ION GNSS 2011
Portland, OR
Sept. 20-23, 2011
Generation and Evaluation of
QZSS L1-SAIF Ephemeris Information
T. Sakai, H. Yamada, S. Fukushima, and K. Ito
Electronic Navigation Research Institute, Japan
ION GNSS 20-23 Sept. 2011 - ENRI
Introduction
SLIDE 1
• 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 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 (PRN 183);
– Implemented L1-SAIF Master Station (L1SMS);
– Need to broadcast ephemeris message to make ranging function available.
 What is the best way to generate ephemeris information for L1-SAIF?
ION GNSS 20-23 Sept. 2011 - ENRI
SLIDE 2
QZSS Concept
GPS/GEO
• Footprint of QZSS orbit;
• Centered 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.
ION GNSS 20-23 Sept. 2011 - ENRI
Inclined Geosynchronous Orbit
SLIDE 3
Apogee
40000km
8:40
15:20
Perigee
32000km
Orbital Planes of QZSS (3 SVs)
Ground Track
• Semi-major axis (42164km) is equal to GEO orbit: synchronous with rotation of
the earth;
• Inclined obit makes ground track ‘8’-figure; Called IGSO or Quasi-Zenith Orbit;
• With three or more satellites on the same ground track, navigation service can
be provided from zenith to regional users at any time.
ION GNSS 20-23 Sept. 2011 - ENRI
SLIDE 4
Space Segment: QZS-1
Mass
4,020kg (wet) 1,802kg (dry)
(NAV Payload：320kg)
Power
Approx. 5.3 kW (EOL)
(NAV Payload: Approx. 1.9kW)
Design Life
10 years
Radiation Cooled TWT
TWSTFT Antenna
Successfully launched on Sept.
11, 2010 and settled on QuasiZenith Orbit (IGSO).
C-band TTC Antenna
Laser Reflector
L1-SAIF Antenna
L-band Helical Array
Antenna
ION GNSS 20-23 Sept. 2011 - ENRI
QZSS Frequency Plan
Signal
Channel
Frequency
L1CD
Bandwidth Min. Rx Power
24 MHz
–163.0 dBW
24 MHz
– 158.25 dBW
24 MHz
– 158.5 dBW
24 MHz
– 160.0 dBW
25 MHz
– 157.9 dBW
25 MHz
– 157.9 dBW
QZS-L1C
L1CP
1575.42 MHz
QZS-L1-C/A
QZS-L2C
1227.6 MHz
L5I
QZS-L5
SLIDE 5
Interoperability
GPS-like supplemental
signals with minimum
modifications from GPS
signals
1176.45 MHz
L5Q
QZS-L1-SAIF
1575.42 MHz
24 MHz
– 161.0 dBW
QZS-LEX
1278.75 MHz
42 MHz
– 155.7 dBW
SBAS-like augmentation
signal (250bps)
QZSS-specific augmentation signal (2kbps)
Find detail in IS-QZSS document.
ION GNSS 20-23 Sept. 2011 - ENRI
QZSS L1-SAIF Signal
SLIDE 6
• 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 position availability.
• Augmentation to GPS L1C/A based on SBAS:
– 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).
ION GNSS 20-23 Sept. 2011 - ENRI
L1-SAIF Signal
QZS satellite
SLIDE 7
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
Receivers
ION GNSS 20-23 Sept. 2011 - ENRI
SLIDE 8
L1-SAIF Message Structure
Preamble
8 bits
Message Type
6 bits
Data Field
212 bits
1 message = 250 bits broadcast every second
Transmitted First
MT
CRC parity
24 bits
Contents
Interval
[s]
MT
Contents
Interval
[s]
0
Test mode
6
17
GEO almanac
300
1
PRN mask
120
18
IGP mask
300
Fast correction & UDRE
60
24
FC & LTC
6
6
UDRE
6
25
Long-term correction
7
Degradation factor for FC
120
26
Ionospheric delay & GIVE 300
9
GEO ephemeris
120
27
SBAS service message
300
10
Degradation parameter
120
28
Clock-ephemeris covariance
120
12
SBAS time information
300
63
Null message
2-5
120
—
ION GNSS 20-23 Sept. 2011 - ENRI
SLIDE 9
L1-SAIF Message (1)
Message Type
Contents
Used by
Status
0
Test mode
SBAS and L1-SAIF
Fixed
1
PRN mask
SBAS and L1-SAIF
Fixed
Fast correction & UDRE
SBAS and L1-SAIF
Fixed
6
UDRE
SBAS and L1-SAIF
Fixed
7
Degradation factor for FC
SBAS and L1-SAIF
Fixed
8
Reserved
SBAS
Fixed
9
GEO ephemeris
SBAS
Fixed
10
Degradation parameter
SBAS and L1-SAIF
Fixed
12
SBAS network time
SBAS
Fixed
17
GEO almanac
SBAS
