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

1. ION GNSS 2011 Portland, OR Sept. 20-23, 2011 Generation and Evaluation of QZSS L1-SAIF Ephemeris Information T. Sakai, H. Yamada, S. Fukushima, 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 satellites on the inclined geosynchronous (quasi-zenith) orbit; • Broadcast GPS-like supplemental signals on three frequencies and two augmentation signals, L1-SAIF and LEX; • The first QZS satellite was successfully launched on Sept. 11, 2010. • 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 position availability; all on L1 single frequency. • ENRI has been developing L1-SAIF signal and experimental facility: • Signal design: SBAS-like message stream on L1 C/A (PRN 183); • Implemented L1-SAIF Master Station (L1SMS); • Need to broadcast ephemeris message to make ranging function available. • What is the best way to generate ephemeris information for L1-SAIF?

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 135E; • Eccentricity 0.075, Inclination 43deg.

4. Inclined Geosynchronous Orbit Apogee 40000km 8:40 15:20 Perigee 32000km Orbital Planes of QZSS (3 SVs) Ground Track • Semi-major axis (42164km) is equal to GEO orbit: synchronous with rotation of the earth; • Inclined obit makes ground track ‘8’-figure; Called IGSO or Quasi-Zenith Orbit; • With three or more satellites on the same ground track, navigation service can be provided from zenith to regional users at any time.

5. 25.3m Radiation Cooled TWT TWSTFT Antenna Successfully launched on Sept. 11, 2010 and settled on Quasi-Zenith Orbit (IGSO). C-band TTC Antenna Laser Reflector L1-SAIF Antenna L-band Helical Array Antenna Space Segment: QZS-1

6. QZSS Frequency Plan Signal Channel Frequency Bandwidth Min. Rx Power Interoperability 1575.42 MHz GPS-like supplemental signals with minimum modifications from GPS signals QZS-L1C L1CD 24 MHz –163.0 dBW L1CP 24 MHz – 158.25 dBW QZS-L1-C/A 24 MHz – 158.5 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-L1-SAIF 1575.42 MHz 24 MHz – 161.0 dBW SBAS-like augmentation signal (250bps) QZS-LEX 1278.75 MHz 42 MHz – 155.7 dBW QZSS-specific augmenta-tion signal (2kbps) Find detail in IS-QZSS document.

7. QZSS L1-SAIF Signal • QZSS broadcasts wide-area augmentation signal: • Called L1-SAIF (Submeter-class Augmentation with Integrity Function); • Designed and developed by ENRI. • L1-SAIF signal offers: • Wide-area differential correction service for improving position accuracy; Target accuracy: 1 meter for horizontal; • Integrity function for safety of mobile users; and • Ranging function for position availability. • Augmentation to GPS L1C/A based on SBAS: • Broadcast on L1 freq. with RHCP; Common antenna and RF front-end; • Modulated by BPSK with C/A code (PRN 183); • 250 bps data rate with 1/2 FEC; message structure is identical with SBAS; • Differences: Large Doppler and additional messages. • Specification of L1-SAIF: See IS-QZSS document (Available at JAXA HP).

8. Ranging Function QZS satellite 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. User GPS Receivers SAIF： Submeter-class Augmentation with Integrity Function

9. L1-SAIF Message Structure Preamble 8 bits Message Type 6 bits Data Field 212 bits CRC parity 24 bits 1 message = 250 bits broadcast every 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 —

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

11. Message Type Contents Used by Status 27 SBAS service message SBAS Fixed 28 Clock-ephemeris covariance SBAS and L1-SAIF 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 SBAS and L1-SAIF Fixed 63 Null message SBAS and L1-SAIF Fixed L1-SAIF Message (2)

12. 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 Example Position Error • 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). Note: Results shown here were obtained with survey-grade antenna and receivers in open sky condition.

