4th AOR W/S on GNSS
Kuala Lumpur, Malaysia
Dec. 8-10, 2012
Multi-GNSS Augmentation
by L1-SAIF Signal: Preliminary Results
Takeyasu Sakai
Electronic Navigation Research Institute
AOR W/S Dec. 2012 - 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.
• 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:
– Signal design: SBAS-like message stream on L1 C/A code (PRN 183);
– Possibility of Multi-GNSS augmentation: combined use of GPS and other
constellations would improve the availability of position solutions.
 Especially where visibility is limited.
 Upgraded L1-SAIF experimental facility and conducted a Multi-GNSS trial.
AOR W/S Dec. 2012 - Slide 2
QZSS Concept
GPS/GEO
• Footprint of QZSS orbit;
• Centered at 135E;
• Eccentricity 0.075, Inclination 43deg.
QZS
• 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.
AOR W/S Dec. 2012 - Slide 3
QZSS L1-SAIF Signal
• QZSS broadcasts wide-area augmentation signal:
– Called L1-SAIF (Submeter-class Augmentation with Integrity Function);
– Augmentation signal for mobile users 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 the SBAS specifications:
– 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 from SBAS: PRN, large Doppler, and some additional messages.
– Developed easily if one has the experience to develop SBAS-capable receiver;
– Specification of L1-SAIF: See IS-QZSS document (Available at JAXA HP).
AOR W/S Dec. 2012 - Slide 4
L1-SAIF Signal Functions
3 Functions by L1-SAIF
QZS satellites
Ranging
Function
GPS Constellation
Error
Correction
Integrity
Function
• 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.
SAIF： Submeter-class Augmentation with Integrity Function
Ranging Signal
User GPS/L1-SAIF
Receivers
AOR W/S Dec. 2012 - Slide 5
ENRI L1-SAIF Master Station
• L1-SAIF Master Station (L1SMS):
– Generates L1-SAIF message stream in realtime and transmits it to QZSS MCS
developed by and installed at JAXA;
– Installed at ENRI, Tokyo; 90km from JAXA Tsukuba Space Center;
– Dual frequency GPS measurements at some locations in Japan necessary to
generate L1-SAIF messages are sent from GEONET in realtime.
GPS
Satellites
GEONET
QZS
Measurements
L1SMS
L1-SAIF
Message
QZSS MCS
GSI Server
ENRI
JAXA TKSC
(Tokyo)
(Tokyo)
(Tsukuba)
AOR W/S Dec. 2012 - Slide 6
L1-SAIF Correction: GPS only
GPS Only Result
6 reference
stations
User location
for this test
L1-SAIF experimental area
Standalone GPS
L1-SAIF Augmentation
Horizontal
Error
1.45 m
Standalone RMS
GPS
Max
6.02 m
RMS
0.29 m
w/ L1-SAIF
Max
1.56 m
System
• Example of user position error at Site
940058 (Takayama);
• Realtime operation with MSAS-like 6
reference stations in Japan;
• Period: 19-23 Jan. 2008 (5 days).
Vertical
Error
2.92 m
8.45 m
0.39 m
2.57 m
Note: Results shown here were obtained with geodeticgrade antenna and receivers at open sky condition.
AOR W/S Dec. 2012 - Slide 7
Adding GLONASS: Motivation
QZS
Augmentation
GPS constellation
Users
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.
AOR W/S Dec. 2012 - Slide 8
GLONASS: Differences from GPS
• FDMA signals:
– Change carrier frequency settings with regard to ranging sources.
• Reference time and coordinates:
– Time: broadcast time offset information by an L1-SAIF message; Avoids increase
of unknowns in user receivers;
– Coordinates: convert PZ-90.02 to WGS-84.
• PRN numbers and insufficient capacity of mask pattern:
– Assign PRN numbers of 38 to 61 as GLONASS slot numbers of 1 to 24;
– Introduce dynamic PRN mask solution to broadcast augmentation information
supporting more than 51 ranging sources, reflecting the actual visibility.
• Missing IOD (Issue of Data):
– IOD is used to identify ephemeris information in order to match ephemerides
between L1-SAIF Master Station and users; Currently using IODE for GPS;
– Identify ephemeris information based on the time of broadcast.
• Satellite position computation: based on PVA as described in GLONASS ICD.
AOR W/S Dec. 2012 - Slide 9
Software Implementation
• ENRI’s L1-SAIF Master Station (L1SMS) Software:
– Generates L1-SAIF 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 of L1SMS for supporting GLONASS and QZSS:
–
–
–
–
Input module: RINEX observation and navigation files containing GLONASS;
Implemented GLONASS extension as explained before, for offline mode of L1SMS;
User-domain receiver software is also upgraded to be GLONASS-capable;
QZSS is also supported as it is taken into account like GPS.
User-side observations
Network GPS
Observables
(RINEX)
Reference station
observations
L1-SAIF
Master Station
(L1SMS) Software
Upgrade for GLONASS
User-Domain
Receiver
Software
SBAS Message Stream
Position Error
Position
Output
AOR W/S Dec. 2012 - Slide 10
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).
AOR W/S Dec. 2012 - Slide 11
PRN Mask Transition
QZSS
GLONASS
GPS
• Showing satellite PRN identifier being
augmented at each epoch;
• 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.
• IODP (issue of Data, PRN Mask)
indicates change of PRN mask.
AOR W/S Dec. 2012 - Slide 12
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.
AOR W/S Dec. 2012 - 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 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.
AOR W/S Dec. 2012 - Slide 14
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.
AOR W/S Dec. 2012 - 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.
AOR W/S Dec. 2012 - Slide 16
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.
AOR W/S Dec. 2012 - Slide 17
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.
AOR W/S Dec. 2012 - Slide 18
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.
AOR W/S Dec. 2012 - Slide 19
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.
AOR W/S Dec. 2012 - Slide 20
Availability vs. Mask Angle
100%
Availability
@ User (b)
• The number of epochs with valid position solution decreases with regard to
increase of mask angle;
• Multi-constellation with QZSS achieves 100% availability even for 40 deg mask.
AOR W/S Dec. 2012 - Slide 21
Conclusion
• Combined use of GPS and GLONASS with L1-SAIF:
– Potential problems and solutions on realizing a Multi-GNSS L1-SAIF, capable of
augmenting GPS, GLONASS, and QZSS simultaneously were investigated;
– L1-SAIF Master Station is upgraded for supporting Multi-GNSS augmentation and
tested successfully; Currently only for offline mode using RINEX files;
– It is confirmed that the performance of L1-SAIF augmentation is certainly improved
by adding GLONASS and QZSS, especially when satellite visibility is limited.
• Ongoing and future works:
– Support of realtime operation mode;
– Broadcast of augmentation information for both GPS and GLONASS on QZS-1
real signal; Plan from the second half of this month;
– Use of GLONASS/QZSS observables in generation of ionospheric correction;
– Further extension to support Galileo.
For further information, contact to sakai@enri.go.jp