0. QZSS L1-SAIF Supporting GPS/GLONASS Multi-Constellation Augmentation
ION ITM 2013
San Diego, CA
Jan , 2013
QZSS L1-SAIF Supporting
GPS/GLONASS Multi-Constellation
Augmentation
T. Sakai, H. Yamada, K. Hoshinoo, and K. Ito
Electronic Navigation Research Institute, Japan

1. Introduction QZSS (Quasi-Zenith Satellite System) program:
ION ITM Jan Slide 1
Introduction
QZSS (Quasi-Zenith Satellite System) program:
Regional navigation service broadcast from high-elevation angle by a combination of three satellites on the inclined geosynchronous (quasi-zenith) orbit;
Broadcast GPS-like supplemental signals on three frequencies and two augmentation signals, L1-SAIF and LEX;
The first QZS satellite was successfully launched on Sept. 11, 2010.
L1-SAIF (Submeter-class Augmentation with Integrity Function) signal offers:
Submeter accuracy wide-area differential correction service;
Integrity function for safety of mobile users; and
Ranging function for improving position availability; all on L1 single frequency.
ENRI has been developing L1-SAIF signal and experimental facility:
Possible to extend to augment GLONASS satellites;
Upgraded to support multi-constellation environment including GPS, GLONASS, and QZSS satellites;
Conducted experiment with broadcast of multi-constellation augmentation.

2. QZSS Concept Broadcast signal from high elevation angle;
ION ITM Jan Slide 2
QZSS Concept
QZS
GPS/GEO
Broadcast signal from high elevation angle;
Applicable to navigation services for mountain area and urban canyon;
Augmentation signal from the zenith could help users to acquire other GPS satellites at any time.
Footprint of QZSS orbit;
Centered at 135E;
Eccentricity 0.075, Inclination 43deg.

3. SAIF： Submeter-class Augmentation with Integrity Function
ION ITM Jan Slide 3
L1-SAIF Signal
Ranging
Function
Error
Correction
Integrity
QZS satellites
GPS Constellation
Ranging Signal
Three functions by a single signal: ranging, error correction (Target accuracy: 1m), and integrity;
User receivers can receive both GPS and L1-SAIF signals with a single antenna and RF front-end;
Message-oriented information transmission: flexible contents.
User GPS
Receivers
SAIF： Submeter-class Augmentation with Integrity Function

4. L1-SAIF Signal QZSS broadcasts wide-area augmentation signal:
ION ITM Jan Slide 4
L1-SAIF Signal
QZSS broadcasts wide-area augmentation signal:
Called L1-SAIF (Submeter-class Augmentation with Integrity Function);
Designed and developed by ENRI.
L1-SAIF signal offers:
Wide-area differential correction service for improving position accuracy; Target accuracy: 1 meter for horizontal;
Integrity function for safety of mobile users; and
Ranging function for improving position availability.
Augmentation to GPS L1C/A based on SBAS techniques:
Broadcast on L1 freq. with RHCP; Common antenna and RF front-end;
Modulated by BPSK with C/A code (PRN 183);
250 bps data rate with 1/2 FEC; Message structure is identical with SBAS;
Differences: Large Doppler and additional messages.
Specification of L1-SAIF: See IS-QZSS document (Available at JAXA HP).

5. Augmentation to GPS Only
ION ITM Jan Slide 5
L1-SAIF Corrections
Standalone GPS
Augmented by L1-SAIF
Example of user position error at Site (Takayama: near center of monitor station network);
Realtime operation with MSAS-like 6 monitor stations;
Period: Jan (5 days);
L1-SAIF provides corrections only;
No L1-SAIF ranging.
Horizontal
Error
Vertical
1.45 m
2.92 m
6.02 m
8.45 m
System
Standalone
GPS
0.29 m
0.39 m
1.56 m
2.57 m
L1-SAIF
RMS
Max
Augmentation to GPS Only
Note: Results shown here were obtained with survey-grade antenna and receiver in open sky condition.

6. GLONASS Support: Motivation
ION ITM Jan Slide 6
GLONASS Support: Motivation
QZSS
L1-SAIF
Augmentation
GPS constellation
Additional Constellation
= GLONASS
Increase of augmented satellites improves availability of position solution;
Chance of robust position information at mountainous areas and urban canyons.
The current SBAS specification already has definition of GLONASS; Easy to support by L1-SAIF.

