T. Sakai, K. Hoshinoo, and K. Ito Electronic Navigation Research Institute, Japan T. Sakai, K. Hoshinoo, and K. Ito Electronic Navigation Research Institute,

T. Sakai, K. Hoshinoo, and K. ItoElectronic Navigation Research Institute, Japan

T. Sakai, K. Hoshinoo, and K. ItoElectronic Navigation Research Institute, Japan

ION ITM 2014ION ITM 2014San Diego, CASan Diego, CA

Jan. 27-29, 2014Jan. 27-29, 2014

Ionospheric Correctionat the Southwestern Islands

for the QZSS L1-SAIF

Ionospheric Correctionat the Southwestern Islands

for the QZSS L1-SAIF

ION ITM Jan. 2014ION ITM Jan. 2014 - Slide - Slide 22

IntroductionIntroduction• QZSS (Quasi-Zenith Satellite System) programQZSS (Quasi-Zenith Satellite System) program::

– Regional navigation service broadcast from high-elevation angle by a combination Regional navigation service broadcast from high-elevation angle by a combination of three or more satellites on the inclined geosynchronous (quasi-zenith) orbit;of three or more satellites on the inclined geosynchronous (quasi-zenith) orbit;

– Broadcast GPS-like supplemental signals on three frequencies and two Broadcast GPS-like supplemental signals on three frequencies and two augmentation signals, L1-SAIF and LEX.augmentation signals, L1-SAIF and LEX.

• L1-SAIF L1-SAIF (Submeter-class Augmentation with Integrity Function) (Submeter-class Augmentation with Integrity Function) signal offers:signal offers:– Submeter accuracy wide-area differential correction service;Submeter accuracy wide-area differential correction service;– Integrity function for safety of mobile users; andIntegrity function for safety of mobile users; and– Ranging function for improving position availability; all on L1 single frequency.Ranging function for improving position availability; all on L1 single frequency.

• ENRI has been developing L1-SAIF signal and experimental facilityENRI has been developing L1-SAIF signal and experimental facility::– L1-SAIF signal achieves good accuracy less than 1 meter in an RMS manner at the L1-SAIF signal achieves good accuracy less than 1 meter in an RMS manner at the

mainland of Japan;mainland of Japan;– Ionosphere disturbance sometimes degrades the position accuracy, especially at Ionosphere disturbance sometimes degrades the position accuracy, especially at

the Southwestern Islands of Japanese territory;the Southwestern Islands of Japanese territory;– In order to improve the accuracy at the southwestern islandsIn order to improve the accuracy at the southwestern islands during ionospheric during ionospheric

storm, we have designed some new L1-SAIF messages and tested them.storm, we have designed some new L1-SAIF messages and tested them.


QZSS ConceptQZSS Concept

• Broadcast signal from high elevation angle;Broadcast signal from high elevation angle;

• Applicable to navigation services for Applicable to navigation services for mountain area and urban canyon;mountain area and urban canyon;

• Augmentation signal from the zenith could Augmentation signal from the zenith could help users to acquire other GPS satellites at help users to acquire other GPS satellites at any time.any time.

• Footprint of QZSS orbit;Footprint of QZSS orbit;• Centered at 135E;Centered at 135E;• Eccentricity 0.075, Inclination 43deg.Eccentricity 0.075, Inclination 43deg.

QZSQZSGPS/GEOGPS/GEO


L1-SAIF SignalL1-SAIF Signal

User GPSUser GPSReceiversReceivers

• Three functions by a single signal: ranging, error Three functions by a single signal: ranging, error correction (Target accuracy: 1m), and integrity;correction (Target accuracy: 1m), and integrity;

• User receivers can receive both GPS and L1-SAIF User receivers can receive both GPS and L1-SAIF signals with a single antenna and RF front-end;signals with a single antenna and RF front-end;

• Message-oriented information transmission: flexible Message-oriented information transmission: flexible contents;contents;

• See IS-QZSS for detail (Available at JAXA HP).See IS-QZSS for detail (Available at JAXA HP).

