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T. Sakai, T. Yoshihara, S. Fukushima, and K. Ito Electronic Navigation Research Institute, Japan

ION ITM 2009 Anaheim, CA Jan. 26-28, 2009. The Ionospheric Correction Processor for SBAS and QZSS L1-SAIF. T. Sakai, T. Yoshihara, S. Fukushima, and K. Ito Electronic Navigation Research Institute, Japan. Introduction. MSAS has been operational since 2007:

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T. Sakai, T. Yoshihara, S. Fukushima, and K. Ito Electronic Navigation Research Institute, Japan

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  1. ION ITM 2009 Anaheim, CA Jan. 26-28, 2009 The Ionospheric Correction Processor for SBAS and QZSS L1-SAIF T. Sakai, T. Yoshihara, S. Fukushima, and K. Ito Electronic Navigation Research Institute, Japan

  2. Introduction • MSAS has been operational since 2007: • SBAS augmentation signal offers: wide-area differential correction, integrity function, and ranging function. • QZSS will broadcast another augmentation signal in 2010: • ENRI is developing L1-SAIF (Submeter-class Augmentation with Integrity Function) on GPS/SBAS L1 frequency; • Upper compatible with SBAS signal; Also offers WADGPS, integrity, and ranging. • Ionosphere is a major problem for both systems: • Developed the Ionospheric Correction Processor (ICP) independent from WADGPS correction processor; • Implemented and integrated with QZSS L1-SAIF Message Generator (L1SMG); Tested successfully.

  3. Part 1 Overview of MSAS and QZSS L1-SAIF Programs

  4. Clock Correction • Same contribution to any user location; • Not a function of location; • Needs fast correction. Ionospheric Correction • Function of user location; • Up to 100 meters; • Vertical structure may be described as a thin shell. Orbit Correction • Different contribution to different user location; • Not a function of user location; but a function of line-of-sight direction; • Long-term correction. Ionosphere Tropospheric Correction • Function of user location, especially height of user; • Up to 20 meters; • Can be corrected enough by a fixed model. Troposphere WADGPS Concept

  5. MSAS Status • Satellite navigation for civil aviation use: • SBAS international standard; • Compatible with US WAAS and European EGNOS. • MSAS facilities: • 2 GEOs: MTSAT-1R (PRN 129) and MTSAT-2 (PRN 137) on orbit; • 6 domestic GMSs and 2 RMSs (Hawaii and Australia) connected with 2 MCSs; • IOC WAAS software with localization. • IOC service since Sept. 27, 2007: • Certified for Enroute to NPA operations as a sole mean navigation; • Stable operation. MTSAT-1R MTSAT-2

  6. @Kawagoe (93011) 08/1/17-19 PRN129 GPS MSAS GPS @Kawagoe (93011) 08/1/17-19 PRN129 MSAS Horizontal Position Accuracy RMS 0.42m MAX 1.64m Vertical Position Accuracy RMS 0.57m MAX 2.34m MSAS Performance

  7. Concerns for MSAS • The current MSAS is built on the IOC WAAS: • As the first satellite navigation system developed by Japan, the design tends to be conservative; • The primary purpose is providing horizontal navigation means to aviation users; Ionopsheric corrections may not be used; • Achieves 100% availability of Enroute to NPA flight modes. • The major concern for vertical guidance is ionosphere: • The ionospheric term is dominant factor of position solution uncertainty; • Necessary to reduce ionospheric uncertainty to provide vertical guidance with reasonable availability.

  8. QZSS Concept QZS GPS/GEO • Signal from high elevation angle • Applicable to navigation services for mountain area and urban canyon • Footprint of QZS orbit • Centered 137E • Eccentricity 0.1, Inclination 45deg

  9. QZSS Signals • Supplement signals: • GPS-compatible L1C/A, L2C, L5, and L1C signals working with GPS; For improving availability of navigation; • With minimum modifications from GPS signal specifications; • Coordination with GPS Wing on broadcasting L1C signal; • JAXA is responsible for all supplement signals. • Augmentation signals: • Augmentation to GPS; Possibly plus Galileo; • L1-SAIF (Submeter-class Augmentation with Integrity Function): compatible with SBAS; reasonable performance for mobile users; • LEX: for experimental purposes; member organizations may use as 2kbps experimental data channel; • ENRI is working for L1-SAIF and JAXA is developing LEX.