Fixed
18
IGP mask
SBAS and L1-SAIF
Fixed
24
Mixed fast/long-term correction
SBAS and L1-SAIF
Fixed
25
Long-term correction
SBAS and L1-SAIF
Fixed
26
Ionospheric delay & GIVE
SBAS and L1-SAIF
Fixed
2 to 5
ION GNSS 20-23 Sept. 2011 - ENRI
SLIDE 10
L1-SAIF Message (2)
Message Type
Contents
27
SBAS service message
28
Clock-ephemeris covariance
Used by
Status
SBAS
Fixed
SBAS and L1-SAIF
Fixed
29 to 51
Undefined
—
—
52
TGP mask
L1-SAIF
Tentative
53
Tropospheric delay
L1-SAIF
Tentative
Advanced Ionospheric delay
L1-SAIF
TBD
56
Intersignal biases
L1-SAIF
Tentative
57
Ephemeris-related parameter
L1-SAIF
TBD
58
QZS ephemeris
L1-SAIF
Tentative
59
QZS almanac
L1-SAIF
TBD
60
Regional information
L1-SAIF
TBD
61
Reserved
L1-SAIF
Tentative
62
Reserved
SBAS and L1-SAIF
Fixed
63
Null message
SBAS and L1-SAIF
Fixed
54 to 55
ION GNSS 20-23 Sept. 2011 - ENRI
SLIDE 11
Example Position Error
• Example of user position error at Site
940058 (Takayama: near center of
monitor station network);
• Realtime operation with MSAS-like 6
monitor stations;
• Period: 19-23 Jan. 2008 (5 days).
Horizontal
Error
Vertical
Error
Standalone RMS
GPS
Max
1.45 m
2.92 m
6.02 m
8.45 m
RMS
0.29 m
0.39 m
Max
1.56 m
2.57 m
System
Standalone GPS
Augmented by L1-SAIF
L1-SAIF
Note: Results shown here were obtained with surveygrade antenna and receivers in open sky condition.
ION GNSS 20-23 Sept. 2011 - ENRI
SLIDE 12
Broadcast Ephemeris for Ranging
• Orbit Information for ranging function:
– Need to broadcast ephemeris information (navigation-grade orbit information)
on QZS signal in order to use it for ranging;
– QZS L1C/A, L2C, L5, and L1C broadcast ephemeris information on their
navigation message similar with GPS; Available since June 2011;
– How about L1-SAIF and LEX signals?
• Option A: Do not broadcast on L1-SAIF, but on other signals:
– L1-SAIF users receive and utilize ephemeris information broadcast on L1C/A,
L2C, L5, or L1C signals;
 Antenna offset may be cancelled by differential correction.
– Users must receive al least two signals (eg. L1-SAIF + L1C/A) to use ranging
function of L1-SAIF signal.
We employ
• Option B: Broadcast ephemeris information on L1-SAIF:
this option
– Users can use QZS for ranging by receiving only L1-SAIF signal;
– Require to design some new message because SBAS GEO ephemeris
message is not applicable; MT58 needs to be designed.
ION GNSS 20-23 Sept. 2011 - ENRI
Ephemeris Message Design
SLIDE 13
• Requirements for ephemeris information:
– Contained by a single message (212 bits);
 GPS legacy-Nav message consumes 420 bits for a set of ephemeris information of a
single GPS SV;
 SBAS GEO ephemeris message consists of only 204 bits; But this is for GEO.
– Available for 600-900 seconds from broadcast;
– SV position error should be within 0.3-0.5m;
 Will be cancelled by applying differential corrections;
 The minimum resolution of corrections is 0.125m.
– Clock correction term should cover a range of 1ms.
 Required by payload developer during discussion of specification.
• Representations in other systems:
– GPS legacy-Nav: Representation by Keplerian 6-element parameters;
– GLONASS: Represented by position, velocity, and acceleration in ECEF;
 Users need numerical integration to obtain the position of the intended moment.
– SBAS: Transmit position, velocity, and acceleration in ECEF.
 No need for numerical integration.
ION GNSS 20-23 Sept. 2011 - ENRI
SLIDE 14
GPS Ephemeris for MEO
Item
Bits
Contents
Item
Bits
toc
16
Epoch time
toe
16
Epoch time
af0
22
Clock corr. (const)
af1
16
af2
Contents
32
Eccentricity
32
Semi-major axis
24
Rate of W
Clock corr. (1st order)
e

A
.