SAIFSAIF ：： Submeter-class Augmentation with Integrity FunctionSubmeter-class Augmentation with Integrity Function

RangingRangingFunctionFunction

ErrorErrorCorrectionCorrection

IntegrityIntegrityFunctionFunction

QZS satellitesQZS satellitesGPS ConstellationGPS Constellation

Ranging SignalRanging Signal


L1-SAIF CorrectionsL1-SAIF Corrections• Example of user position error at Site 940058 Example of user position error at Site 940058

(Takayama: near center of monitor station network);(Takayama: near center of monitor station network);• Realtime operation with MSAS-like 6 monitor Realtime operation with MSAS-like 6 monitor

stations;stations;• Period: 19-23 Jan. 2008 (5 days);Period: 19-23 Jan. 2008 (5 days);• L1-SAIF provides corrections only;L1-SAIF provides corrections only;

No L1-SAIF ranging.No L1-SAIF ranging.

HorizontalHorizontalErrorError

VerticalVerticalErrorError

1.45 m1.45 m 2.92 m2.92 m

6.02 m6.02 m 8.45 m8.45 m

SystemSystem

StandaloneStandaloneGPSGPS

0.29 m0.29 m 0.39 m0.39 m

1.56 m1.56 m 2.57 m2.57 mL1-SAIFL1-SAIF

RMSRMS

MaxMax

RMSRMS

MaxMax

Note: Results shown here were obtained with survey-Note: Results shown here were obtained with survey-grade antenna and receiver in open sky condition.grade antenna and receiver in open sky condition.

Standalone GPSStandalone GPSAugmented by L1-SAIFAugmented by L1-SAIF

Augmentation to GPS OnlyAugmentation to GPS Only


Problem: IonosphereProblem: Ionosphere

IonosphereDensity

(NASA/JPL)

• The largest error source: Ionospheric propagation delay;The largest error source: Ionospheric propagation delay;• Varies on the local time, solar activity, earth magnetic field, and so on;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.Cannot be predicted; Causes large effect in the low magnetic latitude region.


Accuracy at Southwestern IslandAccuracy at Southwestern Island

At Southwestern Island (960735 Wadomari)At Southwestern Island (960735 Wadomari) At Northernmost City (950114 Kitami)At Northernmost City (950114 Kitami)

• During severe ionospheric storm condition (KpDuring severe ionospheric storm condition (Kp~7+)~7+), position accuracy with differential correction largely degrades at the Southwestern Islands;, position accuracy with differential correction largely degrades at the Southwestern Islands;• The effect is not so large at the mainland of Japan;The effect is not so large at the mainland of Japan;• It is confirmed that increase of the number of GMS shows a little improvement.It is confirmed that increase of the number of GMS shows a little improvement.

LT 14:00


Actual Ionosphere CorrectionsActual Ionosphere Corrections

PRN20PRN20

PRN28PRN28

At Southwestern IslandAt Southwestern Island At Northernmost CityAt Northernmost City

• Ionospheric correction continuously differs from the true delay by 5m or more;Ionospheric correction continuously differs from the true delay by 5m or more;• Degradation of position accuracy during storm is due to inaccurate ionospheric correction.Degradation of position accuracy during storm is due to inaccurate ionospheric correction.

5m


L1-SAIF Ionospheric CorrectionL1-SAIF Ionospheric Correction

IGPIGP

IGPIGPIPPIPP

• Vertical ionospheric delay Vertical ionospheric delay information at IGPs ( ) located at information at IGPs ( ) located at 5-degree grid points will be 5-degree grid points will be broadcast to users.broadcast to users.

• User receiver computes vertical User receiver computes vertical ionospheric delays at IPPs with ionospheric delays at IPPs with bilinear interpolation of delays at bilinear interpolation of delays at the surrounding IGPs.the surrounding IGPs.

• Vertical delay is converted to slant Vertical delay is converted to slant delay by multiplying a factor so-delay by multiplying a factor so-called obliquity factor.called obliquity factor.

120 150 1800

30

60

Longitude, E

Latitude, N

15

30

45

0

60

IGP


Thin-Shell IonosphereThin-Shell Ionosphere

• The ionosphere model used by the L1-SAIF;The ionosphere model used by the L1-SAIF;• Ionospheric propagation delay caused at a single point on the thin shell;Ionospheric propagation delay caused at a single point on the thin shell;• The vertical delay is converted into the slant direction via the slant-vertical The vertical delay is converted into the slant direction via the slant-vertical

conversion factor so-called obliquity factor, conversion factor so-called obliquity factor, FF((ELEL).).

