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Takeyasu Sakai, T. Aso, M. Kitamura, K. Hoshinoo, and K. Ito

ION ITM 2016 Monterey, CA Jan. 25-28, 2016. Expanding the Coverage of Local Area Differential Correction. Takeyasu Sakai, T. Aso, M. Kitamura, K. Hoshinoo, and K. Ito Electronic Navigation Research Institute, Japan. Introduction. QZSS (Quasi-Zenith Satellite System) Program :

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Takeyasu Sakai, T. Aso, M. Kitamura, K. Hoshinoo, and K. Ito

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  1. ION ITM 2016 Monterey, CA Jan. 25-28, 2016 Expanding the Coverage of Local Area Differential Correction Takeyasu Sakai, T. Aso, M. Kitamura, K. Hoshinoo, and K. Ito Electronic Navigation Research Institute, Japan

  2. 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 ranging signals on three freq. and some augmentation signals. • Submeter-Level Augmentation Service (SLAS): • The services of the operational QZSS are defined as: • Positioning service, Augmentation service, and Messaging service. • Submeter augmentation is redefined as SLAS: • Broadcast on L1 C/A signal called ‘L1S’. • Contains local-area differential corrections at a number of reference stations. • Expanding the Coverage of Local Corrections: • The messages on L1S signal contain multiple local corrections generated at multiple reference stations; Users have all of them simultaneously. • Potential to improve DGPS positioning accuracy by additionally using differential corrections generated at other reference stations or the WADGPS correction broadcast by QZSS itself or other SBAS system.

  3. QZSS Concept GPS/GEO QZSS-IGSO IGSOs 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 and process other GNSS satellite signals at any time. • IGSOs centered at 135E. • Eccentricity 0.075, inclination 43deg. • Additional GEO.

  4. Now QZSS Program • Phase 1: Experimental System • The QZSS project originally began in 2003. • The first satellite was intended for experimental purposes. The following satellites would be considered after the first one. • The first satellite “Michibiki” was successfully launched in Sept. 2010. • Phase 2: Operational System • In Sept. 2011, it was decided to implement operational QZSS. • Begins the operation with 4-satellite configuration in April 2018, and 7-satellite configuration will follow by 2023. • 3 satellites including a GEO are now in the factory and will be launched in 2017. 2023 7-SV configuration 2018.4 4-SV Configuration 2010.9 Experiment

  5. QZSS Services • Positioning Service • GPS-like L1C/A, L2C, L5, and L1C signals working with GPS. • Improves availability of navigation. • Minimum modifications from GPS signals. QZSS satellites • Augmentation Service • Submeter-Level Augmentation (SLAS): L1S C/A code signal for mobile users. • Centimeter-Level Augmentation (CLAS): Accurate service with carrier-phase. • SBAS service by QZS-3 GEO from 2020: Takes over the current MSAS service. QZS-1: IGSO-1 on orbit QZS-2: IGSO-2 QZS-3: GEO-1 QZS-4: IGSO-3 • Messaging Service • Emergency communication link between mobile terminals and the QZSS center for disaster events. • Served by QZS-3 GEO. in 2017

  6. L1S Submeter Augmentation L1C/A Signal QZSS satellites Ranging Function GPS Constellation L1S Signal Submeter Correction Ranging Signals • Augmentation signal L1S on L1 C/A code. • SBAS-like message structure: 250-bit messages broadcast every second. • Contains multiple differential corrections generated by local DGPS stations. • User receivers can receive both GPS and L1S signals with a single antenna and RF front-end. User GNSS Receivers

  7. L1S DGPS Stations • SLAS: Submeter-Level Augmentation Service • SBAS-like 250 bps message on C/A-code signal called L1S. • User receivers with L1 C/A do not need to equip extra hardware. • Differential Corrections: • L1S broadcasts local-area differential corrections, other than wide-area, for better accuracy in large cities. • As a result of consideration of the shape of Japan. • Corrections are generated at 13 DGPS stations independently. • New messages are being defined for broadcast of such corrections. L1S DGPS Station MSAS GMS (for reference) 200km Radius

  8. DGPS Performance GPS Only DGPS DGPS (1-Out) WADGPS • DGPS performance of test message generated for 2 days, 24 to 25 of Dec. 2015. • Assumes that a user selects the nearest DGPS station. • Condition ‘1-out’ means the nearest station is out and a user selects the second nearest. • Better accuracies than WADGPS correction.

