QZSS L1-SAIF Supporting GPS/GLONASS Multi-Constellation Augmentation ION ITM 2013 San Diego, CA Jan. 28-30, 2013 QZSS L1-SAIF Supporting GPS/GLONASS Multi-Constellation Augmentation T. Sakai, H. Yamada, K. Hoshinoo, and K. Ito Electronic Navigation Research Institute, Japan

Introduction QZSS (Quasi-Zenith Satellite System) program: ION ITM Jan. 2013 - Slide 1 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 improving position availability; all on L1 single frequency. ENRI has been developing L1-SAIF signal and experimental facility: Possible to extend to augment GLONASS satellites; Upgraded to support multi-constellation environment including GPS, GLONASS, and QZSS satellites; Conducted experiment with broadcast of multi-constellation augmentation.

QZSS Concept Broadcast signal from high elevation angle; ION ITM Jan. 2013 - Slide 2 QZSS Concept QZS GPS/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 other GPS satellites at any time. Footprint of QZSS orbit; Centered at 135E; Eccentricity 0.075, Inclination 43deg.

SAIF： Submeter-class Augmentation with Integrity Function ION ITM Jan. 2013 - Slide 3 L1-SAIF Signal Ranging Function Error Correction Integrity QZS satellites GPS Constellation Ranging 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

L1-SAIF Signal QZSS broadcasts wide-area augmentation signal: ION ITM Jan. 2013 - Slide 4 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 improving position availability. Augmentation to GPS L1C/A based on SBAS techniques: 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).

Augmentation to GPS Only ION ITM Jan. 2013 - Slide 5 L1-SAIF Corrections Standalone GPS Augmented by L1-SAIF 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. Horizontal Error Vertical 1.45 m 2.92 m 6.02 m 8.45 m System Standalone GPS 0.29 m 0.39 m 1.56 m 2.57 m L1-SAIF RMS Max Augmentation to GPS Only Note: Results shown here were obtained with survey-grade antenna and receiver in open sky condition.

GLONASS Support: Motivation ION ITM Jan. 2013 - Slide 6 GLONASS Support: Motivation QZSS L1-SAIF Augmentation GPS constellation Additional Constellation = GLONASS Increase of augmented satellites improves availability of position solution; Chance of robust position information at mountainous areas and urban canyons. The current SBAS specification already has definition of GLONASS; Easy to support by L1-SAIF.

Time and Coordinate Systems ION ITM Jan. 2013 - Slide 7 Time and Coordinate Systems GLONASS Time: GLONASS is operating based on its own time system: GLONASS Time; The difference between GPS Time and GLONASS Time must be taken into account for combined use of GPS and GLONASS; The difference is not fixed and slowly changing: about 400ns in July 2012; SBAS broadcasts the difference by Message Type 12; GLONASS-M satellites are transmitting the difference as parameter tGPS in almanac (non-immediate) data: tGPS = tGPS − tGLONASS. PZ-90 Coordinate System: GLONASS ephemeris is derived based on Russian coordinate system PZ-90; The relationship between WGS-84 and the current version of PZ-90 (PZ-90.02) is defined in the SBAS standard as:

GLONASS slot number plus 37 ION ITM Jan. 2013 - Slide 8 PRN Masks PRN Mask: SBAS/L1-SAIF transmits PRN mask information indicating satellites which are currently augmented; PRN number has range of 1 to 210; Up to 51 satellites out of 210 can be augmented simultaneously by the single SBAS/L1-SAIF signal; But, 32 GPS + 24 GLONASS = 56 !!! Solution: Dynamic PRN Mask Actually, PRN mask can change; Controlled by IODP (Issue of Data, PRN Mask); Change PRN mask dynamically to reflect the actual visibility of satellites from the intended service area. PRN definition for SBAS PRN Contents 1 to 37 GPS 38 to 61 GLONASS slot number plus 37 62 to 119 Spare 120 to 138 SBAS 139 to 210

IOD (Issue of Data) IOD indicator along with corrections: ION ITM Jan. 2013 - Slide 9 IOD (Issue of Data) IOD indicator along with corrections: LTC (Long-Term Correction) in SBAS Message Type 24/25 contains orbit and clock corrections; Such corrections depend upon ephemeris data used for position computation; IOD indicates which ephemeris data should be used in receivers. IOD for GPS satellites: For GPS, IOD is just identical with IODE of ephemeris data. Previous Ephemeris IODE=a Next Ephemeris IODE=b LTC IOD=a IOD=b Time

IOD for GLONASS IOD for GLONASS satellites: ION ITM Jan. 2013 - Slide 10 IOD for GLONASS IOD for GLONASS satellites: GLONASS ephemeris has no indicator like IODE of GPS ephemeris; IOD for GLONASS satellites consists of Validity interval (V) and Latency time (L) to identify ephemeris data to be used: 5 MSB of IOD is validity interval, V; 3 LSB of IOD is latency time, L. User receivers use ephemeris data transmitted at a time within the validity interval specified by L and V. Ephemeris Validity Interval L1 V1 Previous Ephemeris Next Ephemeris LTC IOD=V1|L1 V2 IOD=V2|L2 L2 Time

Perturbation terms in ephemeris ION ITM Jan. 2013 - Slide 11 Satellite Position GLONASS ephemeris data: 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 or more. Numerical integration is necessary to compute position of GLONASS satellites; Note: 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). Perturbation terms in ephemeris