7. Time and Coordinate Systems
ION ITM Jan 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 ITM Jan Slide 8
PRN Masks
PRN Mask:
SBAS/L1-SAIF transmits PRN mask
information indicating satellites which are
currently augmented;
PRN number has range of 1 to 210;
Up to 51 satellites out of 210 can be
augmented simultaneously by the single
SBAS/L1-SAIF signal;
But, 32 GPS + 24 GLONASS = 56 !!!
Solution: Dynamic PRN Mask
Actually, PRN mask can change; Controlled by IODP (Issue of Data, PRN Mask);
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 ITM Jan 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 ITM Jan 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 ITM Jan 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. Upgrade of L1SMS L1-SAIF Master Station (L1SMS):
ION ITM Jan Slide 12
Upgrade of L1SMS
L1-SAIF Master Station (L1SMS):
Generates the L1-SAIF message stream and transmits it to JAXA MCS.
Upgrade for supporting GLONASS and QZSS:
Input module: Supports BINEX observables and navigation message records;
Implemented GLONASS extension based on SBAS standards;
User-domain receiver software (MCRX) is also upgraded to be GLONASS-capable;
QZSS is also supported as it is taken into account like GPS.
L1SMS
GEONET
QZS
QZSS MCS
GPS
Measured
Data
L1-SAIF
Message
GSI
ENRI
JAXA
L1-SAIF Signal
L1C/A, L2P
K-band
Closed
Loop
GLONASS
L1SA
L2SA

13. Dynamic PRN Mask Dynamic PRN mask: Transition of PRN mask:
ION ITM Jan Slide 13
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

14. Time offset broadcast to users
ION ITM Jan Slide 14
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

15. Experiment: Monitor Stations
ION ITM Jan Slide 15
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: BINEX
For our experiment:
6 sites for reference stations;
Reference Station (a) to (f)
11 sites for evaluation.
User Station (1) to (11)
Period: 2013/1/6 01:00
to 2013/1/9 23:00 (94 hours).

16. PRN Mask Transition QZSS GLONASS GPS
ION ITM Jan Slide 16
PRN Mask Transition
GPS
GLONASS
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.

17. Elevation Angle GPS GLONASS QZSS PRN Mask Transition 5 deg @ User (7)
ION ITM Jan Slide 17
Elevation Angle
GPS
GLONASS
QZSS
PRN Mask
Transition
5 deg
@ User (7)
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.

18. # of Satellites vs. Mask Angle
ION ITM Jan Slide 18
# of Satellites vs. Mask Angle
16 SVs
9.8 SVs
7.3 SVs
@ User (7)
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 16 satellites at 5 deg and 7.3 satellites even at 40 deg.

19. User Position Error: Mask 5deg
ION ITM Jan Slide 19
User Position Error: Mask 5deg
GPS+GLO+QZS: 0.310m RMS of horizontal error at user location (7);
Looks some limited improvement by using multi-constellation.

20. User Position Error: Mask 30deg
ION ITM Jan Slide 20
User Position Error: Mask 30deg
GPS+GLO+QZS: 0.335m RMS of horizontal error at user location (7);
Multi-constellation offers a good availability even for 30 deg mask.

21. Error vs. User Location: 5 deg
ION ITM Jan Slide 21
Error vs. User Location: 5 deg
North
South
0.421m
0.283m
Expect horizontal accuracy of 0.3 to 0.5m with L1-SAIF augmentation, regardless GLONASS is used or not;
There is a little dependency upon the latitude of user location possibly due to an effect of ionosphere activities.

22. Error vs. User Location: 30 deg
ION ITM Jan Slide 22
Error vs. User Location: 30 deg
North
South
0.425m
The horizontal accuracy is still within a range between 0.3 and 0.5m for the multi-constellation configuration;
The accuracy degrades to 1 or 2.5m for GPS-only single-constellation configuration.

23. Conclusion ENRI has been developing L1-SAIF signal:
ION ITM Jan Slide 23
Conclusion
ENRI has been developing L1-SAIF signal:
Signal design: GPS/SBAS-like L1 C/A code (PRN 183);
Planned as an augmentation to mobile users.
GPS/GLONASS/QZSS multi-constellation support:
L1-SAIF Master Station was upgraded to support GLONASS and QZSS in addition to GPS based on the existing SBAS specifications;
Conducted an experiment with broadcast of L1-SAIF signal containing augmentation information of GPS, GLONASS, and QZSS;
Using multi-constellation it can be expected to maintain a good position accuracy even in higher mask angle conditions representing limited visibility conditions.
Further Investigations will include:
Dynamic PRN mask driven by almanac information;
Use of GLONASS observables in generation of ionospheric corrections;
Considerations of different types of receiver for reference/user stations; and
Extension to Galileo.