W
.
i
14
Rate of i
8
Clock corr. (2nd order)
Crc
16
Harmonic correction
M0
32
Mean anomaly
Crs
16
Harmonic correction
W0
32
Longitude of ascension
Cuc
16
Harmonic correction
w
32
Argument of perigee
Cus
16
Harmonic correction
i0
32
Inclination
Cic
16
Harmonic correction
Dn
16
Mean motion
Cis
16
Harmonic correction
Total
420
Stored in 3 SFs (18s)
ION GNSS 20-23 Sept. 2011 - ENRI
GLONASS Ephemeris for MEO
SLIDE 15
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
Stored in 4 strings (8s)
ION GNSS 20-23 Sept. 2011 - ENRI
SLIDE 16
SBAS Ephemeris for GEO
Item
Bits
Range
Resolution
Contents
t0,GEO
13
0-86384 s
16 s
URA
4
0-15
1
Accuracy indicator
x
30
42950 km
0.08 m
Position X in ECEF
y
30
42950 km
0.08 m
Position Y in ECEF
z
25
6710 km
0.4 m
Position Z in ECEF
dx/dt
17
40.96 m/s
0.625 mm/s
Velocity X in ECEF
dy/dt
17
40.96 m/s
0.625 mm/s
Velocity Y in ECEF
dz/dt
18
524.288 m/s
4 mm/s
Velocity Z in ECEF
d2x/dt2
10
6.4 mm/s2
12.5 mm/s2
Acceleration X in ECEF
d2y/dt2
10
6.4 mm/s2
12.5 mm/s2
Acceleration Y in ECEF
d2z/dt2
10
32 mm/s2
62.5 mm/s2
Acceleration Z in ECEF
aGf0
12
0.9537 ms
2-31 s
Clock correction (const)
aGf1
8
0.11642 ns/s
2-40 s/s
Total
204
Epoch time
Clock correction (1st order)
Stored in 1 msg (1s)
ION GNSS 20-23 Sept. 2011 - ENRI
QZSS SV Position in ECEF
SLIDE 17
QZS-1
Broadcast Nav Msg
on L1C/A PRN 193
2011-08-18
00:00 to 24:00 GPST
GPS/QZSS Rx
@Koganei, Tokyo
Processing by ENRI
24 different IODEs
in 2846 frames
• Computed based on broadcast ephemeris on QZSS L1C/A for day of
2011-08-18;
• QZS-1 was operating normally and healthy except LEX signal.
ION GNSS 20-23 Sept. 2011 - ENRI
QZSS SV Velocity in ECEF
SLIDE 18
QZS-1
Broadcast Nav Msg
on L1C/A PRN 193
2011-08-18
00:00 to 24:00 GPST
GPS/QZSS Rx
@Koganei, Tokyo
Processing by ENRI
24 different IODEs
in 2846 frames
• Velocity of QZS-1 computed from broadcast ephemeris.
ION GNSS 20-23 Sept. 2011 - ENRI
QZSS SV Acceleration in ECEF
SLIDE 19
QZS-1
Broadcast Nav Msg
on L1C/A PRN 193
2011-08-18
00:00 to 24:00 GPST
GPS/QZSS Rx
@Koganei, Tokyo
Processing by ENRI
24 different IODEs
in 2846 frames
• Acceleration of QZS-1 computed from broadcast ephemeris.
ION GNSS 20-23 Sept. 2011 - ENRI
Representation of Acceleration
SLIDE 20
• Reduction of information:
– 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; Enough for L1-SAIF;
– 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).
– This technique is applicable to L1-SAIF ephemeris representation.
Transmit these
perturbation terms
ION GNSS 20-23 Sept. 2011 - ENRI
SLIDE 21
Perturbation Term
QZS-1
Broadcast Nav Msg
on L1C/A PRN 193
2011-08-18
00:00 to 24:00 GPST
GPS/QZSS Rx
@Koganei, Tokyo
Processing by ENRI
24 different IODEs
in 2846 frames
• Acceleration of QZS-1 computed from broadcast ephemeris;
• Perturbation only: Centripetal acceleration term is removed.
ION GNSS 20-23 Sept. 2011 - ENRI
Proposed Message (MT58)
SLIDE 22
Item
Bits
Range
Resolution
Contents
t0,Q
8
0-10740 s
60 s
Epoch time/IODN
URA
4
0-15
1
Accuracy indicator
x
26
42950 km
1.28 m
Position X in ECEF
y
26
42950 km
1.28 m
Position Y in ECEF
z
26
42950 km
1.28 m
Position Z in ECEF
vx
24
4.194 km/s
0.5 mm/s
Velocity X in ECEF
vy
24
4.194 km/s
0.5 mm/s
Velocity Y in ECEF
vz
..
x
..
y
..
z
24
4.194 km/s
0.5 mm/s
Velocity Z in ECEF
5
32 mm/s2
2 mm/s2
Acceleration X in ECEF (only perturbation)
5
32 mm/s2
2 mm/s2
Acceleration Y in ECEF (only perturbation)
5
32 mm/s2
2 mm/s2
Acceleration Z in ECEF (only perturbation)
aGf0
22
1.953 ms
2-30 s
aGf1
13
3.725 ns/s
2-40 s/s
Total
212
Clock correction (const)
Clock correction (1st order)
Stored in 1 msg (1s)
ION GNSS 20-23 Sept. 2011 - ENRI
Generation of Ephemeris
SLIDE 23
• Option A: Generation independent of other systems:
– Need realtime monitor station network:
 Monitor stations also necessary for generation of wide-area differential corrections;
 JAXA’s monitor network covering Japan and Southeast Asia Region is available for
experimental purpose.
