1. Quality Control of Ionospheric Corrections
ION ITM 2015
Dana Point, CA
Jan , 2015
Quality Control of Ionospheric Corrections
for Adoption of Wide Area Augmentation
at the Low Latitude Regions
T. Sakai, T. Aso, K. Hoshinoo, and K. Ito
Electronic Navigation Research Institute, Japan

2. Introduction QZSS (Quasi-Zenith Satellite System) program:
ION ITM Jan. 2015
Introduction
QZSS (Quasi-Zenith Satellite System) program:
Regional navigation service broadcast from high-elevation angle by a combination of three or more 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:
L1-SAIF signal achieves good accuracy less than 1 meter in RMS manner at the mainland of Japan.
Ionosphere disturbance sometimes degrades the position accuracy, especially at the low latitude regions including the southwestern Islands of Japanese territory.
In order to improve the accuracy at the low latitude regions during ionospheric storm, quality control of ionospheric corrections may be introduced.

3. QZSS Concept Broadcast signal from high elevation angle.
ION ITM Jan. 2015
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.

4. Space Segment: QZS-1 L1-SAIF Antenna 25.3m
ION ITM Jan. 2015
Space Segment: QZS-1
L-band Helical Array Antenna
L1-SAIF Antenna
Laser Reflector
C-band TTC Antenna
Radiation Cooled TWT
TWSTFT Antenna
25.3m
Successfully launched on Sept. 11, 2010 and settled on Quasi-Zenith Orbit (IGSO).
Nickname: “Michibiki”
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

5. Broadcast Ranging Signals
ION ITM Jan. 2015
Broadcast Ranging Signals
Supplemental
Signals
Supplemental Signals by JAXA
GPS-like L1C/A, L2C, L5, and L1C signals (PRN 193) working with GPS.
Improves availability of navigation.
Minimum modifications from GPS signals.
QZS-1 satellite
L1-SAIF by ENRI
SBAS-like C/A code (PRN 183) signal on GPS L1 freq.; Reasonable performance for mobile users.
Augmentation to GPS; Potentially plus GLONASS and Galileo.
LEX by JAXA
For carrier-based experimental purposes.
Original CPSK signal on E6 frequency.
Member organizations may use LEX as 2kbps experimental data channel.
Augmentation
Signals

6. SAIF： Submeter-class Augmentation with Integrity Function
ION ITM Jan. 2015
L1-SAIF Signal
Ranging
Function
Error
Correction
Integrity
Information
GPS Constellation
Ranging Signal
L1-SAIF Signal
QZS satellites
SBAS-like augmentation signal on PRN 183.
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.
See IS-QZSS for detail (Available at JAXA HP).
User GPS
Receivers
SAIF： Submeter-class Augmentation with Integrity Function

7. ENRI L1-SAIF Master Station
ION ITM Jan. 2015
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 (monitor stations) necessary to generate L1-SAIF messages are sent from GEONET in realtime.
L1SMS
GEONET
QZS
QZSS MCS
GNSS
Satellites
Measure-
ments
L1-SAIF
Message
GSI Server
(Tokyo)
ENRI
JAXA TKSC
(Tsukuba)
L1-SAIF Signal
Ranging Signal
K-band Uplink

8. L1-SAIF Performance GPS Only Result Horizontal Error Vertical 1.45 m
ION ITM Jan. 2015
L1-SAIF Performance
Standalone GPS
L1-SAIF Augmentation
GPS Only Result
6 reference
stations
User location for this test
L1-SAIF expe- rimental area
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
w/ L1-SAIF
RMS
Max
Example of user position error at Site (Takayama).
Realtime operation with MSAS-like 6 reference stations in Japan.
Period: Jan (5 days).
Note: Results shown here were obtained with geodetic-grade antenna and receivers at open sky condition.

