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

ION ITM 2014 San Diego, CA Jan. 27-29, 2014. Ionospheric Correction at the Southwestern Islands for the QZSS L1-SAIF. T. Sakai, K. Hoshinoo, and K. Ito Electronic Navigation Research Institute, Japan. Introduction. QZSS (Quasi-Zenith Satellite System) program :

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

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  1. ION ITM 2014 San Diego, CA Jan. 27-29, 2014 Ionospheric Correction at the Southwestern Islands for the QZSS L1-SAIF T. Sakai, 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 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 an RMS manner at the mainland of Japan; • Ionosphere disturbance sometimes degrades the position accuracy, especially at the Southwestern Islands of Japanese territory; • In order to improve the accuracy at the southwestern islandsduring ionospheric storm, we have designed some new L1-SAIF messages and tested them.

  3. GPS/GEO QZS QZSS Concept • 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. Ranging Function QZS satellites GPS Constellation Error Correction Ranging Signal Integrity Function L1-SAIF Signal • 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; • See IS-QZSS for detail (Available at JAXA HP). User GPS Receivers SAIF： Submeter-class Augmentation with Integrity Function

  5. System Horizontal Error Vertical Error Standalone GPS Standalone GPS RMS 1.45 m 2.92 m Augmented by L1-SAIF Max 6.02 m 8.45 m L1-SAIF RMS 0.29 m 0.39 m Max 1.56 m 2.57 m L1-SAIF Corrections • 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); • L1-SAIF provides corrections only; • No L1-SAIF ranging. Augmentation to GPS Only Note: Results shown here were obtained with survey-grade antenna and receiver in open sky condition.

  6. 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.

  7. LT 14:00 Accuracy at Southwestern Island At Southwestern Island (960735 Wadomari) At Northernmost City (950114 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 confirmed that increase of the number of GMS shows a little improvement.

  8. PRN28 PRN20 Actual Ionosphere Corrections 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.

  9. 60 60 • Vertical ionospheric delay information at IGPs ( ) located at 5-degree grid points 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 a factor so-called obliquity factor. 45 Latitude, N 30 30 15 IGP 0 IGP 0 IGP 120 150 180 IPP Longitude, E L1-SAIF Ionospheric Correction

  10. Vertical Delay Iv IPP Slant Delay F(EL) • Iv Ionosphere EL Shell Height (350km) Earth Thin-Shell Ionosphere • The ionosphere model used by the L1-SAIF; • 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 conversion factor so-called obliquity factor, F(EL).

  11. 6 H=100km 4 Obliquity Factor H=350km 2 H=1000km 0 15 30 45 Satellite Elevation, deg Obliquity Factor, F(EL) Vertical delay Slant delay Elevation Angle Ionosphere Height Obliquity Factor = Slant / Vertical • 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.

  12. Observe different points if H=600km Observe here if H=350km Limitation due to Iono-Model Shell Height H=350km, EL=25deg Shell Height H=600km, EL=25deg • MCS assumes 2 GMS are observing same location of ionosphere; • However, if true height is not 350km, they are looking at different locations.

  13. Limitation due to Iono-Model • Too Simple Vertical Structure: • Assuming the thin-shell ionosphere at the fixed height of 350km; • 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 model may not represent the horizontal structure as well as vertical. • Obliquity factor may not reflect the true vertical structure of the ionosphere. • Linear Interpolation of Vertical Delays at IGP: • Assumption that the spatial scale of the ionosphere variation is roughly 500km or more; • Small structure cannot, even if observed, be reflected to the delay information. • Need Alternative Ionospheric Correction Methods: • Change assumptions on the ionosphere or avoid error by some way; • Allow definition of new L1-SAIF messages; • Minimize modifications from the current message and correction procedure.

  14. Candidate Methods • Maintain Single-Layer Thin-Shell Ionosphere Model: • Employ widely-used simple model to minimize modifications and to avoid complexity of user receivers; • MT26-like message structure: Share IGP information given by MT18; • Note: MT26 has 7 spare (unused) bits. • Define new message as MT55 (Message Type 55) for this purpose. • Method 1: Variable Ionosphere Height: • Broadcast the peak height of ionosphere in addition to grid delay information. • Method 2: Ionospheric Correction per Satellite: • Generate vertical delay information at the grid points per each GPS satellite. • Method 3: Ionospheric Correction per Direction: • Generate vertical delay information at the grid points per each line-of-sight direction from receiver to satellite.