13. Broadcast Ephemeris for Ranging • Orbit Information for ranging function: • Need to broadcast ephemeris information (navigation-grade orbit information) on QZS signal in order to use it for ranging; • QZS L1C/A, L2C, L5, and L1C broadcast ephemeris information on their navigation message similar with GPS; Available since June 2011; • How about L1-SAIF and LEX signals? • Option A: Do not broadcast on L1-SAIF, but on other signals: • L1-SAIF users receive and utilize ephemeris information broadcast on L1C/A, L2C, L5, or L1C signals; • Antenna offset may be cancelled by differential correction. • Users must receive al least two signals (eg. L1-SAIF + L1C/A) to use ranging function of L1-SAIF signal. • Option B: Broadcast ephemeris information on L1-SAIF: • Users can use QZS for ranging by receiving only L1-SAIF signal; • Require to design some new message because SBAS GEO ephemeris message is not applicable; MT58 needs to be designed. We employ this option

14. Ephemeris Message Design • Requirements for ephemeris information: • Contained by a single message (212 bits); • GPS legacy-Nav message consumes 420 bits for a set of ephemeris information of a single GPS SV; • SBAS GEO ephemeris message consists of only 204 bits; But this is for GEO. • Available for 600-900 seconds from broadcast; • SV position error should be within 0.3-0.5m; • Will be cancelled by applying differential corrections; • The minimum resolution of corrections is 0.125m. • Clock correction term should cover a range of 1ms. • Required by payload developer during discussion of specification. • Representations in other systems: • GPS legacy-Nav: Representation by Keplerian 6-element parameters; • GLONASS: Represented by position, velocity, and acceleration in ECEF; • Users need numerical integration to obtain the position of the intended moment. • SBAS: Transmit position, velocity, and acceleration in ECEF. • No need for numerical integration.

15. .  .  A i W GPS Ephemeris for MEO

16. .. x .. y .. z GLONASS Ephemeris for MEO

17. SBAS Ephemeris for GEO

18. QZSS SV Position in ECEF QZS-1 Broadcast Nav Msg on L1C/A PRN 193 2011-08-18 00:00 to 24:00 GPST GPS/QZSS Rx @Koganei, Tokyo Processing by ENRI 24 different IODEs in 2846 frames • Computed based on broadcast ephemeris on QZSS L1C/A for day of 2011-08-18; • QZS-1 was operating normally and healthy except LEX signal.

19. QZSS SV Velocity in ECEF QZS-1 Broadcast Nav Msg on L1C/A PRN 193 2011-08-18 00:00 to 24:00 GPST GPS/QZSS Rx @Koganei, Tokyo Processing by ENRI 24 different IODEs in 2846 frames • Velocity of QZS-1 computed from broadcast ephemeris.

20. QZSS SV Acceleration in ECEF QZS-1 Broadcast Nav Msg on L1C/A PRN 193 2011-08-18 00:00 to 24:00 GPST GPS/QZSS Rx @Koganei, Tokyo Processing by ENRI 24 different IODEs in 2846 frames • Acceleration of QZS-1 computed from broadcast ephemeris.

21. Representation of Acceleration • Reduction of information: • GLONASS transmits ephemeris information as position, velocity, and acceleration in ECEF; • Navigation-grade ephemeris is provided in 208 bits for a single GLONASS SV; • Broadcast information is valid for 15 minutes; Enough for L1-SAIF; • Centripental acceleration is removed from transmitted information; • These terms can be computed for the specific position and velocity of SV; • GLONASS ICD A.3.1.2 gives the equations below (with some corrections). • This technique is applicable to L1-SAIF ephemeris representation. Transmit these perturbation terms

22. Perturbation Term QZS-1 Broadcast Nav Msg on L1C/A PRN 193 2011-08-18 00:00 to 24:00 GPST GPS/QZSS Rx @Koganei, Tokyo Processing by ENRI 24 different IODEs in 2846 frames • Acceleration of QZS-1 computed from broadcast ephemeris; • Perturbation only: Centripetal acceleration term is removed.