9. Problem: Ionosphere Ionosphere Density (NASA/JPL)
ION ITM Jan. 2015
Problem: Ionosphere
Ionosphere
Density
(NASA/JPL)
The largest error source: Ionospheric propagation delay.
Varies on the local time, solar activity, earth magnetic field, and so on.
Cannot be predicted; Causes large effect in the low magnetic latitude region.

10. Accuracy at Southwestern Island
ION ITM Jan. 2015
Accuracy at Southwestern Island
LT 14:00
At Southwestern Island ( Wadomari)
At Northernmost City ( Kitami)
During severe ionospheric storm condition (Kp~7+), position accuracy with differential correction largely degrades at the Southwestern Islands.
The effect is not so large at the mainland of Japan.
It is known that increase of the number of GMS shows a little improvement.

11. Actual Ionosphere Corrections
ION ITM Jan. 2015
Actual Ionosphere Corrections
PRN20
PRN28 5m
At Southwestern Island
At Northernmost City
Ionospheric correction continuously differs from the true delay by 5m or more.
Degradation of position accuracy during storm is due to inaccurate ionospheric correction.

12. L1-SAIF Ionospheric Correction
ION ITM Jan. 2015
L1-SAIF Ionospheric Correction
120
150
180 30 60
Longitude, E
Latitude, N 15 45
IGP
Vertical ionospheric delay information at IGPs (ionospheric grid point) located with 5-degree interval will be broadcast to users.
User receiver computes vertical ionospheric delays at IPPs with bilinear interpolation of delays at the surrounding IGPs.
Vertical delay is converted to slant delay by multiplying so-called obliquity factor which is a function of elevation angle.
IGP
IPP

13. Message Type 26: Ionospheric Delay Information
ION ITM Jan. 2015
Broadcast Message
MT26: Broadcast Ionospheric Vertical Delay
Contains vertical delay and its uncertainty at IGP.
A MT26 message contains information for 15 IGPs.
Each IGP has two attributes: vertical delay and its uncertainty (GIVE).
Message Type 26: Ionospheric Delay Information
Repeat
Content
Bits
Range
Resolution 1
IGP Band ID 4
0 to 10
IGP Block ID
0 to 13 15
IGP Vertical Delay 9
0 to m
0.125 m
GIVEI
0.3 to 45 m
or Not Monitored
(Table)
IODI 2
0 to 3
Spare 7 —

14. Planar Fit and GIVE GIVE Equation
ION ITM Jan. 2015
Planar Fit and GIVE
Cutoff Radius
Vertical Delay
Fit Plane
IPP
IGP
Planar Fit: An algorithm to estimate vertical delay at an IGP.
Developed for WAAS; MSAS and L1-SAIF employ the same algorithm.
Assume ionospheric vertical delay can be modeled as a plane.
Model parameters are estimated by the least square fit.
GIVE (grid ionosphere vertical error): Uncertainty of the estimation basically computed from residuals of fit and including spatial and temporal threats.
GIVE Equation
Formal Sigma
Spatial Threat
Temporal Threat

15. ION ITM Jan. 2015
Estimation Error
Difference between observation and estimation at user IPPs
Difference, m
4:30
At Northernmost City ( Kitami)
Over 2.5m
Difference, m
4:30
At Southwestern Island ( Wadomari)

16. Quality of Estimation The MCS Can Examine the Quality of Estimation.
ION ITM Jan. 2015
Quality of Estimation
The MCS Can Examine the Quality of Estimation.
Based on the observations used for estimation.
Ex. Storm Detector Algorithm: Chi-Square test for residual errors (already implemented).
Use the difference between observation and estimation for all IPPs:
Ionospheric delay observed at an IPP
observed by MCS; and
The associate delay estimation at the
location of the IPP interpolated based
on surrounding four IGP delay
information generated by the MCS.
The large difference means invalidity
of ionosphere estimation used for
correction at the location of IPP.
Difference, m
| I(fIPP_i, lIPP_i) - IIPP_i | ^
Difference between observation and estimation at the IPPs of MCS