  10. QZSS Frequency Plan Signal Channel Frequency Bandwidth Min. Rx Power QZS-L1C L1CD 1575.42 MHz 24 MHz –163.0 dBW L1CP 24 MHz – 158.25 dBW QZS-L1-C/A 24 MHz – 158.5 dBW QZS-L1-SAIF 24 MHz – 161.0 dBW QZS-L2C 1227.6 MHz 24 MHz – 160.0 dBW QZS-L5 L5I 1176.45 MHz 25 MHz – 157.9 dBW L5Q 25 MHz – 157.9 dBW QZS-LEX 1278.75 MHz 42 MHz – 155.7 dBW Find detail in IS-QZSS document.

  11. QZSS L1-SAIF Signal • QZSS will broadcast wide-area augmentation signal: • Called L1-SAIF (Submeter-class Augmentation with Integrity Function); • Developed by ENRI. • L1-SAIF signal offers: • Wide-area differential corrections for improving position accuracy; Target accuracy: 1 meter for horizontal; • Integrity function for safety of mobile users; and • Ranging function to improve signal availability. • Interoperable with GPS L1C/A and fully compatible with SBAS: • Broadcast on L1 freq. with RHCP; Common antenna and RF front-end; • Modulated by BPSK with C/A code; • 250 bps data rate with 1/2 FEC; message structure is same as SBAS.

  12. SBAS/L1-SAIF Message Structure Preamble 8 bits Message Type 6 bits Data Field 212 bits CRC parity 24 bits 250 bits per second Transmitted First MT Contents Interval [s] MT Contents Interval [s] 0 Test mode 6 17 GEO almanac 300 1 PRN mask 120 18 IGP mask 300 2～5 Fast correction & UDRE 60 24 FC & LTC 6 6 UDRE 6 25 Long-term correction 120 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 —

  13. SBAS/L1-SAIF Message (1) Message Type Contents Compatibility Status 0 Test mode Both Fixed 1 PRN mask Both Fixed 2 to 5 Fast correction & UDRE Both Fixed 6 UDRE Both Fixed 7 Degradation factor for FC Both Fixed 8 Reserved SBAS Fixed 9 GEO ephemeris SBAS Fixed 10 Degradation parameter Both Fixed 12 SBAS network time SBAS Fixed 17 GEO almanac SBAS Fixed 18 IGP mask Both Fixed 24 Mixed fast/long-term correction Both Fixed 25 Long-term correction Both Fixed 26 Ionospheric delay & GIVE Both Fixed

  14. SBAS/L1-SAIF Message (2) Message Type Contents Compatibility Status 27 SBAS service message SBAS Fixed 28 Clock-ephemeris covariance Both Fixed 29 to 51 (Undefined) — — 52 TGP mask L1-SAIF Tentative 5３ Tropospheric delay L1-SAIF Tentative 54 to 55 (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 Both Fixed 63 Null message Both Fixed

  15. Part 2 Ionospheric Correction Processor (ICP) and L1-SAIF Message Generator

  16. QZS GPS L1C/A, L2P L1-SAIF Signal K-band L1C/A, L2P Closed Loop Measured Data L1-SAIF Message GEONET L1SMS QZSS MCS GSI ENRI JAXA ENRI L1SMS • L1-SAIF Master Station (L1SMS): • Generates L1-SAIF message stream in realtime and transmits them to QZSS MCS developed by JAXA; • Installed at ENRI, Tokyo; • Subsystems: GEONET Server, Primary Receiver, Interface Processor, Message Generator, Ionosphere Processor, Troposphere Processor, and Batch Processor.

  17. L1SMS Subsystems (1) • GEONET Server: • Receives dual frequency measurement from GEONET operated by Geographical Survey Institute (GSI), Japan; • Output rate: 1 sample per second (1 Hz); In native binary format of receivers; Latency is less than 2 seconds; • 5 servers for 1,000 GEONET stations distributed all over Japan. • Primary Receiver: • Installed inside L1SMS with connection via Ethernet LAN; • Provides measurements for immediate response to satellite failure to ensure integrity function; • Collects navigation message every subframe; • Provides the actual time to the message generator; • Currently NovAtel OEM-3 MiLLennium-STD.

  18. L1SMS Subsystems (2) • Interface Processor: • Distributes GPS measurement data stream to other processors; • Other subsystem processors access to this processor for measurements to avoid generating lots of direct connections to GEONET Server and Primary Receiver; • Also relays L1-SAIF message packets from Message Generator to QZSS MCS at JAXA. • Message Generator (L1SMG): • Generates L1-SAIF message with clock and orbit corrections; • Variable configuration of monitor stations; • Accepts several types of receiver: RINEX, NovAtel, Trimble, JAVAD; • Standard planar fit algorithm for ionospheric correction; Identical with WAAS/MSAS ionospheric corrections; • Standard correction model for troposphere.