– Need some orbit determination software;
– Independent of other systems.
 However, observations from monitor network are provided by JAXA MCS.
• Option B: Reconstruction from MCS products:
– L1-SAIF is broadcast from QZS-1 SV.
We employ
this option
 QZS-1 itself broadcasts ephemeris information on L1C/A signal;
– Ephemeris information can be generated based on L1C/A ephemeris;
 Reconstruction from Keplerian representation in L1C/A legacy Nav message into
ECEF PVA representation for MT58;
 L1C/A and L1-SAIF are broadcast from different antennas; Antenna offset may be
cancelled by differential corrections.
– Users still do not need to receive any signals other than L1-SAIF.
ION GNSS 20-23 Sept. 2011 - ENRI
Ephemeris Reconstruction
SLIDE 24
QZS-1
JAXA MCS
Decode
ephemeris
Format
into MT58
Compute
PVA
L1-C/A Users
Ephemeris Reconstruction
L1-SAIF Master Station at ENRI
L1-SAIF Users
ION GNSS 20-23 Sept. 2011 - ENRI
SV Position Error for PVA
Quantization of corrections
SLIDE 25
QZS-1
Reconstructed from
broadcast Nav Msg
on L1C/A PRN 193
2011-08-18
00:00 to 24:00 GPST
Processing by ENRI
Assumed user location:
@ENRI, Tokyo
• Integration error in user receiver with regard to time after broadcast of PVA;
• LOS component including satellite clock offset;
• SV PVA is computed from broadcast ephemeris on QZSS L1C/A.
ION GNSS 20-23 Sept. 2011 - ENRI
SV Position Error for MT58
SLIDE 26
QZS-1
Reconstructed from
broadcast Nav Msg
on L1C/A PRN 193
2011-08-18
00:00 to 24:00 GPST
Processing by ENRI
Assumed user location:
@ENRI, Tokyo
• PVA is formatted into proposed MT58 format and integrated;
• Initial position error is dominant component because of its resolution of
1.28m; Need some correction.
ION GNSS 20-23 Sept. 2011 - ENRI
Correction by LTC Message
SBAS GEO
PRN 120-138
Measured
Pseudorange
QZSS L1-SAIF
PRN 183
Measured
Pseudorange
MT9
GEO Ephemeris
Orbit
Clock
(0.125m)
MT2-5
Fast Correction
(FC)
Clock
(0.125m)
Corr.
Resolution
MT58
QZS Ephemeris
Orbit
Clock
(1.28m)
MT2-5
Fast Correction
(FC)
Clock
(0.125m)
Corr.
SLIDE 27
Corrected
Pseudorange
Corrected
Pseudorange
Clock & Orbit Corr.
(0.125m)
MT24/25
• Apply MT24/25 LTC orbit and
Long-Term
clock correction like GPS SVs;
Correction (LTC)
• LTC has 0.125m resolution.
ION GNSS 20-23 Sept. 2011 - ENRI
SLIDE 28
Applying LTC Correction
QZS-1
Reconstructed from
broadcast Nav Msg
on L1C/A PRN 193
2011-08-18
00:00 to 24:00 GPST
Processing by ENRI
Assumed user location:
@ENRI, Tokyo
• Assumed that initial position error is corrected by LTC (long-term
correction) in MT24/25 messages;
• Range error is within 0.3m for 300s integration.
 Note that ephemeris message of SBAS has timeout interval of 240-360s.
ION GNSS 20-23 Sept. 2011 - ENRI
Conclusion
SLIDE 29
• 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;
– Implemented L1-SAIF Master Station (L1SMS) which generates augmentation
message stream in realtime and transmit it to QZSS MCS.
• QZSS ephemeris message:
–
–
–
–
Need to broadcast ephemeris information to make ranging function available;
For L1-SAIF signal, MT 58 is designed to broadcast ephemeris information;
MT 58 provides SV position, velocity, and acceleration in ECEF;
Integration error is within 0.3m when combined with LTC correction.
• Ongoing work:
– Upgrade L1SMS to generate corrections for L1-SAIF signal itself;
– User side consideration of combined use of QZSS and MSAS.