17. Quality Control of Corrections
ION ITM Jan. 2015
Quality Control of Corrections
Action to Detected Invalidity.
Due to the equatorial anomaly, irregularity of ionosphere may exist in south side.
In case that the difference at an IPP is larger than the threshold, all IGPs in the south from the IPP are set to:
‘Not Monitored’ (NM)
with threshold THNM (QC-NM); or
Maximum GIVE (45m)
with threshold TH45 (QC-45).
Difference, m
QC-45: IGPs in the south from 20N are set to 45m due to two IPPs (red circled) having the difference larger than TH45 = 3.0m.
QC-NM: IGPs in the south from 15N are set to NM due to an IPP (black circled) having the difference larger than THNM = 5.0m.

18. Quality Control of Corrections
ION ITM Jan. 2015
Quality Control of Corrections
For All IPPs
For All IGPs
For All IGPs
>
fIGP_k : fIPP_i
Compute Estimation for IPP i:
I(fIPP_i, lIPP_i) ^
Planar Fit Algorithm
for IGP k
<
|I(fIPP_i, lIPP_i)-IIPP_i| : TH ^
Set GIVE = NM or 45m
for IGP k
>
<
Observation
Estimation
Conventional
Planar Fit
Implementation
New: Quality Control Algorithm

19. Experiment Setup Observation Data from GEONET:
ION ITM Jan. 2015
Experiment Setup
Observation Data from GEONET:
GPS network operated by Geospatial Information Authority of Japan.
Archive of 30-sec interval data.
L1-SAIF Master Station:
Runs in offline mode with RINEX files.
Using 6 GEONET stations (a) to (f) near MSAS/L1-SAIF GMS (Ground Monitor Station) locations to generate augmentation message.
User Stations:
5 stations from North to South: (1) to (5) for performance evaluation.
Compute position estimates with the L1-SAIF message output from L1-SAIF Master Station

20. Baseline Performance At Southwestern Island (5) Wadomari
ION ITM Jan. 2015
Baseline Performance
At Southwestern Island (5) Wadomari
At Northernmost City (1) Kitami
Period: 2011/10/23 to 2011/10/26 (severe storm: Kp~7+)
L1SMS Parameters: # of GMS = 6, Iono Correction = Planar Fit

21. Quality Control (QC-45) At Southwestern Island (5) Wadomari
ION ITM Jan. 2015
Quality Control (QC-45)
At Southwestern Island (5) Wadomari
At Northernmost City (1) Kitami
Quality Control with TH45=1.0m (QC-45) and THNM= (No QC-NM).
Possibility of some improvement at southwestern islands.
Availability of position solution is not affected.

22. Quality Control (QC-45 and QC-NM)
ION ITM Jan. 2015
Quality Control (QC-45 and QC-NM)
At Southwestern Island (5) Wadomari
At Northernmost City (1) Kitami
Quality Control with TH45=2.0m (QC-45) and THNM=3.0m (QC-NM).
Improvement is not clear.
Availability of position solution is degraded.

23. Effects of Quality Control
ION ITM Jan. 2015
Effects of Quality Control
10%
No loss of Availability
Apply QC-45 Only
Apply QC-45 and QC-NM
QC-45 (Quality Control with setting Max GIVE) has a potential of 10% improvement.
QC-NM (Quality Control with setting Not Monitored) has unclear improvement and degraded availability.

24. Conclusion ENRI has been developing L1-SAIF signal:
ION ITM Jan. 2015
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.
Ionosphere disturbance is a concern:
L1-SAIF signal achieves good accuracy less than 1 meter in RMS manner at the mainland of Japan.
Ionosphere disturbance sometimes degrades the position accuracy, especially at the Southwestern Islands of Japanese territory.
This time we tried Quality Control of ionospheric corrections based on the difference between observation and estimation at IPPs.
Expected performance improvement: ~10% in RMS.
Further Investigations will include:
Other methods to improve the quality of ionospheric corrections for wide area augmentation.
Consideration of using external data sources (space weather and other sensors).
Performance improvement at other Asian Countries.