EarthEarth

IonosphereIonosphere

ELEL

Vertical DelayVertical Delay

IIvvSlant DelaySlant Delay

FF((ELEL) ) •• I Ivv

Shell HeightShell Height(350km)(350km)

IPPIPP


Obliquity Factor, F(EL)Obliquity Factor, F(EL)

• Slant-vertical conversion factor as a function of the elevation angle;Slant-vertical conversion factor as a function of the elevation angle;• Also a function of the shell height; The current L1-SAIF specifies the shell height of 350 km.Also a function of the shell height; The current L1-SAIF specifies the shell height of 350 km.

00 1515 3030 4545

22

44

66

Obliquity Factor

Obliquity Factor

Satellite Elevation, degSatellite Elevation, deg

H=100kmH=100km

H=1000kmH=1000km

H=350kmH=350km

VerticalVerticaldelaydelay

Slant delaySlant delay

ElevationElevationAngleAngle

IonosphereIonosphereHeightHeight

Obliquity Factor = Slant / VerticalObliquity Factor = Slant / Vertical


Limitation due to Iono-ModelLimitation due to Iono-Model

Observe hereObserve hereif H=350kmif H=350km

ObserveObservedifferentdifferentpoints if points if

H=600kmH=600km

Shell Height H=350km, EL=25degShell Height H=350km, EL=25deg Shell Height H=600km, EL=25degShell Height H=600km, EL=25deg

• MCS assumes 2 GMS are observing same location of ionosphere;MCS assumes 2 GMS are observing same location of ionosphere;• However, if true height is not 350km, they are looking at different locations.However, if true height is not 350km, they are looking at different locations.


Limitation due to Iono-ModelLimitation due to Iono-Model• Too Simple Vertical Structure:Too Simple Vertical Structure:

– Assuming the thin-shell ionosphere at the fixed height of 350km;Assuming the thin-shell ionosphere at the fixed height of 350km;

– IPP location may differ from the actual point with the peak density; Essentially, IPP location may differ from the actual point with the peak density; Essentially, the ionospheric delay is caused over a certain distance within ionosphere;the ionospheric delay is caused over a certain distance within ionosphere;

The model may not represent the horizontal structure as well as vertical.The model may not represent the horizontal structure as well as vertical.

– Obliquity factor may not reflect the true vertical structure of the ionosphere.Obliquity factor may not reflect the true vertical structure of the ionosphere.

• Linear Interpolation of Vertical Delays at IGP:Linear Interpolation of Vertical Delays at IGP:– Assumption that the spatial scale of the ionosphere variation is roughly Assumption that the spatial scale of the ionosphere variation is roughly 500km 500km

or more;or more;

– Small structure cannot, even if observed, be reflected to the delay information.Small structure cannot, even if observed, be reflected to the delay information.

• Need Alternative Ionospheric Correction Methods:Need Alternative Ionospheric Correction Methods:– Change assumptions on the ionosphere or avoid error by some way;Change assumptions on the ionosphere or avoid error by some way;

– Allow definition of new L1-SAIF messages;Allow definition of new L1-SAIF messages;

– Minimize modifications from the current message and correction procedure.Minimize modifications from the current message and correction procedure.


Candidate MethodsCandidate Methods• Maintain Single-Layer Thin-Shell Ionosphere Model:Maintain Single-Layer Thin-Shell Ionosphere Model:

– Employ widely-used simple model to minimize modifications and to avoid Employ widely-used simple model to minimize modifications and to avoid complexity of user receivers;complexity of user receivers;

– MT26-like message structure: Share IGP information given by MT18;MT26-like message structure: Share IGP information given by MT18; Note: MT26 has 7 spare (unused) bits.Note: MT26 has 7 spare (unused) bits.

– Define new message as MT55 (Message Type 55) for this purpose.Define new message as MT55 (Message Type 55) for this purpose.

• Method 1: Variable Ionosphere HeightMethod 1: Variable Ionosphere Height::– Broadcast the peak height of ionosphere in addition to grid delay information.Broadcast the peak height of ionosphere in addition to grid delay information.

• Method 2: Ionospheric Correction per SatelliteMethod 2: Ionospheric Correction per Satellite::– Generate vertical delay information at the grid points per each GPS satellite.Generate vertical delay information at the grid points per each GPS satellite.