  19. L1SMS Subsystems (3) • Ionospheric Correction Processor (ICP): • Generates ionospheric correction and integrity information based on vast number of monitor stations; • Tested with realtime measurements from up to 200 monitor stations; • IGP location is not fixed and identified by QUERY command; • This processor is optional; If not exist, L1SMG employs its own standard algorithm. • Tropospheric Correction Processor (under development): • Estimates atmospheric condition and generates tropospheric delay; • Semi-realtime estimation: latency is less than 5 min; • Formats delay information into vertical delay at TGP (tropospheric grid point) like IGP for ionosphere; • Also optional; If not exist, standard troposphere model is used.

  20. L1SMS Subsystems (4) • Batch Processor: • Estimates satellite and receiver hardware biases so-called Inter-frequency bias or L1/L2 bias; • Runs on daily basis; Constructs model of ionosphere based on measurements for at least two days and performs estimation; • Provides stable and accurate estimation in comparison with a realtime sequential processing. • Data Storage Server: • Very large capacity storage with RAID configuration; • Holds input measurements and resulted message stream for several months (depending on the number of monitor stations).

  21. Message Generator Ionosphere Processor I/F Storage Storage Storage Router to GEONET GEONET Server UPS UPS L1SMS Installed at ENRI

  22. GEONET TCP/IP Dual Freq. Ant. Observation File (RINEX) via FTP Message Output via TCP/IP GEONET Server Primary Receiver Batch Processor (IFB Estimation) Interface Processor IFB Estimates Ionosphere Processor Troposphere Processor Message Generator (L1SMG) L1SMS Batch Subsystem L1SMS Realtime Subsystems Configuration of L1SMS

  23. Dual Freq. Ant. Primary Receiver GEONET Server GMS measurement (6 stations) Time and NAV Message IMS measurement (200 stations) Input Module Input Module QUERY Clock and Orbit Correction Ionospheric Correction (Standard Planar Fit) Ionospheric Correction Module Messaging Module Input from Iono Processor RESPONSE L1-SAIF Message Generator (L1SMG) Ionospheric Correction Processor (ICP) Message Log Message Output Message Generator (L1SMG)

  24. Command and Response QUERY Command RESPONSE Message • Each command and message is packed with Header and CRC; • QUERY command identifies location of each IGP; ICP computes vertical delay for each IGP location separately; • If Message Generator could not receive any response from ICP for 150 seconds, the command shall be timed out.

  25. Realtime Operation Test GMS Stations (6) for L1SMG IMS Station (200) for ICP Evaluation Locations (14) L1-SAIF Experimental Area • Tested performance of the ICP Implemented as a subsystem of L1SMS; running with L1SMG; • Analyzed user position error at 14 evaluation locations; Numbered from North to South; • Used GEONET stations as all monitor stations and evaluation sites.

  26. Results – Position Error Sample L1-SAIF Augmentation MSAS Augmentation Standalone GPS • Example of user positioning error at Site #5 93022 Choshi (East of Tokyo); • ICP: 200 IMS, 5-deg IGP, 0th Order Fit; • Period: 16-21 Jan. 2009 (5 days).

  27. L1SMG ICP Connection Computation Time 0.16 s / IGP ACK Request 42 IGPs (5 deg) QUERY Command 7 seconds for 42 IGPs RESPONSE Message Delay and GIVE Cache for 42 IGPs Request 143 IGPs (2.5 deg) QUERY Command 16 seconds for 101 IGPs Delay and GIVE RESPONSE Message Computation Time

  28. Test Cases • Rmax, Nmax, and Nmin are parameters to select ionospheric pierce point (IPP) measurements to be used for estimate the IGP delay; • The ICP collects IPPs within smaller radius because 200 IMS stations provide vast number of measurements.

  29. Location vs. Horizontal Accuracy • ICP improves position accuracy in the Southern Region; • First order estimation is better to ensure accuracy.

  30. Location vs. Vertical Accuracy • 1 meter accuracy is achievable even for vertical direction; • Note that these results associate with solar minimum phase.

  31. Conclusion • ENRI has been developing QZSS L1-SAIF signal: • L1-SAIF augmentation signal on GPS/SBAS L1 frequency; • Signal design: upper compatible with SBAS. • Development of Ionospheric Correction Processor (ICP): • Improves accuracy of WADGPS such as SBAS and L1-SAIF; • Implemented as a subsystem of L1-SAIF Master Station; Respond to QUERY command issued from L1-SAIF Message Generator; • Achievable accuracy: 0.2-0.3m horizontal at center of Japan; 0.5-0.7m at the edge of service area; Note: nominal condition of solar minimum phase. • Future works will include: • Verify the performance during ionospheric storm condition; • Consider other L1-SAIF message formats for ionospheric correction. • Contact: sakai@enri.go.jp

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