  15. Existing Message Type 26 • MT26: Broadcast Ionospheric Vertical Delay • Contains vertical delay information at IGP; • A MT26 message contains information at 15 IGPs; Message Type 26: Ionospheric Delay Information

  16. New Message Design (1) • Method 1: Variable Ionosphere Height: • Broadcast the peak height of ionosphere in addition to grid delay information; • Both MCS and user receivers need to compute the ionospheric pierce point and the obliquity factor appropriately for given peak height; • MT55 contains the information of the peak height of the ionosphere. Identical to MT26 Peak Height of Ionosphere 00: 350 km 01: 250 km 10: 600 km 11: 1,000 km Message Type 55 (1): Advanced Ionospheric Correction

  17. New Message Design (2) • Method 2: Ionospheric Correction per Satellite: • Generate every grid delay information for each ranging source satellite in view; • MT55 contains an identification of satellite; • 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. • May need more measurements (ground stations) for this correction. Identical to MT26 SV ID (PRN-1) Message Type 55 (2): Advanced Ionospheric Correction

  18. 010 001 101 011 100 New Message Design (3) Example Definition of LOS Direction • Method 3: Ionospheric Correction per Direction: • Generate every grid delay information for each line-of-sight • direction from receiver to satellite (azimuth and elevation angle); • Divide the sky into, for example, 5 directions; • MT55 contains the information of the direction. • Also may need more measurements (ground stations) for this correction. Identical to MT26 LOS Direction 000: All 001: Zenith 010: North 011: East 100: South 101: West Message Type 55 (3): Advanced Ionospheric Correction

  19. GPS Satellites QZS Nav Message L1-SAIF Signal K-band Uplink Ranging Signal Measure- ments L1-SAIF Message GEONET L1SMS QZSS MCS GSI Server (Tokyo) ENRI (Tokyo) JAXA TKSC (Tsukuba) Experiment: Configuration • Experiment Using L1-SAIF Master Station (L1SMS): • Upgrade to support new messages (MT55) for Methods (1) to (3); • For this experiment, L1SMS operates in off-line mode; No realtime connection to GEONET and QZSS MCS; RINEX files from GEONET; • Evaluate augmentation performance of new messages by receiver software also upgraded to support MT55. Evaluation by User Receiver Software Operates in Off-Line Mode

  20. Experiment: Configuration • Upgrade of L1-SAIF Master Station (L1SMS): • Support new messages (MT55) for Methods (1) to (3); • Accept additional measurements from IMS (Ionosphere Monitor Station) sites to increase the number of measurements (IPPs) for Method (2) and (3); • User receiver software is also upgraded to decode and apply MT55. GPS Satellites Performance Evaluation GMS Data L1SMS L1-SAIF Message Clock/Orbit Corrections Ranging Signal Receiver Software GMS+IMS Data Ionosphere Corrections MT26/55 User Algorithms GEONET GMS/IMS Measurements RINEX Files Upgraded for MT55 User Measurements

  21. Experiment: Monitor Stations • Observation Data from GEONET: • GPS network operated by Geospatial Information Authority of Japan; • Survey-grade receivers over 1,200 stations within Japanese territory. • Monitor Stations for Experiment: • 6 GMS (Ground Monitor Station) near MSAS GMS locations for clock/orbit and ionospheric corrections; • 8 IMS (Ionosphere Monitor Station) for Method (2) and (3) ionospheric corrections. • User Stations: • Selected 5 stations from North to South: (1) to (5) for performance evaluation.

  22. LT 14:00 Baseline Performance At Southwestern Island (User #4) At Northernmost City (User #1) • 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; • All corrections are derived by measurements from 6 GMS.

  23. Variable Ionosphere Height LT 14:00 At Southwestern Island (User #4) At Northernmost City (User #1) • 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; • All corrections are derived by measurements from 6 GMS.

  24. Variable Ionosphere Height Storm Condition (11/10/23 to 11/10/26) 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 South; • The effect is not so large; Need more investigation.

  25. Iono-Correction per Satellite LT 14:00 At Southwestern Island (User #4) At Northernmost City (User #1) • Reduces position error by roughly 40% at the Southwestern Islands, while maintains the accuracy at other regions; • Clock/Orbit corrections by 6 GMS; Ionospheric corrections by 6 GMS + 8 IMS.

  26. Iono-Correction per Direction LT 14:00 At Southwestern Island (User #4) At Northernmost City (User #1) • This method also has a capability to reduce position error at the Southwestern Islands; • Desirable behavior at other regions; • Clock/Orbit corrections by 6 GMS; Ionospheric corrections by 6 GMS + 8 IMS.

  27. Iono-Correction per SV/Direction Storm Condition (11/10/23 to 11/10/26) Quiet Condition (12/7/22 to 12/7/24) • 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 advantage.

  28. 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 an RMS manner at the mainland of Japan; • Ionosphere disturbance sometimes degrades the position accuracy, especially at the Southwestern Islands of Japanese territory; • In order to improve the accuracy at the southwestern islands during ionospheric storm, we have designed some new L1-SAIF messages and tested them. • Method (3) corrections per direction has a good property. • Further Investigations will include: • Validation of performance against historical storm events at many locations; • Performance at other Asian Countries; • More investigation of other correction methods against ionospheric disturbances.

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