SBAS IWG/25
St. Petersburg, Russia
June 25-27, 2013
GPS/GLONASS
Multi-Constellation SBAS Trial
Takeyasu Sakai
Electronic Navigation Research Institute
IWG/25 June 2013 - 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.
IWG/25 June 2013 - 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.
IWG/25 June 2013 - 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
IWG/25 June 2013 - 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)
IWG/25 June 2013 - 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-90.11 ?
IWG/25 June 2013 - 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 !!!
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
• 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.
IWG/25 June 2013 - 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
Time
LTC
IOD=a
LTC
IOD=a
LTC
IOD=a
LTC
IOD=b
LTC
IOD=b
IWG/25 June 2013 - 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.
Previous Ephemeris
Next Ephemeris
Time
LTC
IOD=V1|L1
Ephemeris Validity
Interval
V1
L1
LTC
IOD=V2|L2
Ephemeris Validity
Interval
V2
L2
IWG/25 June 2013 - 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.3.1.2 gives the equations below (with some corrections).
Perturbation
terms in
ephemeris
IWG/25 June 2013 - 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
User
Location
 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).
IWG/25 June 2013 - Slide 11
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 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.
IWG/25 June 2013 - 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.
IWG/25 June 2013 - 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.
IWG/25 June 2013 - 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.
IWG/25 June 2013 - 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.
IWG/25 June 2013 - 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.
IWG/25 June 2013 - 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.
IWG/25 June 2013 - Slide 18
Vertical Protection Level
Reduce
GPS only
GPS+GLO+QZS
@ User (b)
• 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; This means improvement of availability of navigation.
IWG/25 June 2013 - 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.