• Method 3: Ionospheric Correction per DirectionMethod 3: Ionospheric Correction per Direction::– Generate vertical delay information at the grid points per each line-of-sight Generate vertical delay information at the grid points per each line-of-sight

direction from receiver to satellite.direction from receiver to satellite.


Existing Message Type 26Existing Message Type 26• MT26: Broadcast Ionospheric Vertical DelayMT26: Broadcast Ionospheric Vertical Delay

– Contains vertical delay information at IGP;Contains vertical delay information at IGP;– A MT26 message contains information at 15 IGPs;A MT26 message contains information at 15 IGPs;

Repeat Content Bits Range Resolution

1 IGP Band ID 4 0 to 10 1

1 IGP Block ID 4 0 to 13 1

15

IGP Vertical Delay

9 0 to 63.875 m 0.125 m

GIVEI 4 (Table) —

1 IODI 2 0 to 3 1

1 Spare 7 — —

Message Type 26: Ionospheric Delay InformationMessage Type 26: Ionospheric Delay Information


New Message Design (1)New Message Design (1)• Method 1: Variable Ionosphere HeightMethod 1: Variable Ionosphere Height::

– Broadcast the peak height of ionosphere in addition to grid delay information;Broadcast the peak height of ionosphere in addition to grid delay information;

– Both MCS and user receivers need to compute the ionospheric pierce point Both MCS and user receivers need to compute the ionospheric pierce point and the obliquity factor appropriately for given peak height;and the obliquity factor appropriately for given peak height;

– MT55 contains the information of the peak height of the ionosphere.MT55 contains the information of the peak height of the ionosphere.




15

IGP Vertical Delay

9 0 to 63.875 m 0.125 m

GIVEI 4 (Table) —

1 IODI 2 0 to 3 1

1 Peak Height 2 (Table) —

1 Spare 5 — —

00: 350 km00: 350 km01: 250 km01: 250 km10: 600 km10: 600 km11: 1,000 km11: 1,000 km

Peak Height ofPeak Height ofIonosphereIonosphere

Identical to MT26Identical to MT26

Message Type 55 (1): Advanced Ionospheric CorrectionMessage Type 55 (1): Advanced Ionospheric Correction


New Message Design (2)New Message Design (2)• Method 2: Ionospheric Correction per SatelliteMethod 2: Ionospheric Correction per Satellite::

– Generate every grid delay information for each ranging source satellite in view;Generate every grid delay information for each ranging source satellite in view;– MT55 contains an identification of satellite;MT55 contains an identification of satellite;

Satellite ID requires at least 8 bits, however, we have only 7 spare bits in MT26;Satellite ID requires at least 8 bits, however, we have only 7 spare bits in MT26; Here we use only GPS satellites for the experimental purpose.Here we use only GPS satellites for the experimental purpose.

– May need more measurements (ground stations) for this correction.May need more measurements (ground stations) for this correction.




15

IGP Vertical Delay

9 0 to 63.875 m 0.125 m

GIVEI 4 (Table) —

1 IODI 2 0 to 3 1

1 SV ID 5 1 to 32 1

1 Spare 2 — —

SV IDSV ID(PRN(PRN--1)1)




Example DefinitionExample Definitionof LOS Directionof LOS Direction

New Message Design (3)New Message Design (3)• Method 3: Ionospheric Correction per DirectionMethod 3: Ionospheric Correction per Direction::

– Generate every grid delay information for each line-of-sightGenerate every grid delay information for each line-of-sightdirection from receiver to satellite (azimuth and elevation angle);direction from receiver to satellite (azimuth and elevation angle);

– Divide the sky into, for example, 5 directions;Divide the sky into, for example, 5 directions; MT55 contains the information of the direction.MT55 contains the information of the direction.

– Also may need more measurements (ground stations) for this correction.Also may need more measurements (ground stations) for this correction.