Upgrade of L1SMS L1-SAIF Master Station (L1SMS): ION ITM Jan. 2013 - Slide 12 Upgrade of L1SMS L1-SAIF Master Station (L1SMS): Generates the L1-SAIF message stream and transmits it to JAXA MCS. Upgrade for supporting GLONASS and QZSS: Input module: Supports BINEX observables and navigation message records; Implemented GLONASS extension based on SBAS standards; User-domain receiver software (MCRX) is also upgraded to be GLONASS-capable; QZSS is also supported as it is taken into account like GPS. L1SMS GEONET QZS QZSS MCS GPS Measured Data L1-SAIF Message GSI ENRI JAXA L1-SAIF Signal L1C/A, L2P K-band Closed Loop GLONASS L1SA L2SA

Dynamic PRN Mask Dynamic PRN mask: Transition of PRN mask: ION ITM Jan. 2013 - Slide 13 Dynamic PRN Mask Dynamic PRN mask: Changes PRN mask dynamically to reflect the actual visibility of satellites; Set PRN masks ON for satellites whose pseudorange observations are available; Not based on prediction by almanac information not provided by RINEX; Semi-dynamic PRN mask: Fix masks ON for GPS and QZSS, and change dynamically only for GLONASS to reduce receiver complexity. Transition of PRN mask: Periodical update of PRN mask: updates every 30 minutes; Transition time 180s is given to users to securely catch the new PRN mask. FC PRN Mask (IODP=i) PRN Mask (IODP=i+1) tcutover 180s LTC Corrections before cutover Corrections after cutover Transition time Cutover

Time offset broadcast to users ION ITM Jan. 2013 - Slide 14 GLONASS Time Offset Realtime computation: Computes as the difference between receiver clocks for a group of GPS satellites (and QZSS) and the other group of GLONASS satellites; Enough accuracy with a filter of long time constant; Need no almanac information broadcast by GLONASS satellites; Transmitted to users via Message Type 12 of SBAS. t tGPS tGLONASS tR DtGPS DtGLONASS BGLONASS ^ BGPS True Time GLONASS System Time GPS Receiver -daGLONASS Receiver clock for GPS satellites Time offset broadcast to users

Experiment: Monitor Stations ION ITM Jan. 2013 - Slide 15 Experiment: Monitor Stations Recently Japanese GEONET began to provide GLONASS and QZSS observables in addition to GPS; Currently more than 150 stations are GLONASS/QZSS-capable; Data format: BINEX For our experiment: 6 sites for reference stations; Reference Station (a) to (f) 11 sites for evaluation. User Station (1) to (11) Period: 2013/1/6 01:00 to 2013/1/9 23:00 (94 hours).

PRN Mask Transition QZSS GLONASS GPS ION ITM Jan. 2013 - Slide 16 PRN Mask Transition GPS GLONASS QZSS Reflecting our implementation, PRN mask is updated periodically at every 30 minutes; Semi-dynamic PRN mask: GPS and QZSS satellites are always ON in the masks; PRN masks for GLONASS satellites are set ON if the satellite are visible and augmented; Stair-like shape: because the slot number of GLONASS satellites are assigned increasingly along with the orbit. IODP (issue of Data, PRN Mask) indicates change of PRN mask.

Elevation Angle GPS GLONASS QZSS PRN Mask Transition 5 deg @ User (7) ION ITM Jan. 2013 - Slide 17 Elevation Angle GPS GLONASS QZSS PRN Mask Transition 5 deg @ User (7) Rising satellites appear at 5-12 deg above the horizon; Latency due to periodical update of PRN mask; However, GPS satellites also have similar latency; Not a major problem because low elevation satellites contribute a little to improve position accuracy.

# of Satellites vs. Mask Angle ION ITM Jan. 2013 - Slide 18 # of Satellites vs. Mask Angle 16 SVs 9.8 SVs 7.3 SVs @ User (7) Introducing GLONASS satellites increases the number of satellites in roughly 75%; QZSS increases a satellite almost all day by only a satellite on the orbit, QZS-1 "Michibiki" Multi-constellation with QZSS offers 16 satellites at 5 deg and 7.3 satellites even at 40 deg.

User Position Error: Mask 5deg ION ITM Jan. 2013 - Slide 19 User Position Error: Mask 5deg GPS+GLO+QZS: 0.310m RMS of horizontal error at user location (7); Looks some limited improvement by using multi-constellation.

User Position Error: Mask 30deg ION ITM Jan. 2013 - Slide 20 User Position Error: Mask 30deg GPS+GLO+QZS: 0.335m RMS of horizontal error at user location (7); Multi-constellation offers a good availability even for 30 deg mask.

Error vs. User Location: 5 deg ION ITM Jan. 2013 - Slide 21 Error vs. User Location: 5 deg North South 0.421m 0.283m Expect horizontal accuracy of 0.3 to 0.5m with L1-SAIF augmentation, regardless GLONASS is used or not; There is a little dependency upon the latitude of user location possibly due to an effect of ionosphere activities.

Error vs. User Location: 30 deg ION ITM Jan. 2013 - Slide 22 Error vs. User Location: 30 deg North South 0.425m The horizontal accuracy is still within a range between 0.3 and 0.5m for the multi-constellation configuration; The accuracy degrades to 1 or 2.5m for GPS-only single-constellation configuration.

Conclusion ENRI has been developing L1-SAIF signal: ION ITM Jan. 2013 - Slide 23 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. GPS/GLONASS/QZSS multi-constellation support: L1-SAIF Master Station was upgraded to support GLONASS and QZSS in addition to GPS based on the existing SBAS specifications; Conducted an experiment with broadcast of L1-SAIF signal containing augmentation information of GPS, GLONASS, and QZSS; Using multi-constellation it can be expected to maintain a good position accuracy even in higher mask angle conditions representing limited visibility conditions. Further Investigations will include: Dynamic PRN mask driven by almanac information; Use of GLONASS observables in generation of ionospheric corrections; Considerations of different types of receiver for reference/user stations; and Extension to Galileo.