23. .. x .. y .. z Proposed Message (MT58)

24. Generation of Ephemeris • Option A: Generation independent of other systems: • Need realtime monitor station network: • Monitor stations also necessary for generation of wide-area differential corrections; • JAXA’s monitor network covering Japan and Southeast Asia Region is available for experimental purpose. • Need some orbit determination software; • Independent of other systems. • However, observations from monitor network are provided by JAXA MCS. • Option B: Reconstruction from MCS products: • L1-SAIF is broadcast from QZS-1 SV. • QZS-1 itself broadcasts ephemeris information on L1C/A signal; • Ephemeris information can be generated based on L1C/A ephemeris; • Reconstruction from Keplerian representation in L1C/A legacy Nav message into ECEF PVA representation for MT58; • L1C/A and L1-SAIF are broadcast from different antennas; Antenna offset may be cancelled by differential corrections. • Users still do not need to receive any signals other than L1-SAIF. We employ this option

25. Ephemeris Reconstruction QZS-1 L1-SAIF (PRN 183) Ephemeris in MT58 Uplink QZS-L1C/A QZS-L1C/A (PRN 193) Ephemeris in Legacy Nav JAXA MCS Decode ephemeris Format into MT58 L1-SAIF Users Compute PVA L1-C/A Users Ephemeris Reconstruction L1-SAIF Master Station at ENRI

26. Quantization of corrections SV Position Error for PVA QZS-1 Reconstructed from broadcast Nav Msg on L1C/A PRN 193 2011-08-18 00:00 to 24:00 GPST Processing by ENRI Assumed user location: @ENRI, Tokyo • Integration error in user receiver with regard to time after broadcast of PVA; • LOS component including satellite clock offset; • SV PVA is computed from broadcast ephemeris on QZSS L1C/A.

27. SV Position Error for MT58 QZS-1 Reconstructed from broadcast Nav Msg on L1C/A PRN 193 2011-08-18 00:00 to 24:00 GPST Processing by ENRI Assumed user location: @ENRI, Tokyo • PVA is formatted into proposed MT58 format and integrated; • Initial position error is dominant component because of its resolution of 1.28m; Need some correction.

28. Correction by LTC Message MT9 GEO Ephemeris MT2-5 Fast Correction (FC) SBAS GEO PRN 120-138 Orbit Clock Clock Corr. (0.125m) (0.125m) Measured Pseudorange Corrected Pseudorange Resolution MT58 QZS Ephemeris MT2-5 Fast Correction (FC) QZSS L1-SAIF PRN 183 Orbit Clock Clock Corr. (1.28m) (0.125m) Measured Pseudorange Corrected Pseudorange Clock & Orbit Corr. (0.125m) MT24/25 Long-Term Correction (LTC) • Apply MT24/25 LTC orbit and clock correction like GPS SVs; • LTC has 0.125m resolution.

29. Applying LTC Correction QZS-1 Reconstructed from broadcast Nav Msg on L1C/A PRN 193 2011-08-18 00:00 to 24:00 GPST Processing by ENRI Assumed user location: @ENRI, Tokyo • Assumed that initial position error is corrected by LTC (long-term correction) in MT24/25 messages; • Range error is within 0.3m for 300s integration. • Note that ephemeris message of SBAS has timeout interval of 240-360s.

30. 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; • Implemented L1-SAIF Master Station (L1SMS) which generates augmentation message stream in realtime and transmit it to QZSS MCS. • QZSS ephemeris message: • Need to broadcast ephemeris information to make ranging function available; • For L1-SAIF signal, MT 58 is designed to broadcast ephemeris information; • MT 58 provides SV position, velocity, and acceleration in ECEF; • Integration error is within 0.3m when combined with LTC correction. • Ongoing work: • Upgrade L1SMS to generate corrections for L1-SAIF signal itself; • User side consideration of combined use of QZSS and MSAS.