
Period: 12/7/18 – 12/7/20 (3 days).

18. PRN Mask Transition QZSS GLONASS GPS
ION GNSS Sept Slide 18
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 for GLONASS satellites are set ON if the satellite are visible and augmented;
Stair-like shape: because the slot number of GLONASS satellites are assigned increasingly along with the orbit.
IODP (issue of Data, PRN Mask) indicates change of PRN mask.
GLONASS
GPS

19. Elevation Angle GPS GLONASS QZSS PRN Mask Transition 5 deg @ Tokyo
ION GNSS Sept Slide 19
Elevation Angle
GPS
GLONASS
QZSS
PRN Mask
Transition
5 deg
@ Tokyo
Rising satellites appear at 5-12 deg above the horizon; Latency due to periodical update of PRN mask;
However, GPS satellites also have similar latency; Not a major problem because low elevation satellites contribute a little to improve position accuracy.

20. # of Satellites vs. Mask Angle
ION GNSS Sept Slide 20
# of Satellites vs. Mask Angle
17 SVs
9.8 SVs
7.4 SVs
@ User (b)
Introducing GLONASS satellites increases the number of satellites in roughly 75%;
QZSS increases a satellite almost all day by only a satellite on the orbit, QZS-1 "Michibiki"
Multi-constellation with QZSS offers 17 satellites at 5 deg and 9.8 satellites even at 30 deg.

21. Availability vs. Mask Angle
ION GNSS Sept Slide 21
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.

22. DOP vs. Mask Angle HDOP = 2.3 @ User (b)
ION GNSS Sept Slide 22
DOP vs. Mask Angle
HDOP = 2.3
@ User (b)
GLONASS-only users suffer poor geometries;
Multi-constellation with QZSS offers HDOP of 2.3 even for 40 deg mask.

23. User Position Error: Mask 5deg
ION GNSS Sept Slide 23
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.

24. User Position Error: Mask 30deg
ION GNSS Sept Slide 24
User Position Error: Mask 30deg
GPS+GLO+QZS: 0.372m RMS of horizontal error at user location (b);
Multi-constellation offers a good availability even for 30 deg mask.

25. RMS Error vs. Mask: User (a)
ION GNSS Sept Slide 25
RMS Error vs. Mask: User (a)
0.528m
@ User (a)
Northernmost user location;
Multi-constellation provides robust position information through mask angle of 5 to 40 deg.

26. RMS Error vs. Mask: User (b)
ION GNSS Sept Slide 26
RMS Error vs. Mask: User (b)
0.602m
@ User (b)
User location near the centroid of reference station network;
For vertical direction, 10 deg mask shows the best accuracy except GLONASS only case.

27. RMS Error vs. Mask: User (c)
ION GNSS Sept Slide 27
RMS Error vs. Mask: User (c)
0.588m
@ User (c)
Southernmost user location;
There is little dependency upon user location; possibly because ionosphere condition is quiet for the period of this experiment.

28. Vertical Protection Level
ION GNSS Sept Slide 28
Vertical Protection Level
Reduce
GPS only
GPS+GLO+QZS
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; Means improvement of availability of navigation.

29. Conclusion Combined use of GPS and GLONASS with SBAS:
ION GNSS Sept Slide 29
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:
Support of realtime operation mode;
Realtime operation test broadcasting augmentation information for both GPS and GLONASS on QZSS L1-SAIF augmentation channel;
Use GLONASS observables in generation of ionospheric correction;
Mixed use of different types of receiver for reference stations;
Further extension to support Galileo.