ION GNSS 2012
Nashville, TN
Sept. 17-21, 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
ION GNSS Sept. 2012 - 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.
ION GNSS Sept. 2012 - Slide 2
Concept of SBAS
Geostationary
Satellites
GPS
Satellites
Augmentation
Signal
Accuracy
Integrity
Ranging
Ranging
Signal
Users
Uplink
Stations
Ground Monitor
Stations
ION GNSS Sept. 2012 - 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.
ION GNSS Sept. 2012 - Slide 4
Part 1
GLONASS
in the Current SBAS Standard
ION GNSS Sept. 2012 - 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
ION GNSS Sept. 2012 - 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)
ION GNSS Sept. 2012 - 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:
ION GNSS Sept. 2012 - 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 !!!
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
Spare
• 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.
ION GNSS Sept. 2012 - 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
Time
LTC
IOD=a
LTC
IOD=a
LTC
IOD=a
LTC
IOD=b
LTC
IOD=b
ION GNSS Sept. 2012 - 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.
Previous Ephemeris
Next Ephemeris
Time
LTC
IOD=V1|L1
Ephemeris Validity
Interval
V1
L1
LTC
IOD=V2|L2
Ephemeris Validity
Interval
V2
L2
ION GNSS Sept. 2012 - 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.3.1.2 gives the equations below (with some corrections).
Perturbation
terms in
ephemeris
ION GNSS Sept. 2012 - Slide 12
GLONASS Ephemeris
Item
Bits
Range
Resolution
Contents
tb
7
15-1425 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
27
27000 km
2-11 km
Position Y in ECEF
z
27
27000 km
2-11 km
Position Z in ECEF
vx
24
4.3 km/s
2-20 km/s
Velocity X in ECEF
vy
24
4.3 km/s
2-20 km/s
Velocity Y in ECEF
vz
..
x
..
y
..
z
24
4.3 km/s
2-20 km/s
Velocity Z in ECEF
5
6.2 mm/s2
2-30 km/s2
Acceleration X in ECEF (only perturbation)
5
6.2 mm/s2
2-30 km/s2
Acceleration Y in ECEF (only perturbation)
5
6.2 mm/s2
2-30 km/s2
Acceleration Z in ECEF (only perturbation)
Total
208
ION GNSS Sept. 2012 - Slide 13
Part 2
Implementation and Experiment
ION GNSS Sept. 2012 - 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.
User-side observations
Network GPS
observables
Reference station
observations
Software SBAS
Simulator
(RTWAD)
User-Domain
Receiver
Software
SBAS Message Stream
Position Error
Position
Output
ION GNSS Sept. 2012 - 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.
Transition time
180s
tcutover
PRN Mask (IODP=i)
FC
FC
LTC
FC
Cutover
PRN Mask (IODP=i+1)
FC
LTC
Corrections before cutover
FC
FC
LTC
FC
Corrections after cutover
FC
ION GNSS Sept. 2012 - 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.
True
Time
GLONASS
System Time
t
tGLONASS
GPS
System Time
Receiver clock for
GPS satellites
tR
^
B
GLONASS
DtGLONASS
DtGPS
Time offset
broadcast to users
tGPS
Receiver
Time
-daGLONASS
^
B
GPS
Receiver clock for
GPS satellites
ION GNSS Sept. 2012 - 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).
ION GNSS Sept. 2012 - Slide 18
PRN Mask Transition
QZSS
GLONASS
GPS
• 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.
ION GNSS Sept. 2012 - 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.
ION GNSS Sept. 2012 - 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.
ION GNSS Sept. 2012 - 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.
ION GNSS Sept. 2012 - 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.
ION GNSS Sept. 2012 - 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.
ION GNSS Sept. 2012 - 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.
ION GNSS Sept. 2012 - 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.
ION GNSS Sept. 2012 - 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.
ION GNSS Sept. 2012 - 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.
ION GNSS Sept. 2012 - Slide 28
Vertical Protection Level
Reduce
GPS only
GPS+GLO+QZS
• Protection levels mean the confidence limit at 99.99999% 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.
ION GNSS Sept. 2012 - 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.