  9. Expanding DGPS Coverage • Expanding the Coverage of Local Corrections: • The messages on L1S contain local corrections generated at 13 DGPS stations; • Users have all of them simultaneously. • Potential to improve positioning accuracy by using: • Differential corrections at other DGPS stations; or • WADGPS correction broadcast by QZSS itself or other SBAS system. • Using Corrections at Other DGPS Stations: • Generates the virtual station inside the user receiver based on corrections at some DGPS stations surrounding the user by: • Weighted-average of corrections or planar estimation of corrections. • Using WADGPS Correction: • WADGPS correction also available for the user receiver. • Broadcast by QZSS itself or other SBAS systems. • Make a modification to the DGPS corrections based on the difference of WADGPS correction between user location and DGPS station. • Full correction or ionosphere-only correction. • Note: Tropospheric delay should be properly excluded from these processing. • It depends upon altitude of user receiver; Can be cancelled by the model.

  10. Virtual Station DGPS • Using Corrections at Other DGPS Stations: • Generates virtual stations inside the user receiver based on corrections at some DGPS stations surrounding the user by: • Weighted-average of corrections • or, Planar estimation of corrections. PRC for user PRC (pseudorange correction) for user: where weighting factor is: PRC of station i Location of user Location of station i Modeling correction values as: PRC for user is estimated (by weighted least-square method) as: Latitude offset from user location Longitude offset from user location

  11. Delta WADGPS Correction • Using WADGPS Correction: • WADGPS correction also available for the user receiver. • Broadcast by QZSS itself or other SBAS systems. • Make a modification to the DGPS corrections based on the difference of WADGPS correction between user location and DGPS station. • Full correction or ionosphere-only correction, because the dominant factor is ionosphere. Local DGPS correction at station i Difference of WADGPS correction between user location and DGPS station i • WADGPS correction at location x : • Full correction with: FC, LTC, and IC • Ionosphere-only correction with: IC

  12. Experimental Trial • Observation Data from GEONET: • GPS network operated by Geospatial Information Authority of Japan. • Archive of 30-sec interval data. • Selected GEONET sites near to stations planned for QZSS-SLAS. • Corrections generated for this experiment: • Local DGPS corrections at 13 stations (a) to (m). • WADGPS correction for the whole area. • Both for 2 days, 24 and 25 Dec. 2015. • User Stations: • 7 stations from North to South: (1) to (7) for performance evaluation. • Compute position estimates with local DGPS corrections and coverage expansion methods.

  13. Baseline Performance GPS Only DGPS DGPS (1-Out) WADGPS • DGPS and WADGPS performance for 2 days. • For DGPS, assumes that a user selects the nearest DGPS station. • Condition ‘1-out’ means the nearest station is out and a user selects the second nearest.

  14. Position Error at Location (4) DGPS @User 4 WADGPS At Location (4) • User (4) applies the DGPS correction generated by Station (c). • Better accuracy than WADGPS correction.

  15. Virtual Station DGPS DGPS WADGPS VSTN WGT-AVG VSTN PLANAR • Virtual station DGPS based on weighted-average and planar estimation. • Improvement at location (3) and (4); Both inside reference network. • Large error at Location (5) and (6); Need more investigation.

  16. Position Error at Location (4) DGPS @User 4 WADGPS VSTN WGT-AVG At Location (4) • Virtual station with weighted-average: Stable solution at Location (4). • Planar estimation shows similar result with weighted-average.

  17. Delta WADGPS Correction DGPS WADGPS Delta WADGPS Delta WADGPS (Iono Only) • Delta WADGPS with full correction and ionosphere-only correction. • The performance is similar with DGPS. • Small effect of localization by WADGPS correction. • Distance between user and the nearest DGPS station seems small enough.

  18. Position Error at Location (4) DGPS @User 4 WADGPS Delta WADGPS At Location (4) • Delta WADGPS correction: Similar with DGPS result. • Results by full correction and ionosphere-only correction are almost identical.

  19. Virtual Station: 1-Station Out DGPS WADGPS VSTN WGT-AVG VSTN PLANAR • The performance in case that the nearest station for each user location is out. • Position accuracy approaches to case of WADGPS. • The characteristics of virtual station DGPS does not change. • Improvement inside reference network but large error at (5) and (6).

  20. Delta WADGPS: 1-Station Out DGPS WADGPS Delta WADGPS Delta WADGPS (Iono Only) • The performance in case that the nearest station for each user location is out. • Delta WADGPS again shows the performance similar with DGPS • Small effect of localization by WADGPS correction. • Need investigation for cases during the solar-high ionosphere disturbance events.

  21. Conclusion • 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 ranging signals on three freq. and some augmentation signals. • SLAS: Submeter-level augmentation service: • Broadcast on L1 C/A signal called L1S. • Contains local-area differential corrections at a number of reference stations. • Expanding the coverage of local corrections: • Virtual Station: Improves position accuracy inside reference network. • Not good outside reference network; Need more investigation. • Delta WADGPS: Similar performance with local DGPS corrections. • Needs only ionospheric corrections. • Further investigations would include: • Performance of Delta WADGPS method well-outside reference network. • Response against the solar-high ionosphere disturbance events.

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