15

IGP Vertical Delay

9 0 to 63.875 m 0.125 m

GIVEI 4 (Table) —

1 IODI 2 0 to 3 1

1 Direction 3 (Table) —

1 Spare 4 — —

000: All000: All001: Zenith001: Zenith010: North010: North011: East011: East100: South100: South101: West101: West

LOS DirectionLOS Direction



010010

100100

101101 011011001001

Experiment: ConfigurationExperiment: ConfigurationION ITM Jan. 2014ION ITM Jan. 2014 - Slide - Slide 1919

L1SMSL1SMSGEONETGEONET

QZSQZS

QZSS MCSQZSS MCS

GPSGPSSatellitesSatellites

Measure-Measure-mentsments

L1-SAIFL1-SAIFMessageMessage

GSI ServerGSI Server(Tokyo)(Tokyo)

ENRIENRI(Tokyo)(Tokyo)

JAXA TKSCJAXA TKSC(Tsukuba)(Tsukuba)

L1-S

AIF S

ignal

L1-S

AIF S

ignalNav Message

Nav MessageRan

ging

Signal

Rangin

g Signa

l

K-band Uplink

K-band Uplink

Operates in Off-Line Mode

Evaluation by User Receiver

Software

• Experiment Using L1-SAIF Master Station (L1SMS):Experiment Using L1-SAIF Master Station (L1SMS):– Upgrade to support new messages (MT55) for Methods (1) to (3);Upgrade to support new messages (MT55) for Methods (1) to (3);

– For this experiment, L1SMS operates in off-line mode; No realtime connection to For this experiment, L1SMS operates in off-line mode; No realtime connection to GEONET and QZSS MCS; RINEX files from GEONET;GEONET and QZSS MCS; RINEX files from GEONET;

– Evaluate augmentation performance of new messages by receiver software also Evaluate augmentation performance of new messages by receiver software also upgraded to support MT55.upgraded to support MT55.

Experiment: ConfigurationExperiment: ConfigurationION ITM Jan. 2014ION ITM Jan. 2014 - Slide - Slide 2020

• Upgrade of L1-SAIF Master Station (L1SMS):Upgrade of L1-SAIF Master Station (L1SMS):– Support new messages (MT55) for Methods (1) to (3);Support new messages (MT55) for Methods (1) to (3);

– Accept additional measurements from IMS (Ionosphere Monitor Station) sites to Accept additional measurements from IMS (Ionosphere Monitor Station) sites to increase the number of measurements (IPPs) for Method (2) and (3);increase the number of measurements (IPPs) for Method (2) and (3);

– User receiver software is also upgraded to decode and apply MT55.User receiver software is also upgraded to decode and apply MT55.

L1SMSL1SMS

ReceiverReceiverSoftwareSoftware

L1-SAIFL1-SAIFMessageMessage

UpgradedUpgradedfor MT55for MT55

UserUserAlgorithmsAlgorithms

PerformancePerformanceEvaluationEvaluation

GPSGPSSatellitesSatellites

GMS/IMSGMS/IMSMeasurementsMeasurementsRINEXRINEX

FilesFiles

GEONETGEONET

Ranging Signal

Ranging Signal

GMSGMSDataData

GMS+IMSGMS+IMSDataData

UserUserMeasurementsMeasurements

MT26/55MT26/55

Clock/OrbitClock/OrbitCorrectionsCorrections

IonosphereIonosphereCorrectionsCorrections


Experiment: Monitor StationsExperiment: Monitor Stations• Observation Data from GEONETObservation Data from GEONET::

– GPS network operated by Geospatial GPS network operated by Geospatial Information Authority of Japan; Information Authority of Japan;

– Survey-grade receivers over 1,200 Survey-grade receivers over 1,200 stations within Japanese territory.stations within Japanese territory.

• Monitor Stations for ExperimentMonitor Stations for Experiment::– 6 GMS (Ground Monitor Station) near 6 GMS (Ground Monitor Station) near

MSAS GMS locations for clock/orbit MSAS GMS locations for clock/orbit and ionospheric corrections;and ionospheric corrections;

– 8 IMS (Ionosphere Monitor Station) for 8 IMS (Ionosphere Monitor Station) for Method (2) and (3) ionospheric Method (2) and (3) ionospheric corrections.corrections.

• User StationsUser Stations::– Selected 5 stations from North to Selected 5 stations from North to

South: (1) to (5) for performance South: (1) to (5) for performance evaluation.evaluation.


Baseline PerformanceBaseline Performance

LT 14:00

At Southwestern Island (User #4)At Southwestern Island (User #4) At Northernmost City (User #1)At Northernmost City (User #1)

• During severe ionospheric storm condition (KpDuring severe ionospheric storm condition (Kp~7+)~7+), position accuracy with differential correction largely degrades at the Southwestern , position accuracy with differential correction largely degrades at the Southwestern Islands;Islands;

• The effect is not so large at the mainland of Japan;The effect is not so large at the mainland of Japan;• All corrections are derived by measurements from 6 GMS.All corrections are derived by measurements from 6 GMS.


Variable Ionosphere HeightVariable Ionosphere Height

LT 14:00


• The ionosphere shell height of 600 km improves position accuracy a little;The ionosphere shell height of 600 km improves position accuracy a little;• However, some degradation is observed at the north and during quiet conditions; The effect is limited;However, some degradation is observed at the north and during quiet conditions; The effect is limited;• All corrections are derived by measurements from 6 GMS.All corrections are derived by measurements from 6 GMS.


Variable Ionosphere HeightVariable Ionosphere Height

Storm Condition (11/10/23 to 11/10/26)Storm Condition (11/10/23 to 11/10/26) Quiet Condition (12/7/22 to 12/7/24)Quiet Condition (12/7/22 to 12/7/24)

• The ionosphere shell height of 600 km may improve the balance of the accuracy between North and The ionosphere shell height of 600 km may improve the balance of the accuracy between North and South;South;

• The effect is not so large; Need more investigation.The effect is not so large; Need more investigation.


Iono-Correction per SatelliteIono-Correction per Satellite

LT 14:00


• Reduces position error by roughly 40% at the Southwestern Islands, while maintains the accuracy at Reduces position error by roughly 40% at the Southwestern Islands, while maintains the accuracy at other regions;other regions;

• Clock/Orbit corrections by 6 GMS; Ionospheric corrections by 6 GMS + 8 IMS.Clock/Orbit corrections by 6 GMS; Ionospheric corrections by 6 GMS + 8 IMS.


Iono-Correction per DirectionIono-Correction per Direction

LT 14:00


• This method also has a capability to reduce position error at the Southwestern Islands;This method also has a capability to reduce position error at the Southwestern Islands;• Desirable behavior at other regions;Desirable behavior at other regions;• Clock/Orbit corrections by 6 GMS; Ionospheric corrections by 6 GMS + 8 IMS.Clock/Orbit corrections by 6 GMS; Ionospheric corrections by 6 GMS + 8 IMS.


Iono-Correction per SV/DirectionIono-Correction per SV/Direction

Storm Condition (11/10/23 to 11/10/26)Storm Condition (11/10/23 to 11/10/26) Quiet Condition (12/7/22 to 12/7/24)Quiet Condition (12/7/22 to 12/7/24)

• These methods have similar performance on ionospheric corrections;These methods have similar performance on ionospheric corrections;• In terms of the number of messages to be broadcast, Method (3) correction per direction has the In terms of the number of messages to be broadcast, Method (3) correction per direction has the

advantage.advantage.


ConclusionConclusion• ENRI has been developing L1-SAIF signalENRI has been developing L1-SAIF signal::

– Signal design: GPS/SBAS-like L1 C/A code (PRN 183);Signal design: GPS/SBAS-like L1 C/A code (PRN 183);

– Planned as an augmentation to mobile users.Planned as an augmentation to mobile users.

• Ionosphere disturbance is a concernIonosphere disturbance is a concern::– L1-SAIF signal achieves good accuracy less than 1 meter in an RMS manner at L1-SAIF signal achieves good accuracy less than 1 meter in an RMS manner at

the mainland of Japan;the mainland of Japan;– Ionosphere disturbance sometimes degrades the position accuracy, especially at Ionosphere disturbance sometimes degrades the position accuracy, especially at

the Southwestern Islands of Japanese territory;the Southwestern Islands of Japanese territory;– In order to improve the accuracy at the southwestern islands during ionospheric In order to improve the accuracy at the southwestern islands during ionospheric

storm, we have designed some new L1-SAIF messages and tested them.storm, we have designed some new L1-SAIF messages and tested them. Method (3) corrections per direction has a good property.Method (3) corrections per direction has a good property.

• Further Investigations will include:Further Investigations will include:– Validation of performance against historical storm events at many locations;Validation of performance against historical storm events at many locations;

– Performance at other Asian Countries;Performance at other Asian Countries;

– More investigation of other correction methods against ionospheric disturbances.More investigation of other correction methods against ionospheric disturbances.

Documents

T. Sakai, K. Hoshinoo, and K. Ito Electronic Navigation Research Institute, Japan T. Sakai, K. Hoshinoo, and K. Ito Electronic Navigation Research Institute,