GPS/GLONASS Multi-Constellation SBAS Trial SBAS IWG/25 St. Petersburg, Russia June 25-27, 2013 GPS/GLONASS Multi-Constellation SBAS Trial Takeyasu Sakai Electronic Navigation Research Institute

Introduction Combined use of GPS and GLONASS with SBAS augmentation: IWG/25 June 2013 - Slide 1 Introduction Combined use of GPS and GLONASS with SBAS augmentation: GPS/GLONASS-capable receivers are now widely available; SBAS (satellite-based augmentation system) is an international standard of the augmentation system; US WAAS, Japanese MSAS, and European EGNOS are already operational; All operational SBAS are augmenting only GPS; To improve availability of SBAS-augmented position information, a possible way is extending SBAS to support an additional constellation, e.g., GLONASS. Possibility of Multi-Constellation SBAS (MC SBAS): SBAS specification already has definitions necessary to augment GLONASS; Investigating advantages of using GLONASS, we have implemented SBAS simulator capable of augmenting both GPS and GLONASS simultaneously; It is confirmed that introducing GLONASS improves availability and robustness of position information especially where visibility is limited.

Additional Constellation IWG/25 June 2013 - Slide 2 Motivation SBAS GEO Augmentation GPS Constellation Additional Constellation = GLONASS Increase of augmented satellites improves availability of position solution; Also possibly reduce protection levels; Improve availability of navigation; Chance of robust position information at mountainous areas and urban canyons.

The SBAS standard in the Annex to the Civil Aviation Convention IWG/25 June 2013 - Slide 3 Current SBAS Standard Already has definition of GLONASS: The SBAS standard is documented as the ICAO SARPS; GLONASS L1 CSA (channel of standard accuracy) signal has already been described in the SBAS standard based on GLONASS ICD; SBAS signal is also able to contain information on GLONASS satellites. Differences from GPS in terms of SBAS augmentation: (1) FDMA signals; (2) Reference time and coordination system; (3) PRN mask numbers; (4) Missing IOD for ephemeris; and (5) Satellite position computation. The SBAS standard in the Annex to the Civil Aviation Convention

(1) FDMA Signals FCN (Frequency Channel Number): IWG/25 June 2013 - Slide 4 (1) FDMA Signals FCN (Frequency Channel Number): GLONASS ICD defines FCN of –7 to +13; Historically 0 to +13 were used; After 2005 the range of FCN shifts to –7 to +6; FCN cannot be used for identification of satellites; two satellites share the same FCN. Difference of carrier frequency affects: Carrier smoothing: Wave length per phase cycle is dependent upon carrier frequency. Ionospheric corrections: Ionospheric propagation delay is inversely proportional to square of carrier frequency. (GLONASS ICD v5.0)

(2) Time and Coordinate Systems IWG/25 June 2013 - Slide 5 (2) 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 the equation: No need for PZ-90.11 ?

GLONASS slot number plus 37 IWG/25 June 2013 - Slide 6 (3) PRN Mask PRN Mask: SBAS transmits PRN mask information indicating satellites which are augmented by the SBAS; PRN number has range of 1 to 210; Up to 51 satellites out of 210 can be augmented simultaneously by the single SBAS signal; But, 32 GPS + 24 GLONASS = 56 !!! A solution: Dynamic PRN Mask Actually, PRN mask can change; Controlled by IODP (Issue of Data, PRN Mask); RTCA MOPS states this occurs “infrequently” while ICAO SARPS does not. Change PRN mask dynamically (for GLONASS satellites only; semi-dynamic PRN masking) to reflect the actual visibility from the intended service area; This is a tentative implementation for this MC-SBAS trial by ENRI. 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

(4) IOD (Issue of Data) IOD indicator along with corrections: IWG/25 June 2013 - Slide 7 (4) 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: IWG/25 June 2013 - Slide 8 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 IWG/25 June 2013 - Slide 9 (5) 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

IWG/25 June 2013 - Slide 10 MC-SBAS Experiment ENRI’s software SBAS simulator is upgraded to support GLONASS and Japan’s QZSS constellations. QZSS currently contains only 1 IGSO broadcasting PRN 193 on L1C/A; The software generates the complete SBAS message stream based on input measurements given as RINEX files. GNSS receiver network: GEONET More than 1,200 stations are GLONASS/ QZSS-capable; Data format: RINEX 2.12 observation and navigation files. Monitor stations for this experiment: 8 Reference Stations: (1) to (8). 3 User Stations: (a) to (c); In this presentation, discussion for user (b) only. Period: 2012/7/18 to 2012/7/20 (3 days). User Location

PRN Mask Transition QZSS GLONASS GPS IWG/25 June 2013 - Slide 11 PRN Mask Transition 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 are set ON for GLONASS satellites visible from 1 or more stations; Set OFF if not visible. IODP (issue of Data, PRN Mask) indicates change of PRN mask at every 30 minutes. GLONASS GPS

Elevation Angle GPS GLONASS QZSS PRN Mask Transition 5 deg @ User (b) IWG/25 June 2013 - Slide 12 Elevation Angle GPS GLONASS QZSS PRN Mask Transition 5 deg @ User (b) Rising satellites appear at 5-12 deg above the horizon; Latency due to periodical update of PRN mask without prediction by almanac; However, GPS satellites also have similar latency; The latency of GLONASS satellites would not be a major problem.

# of Satellites vs. Mask Angle IWG/25 June 2013 - Slide 13 # of Satellites vs. Mask Angle 17 SVs 9.8 SVs 7.4 SVs @ User (b) Introducing GLONASS satellites increases the number of satellites roughly 75%; QZSS increases a satellite almost all day by only a satellite on the orbit, QZS-1; Multi-constellation with QZSS offers 17 satellites for 5 deg mask angle and 9.8 satellites even for 30 deg.

Availability vs. Mask Angle IWG/25 June 2013 - Slide 14 Availability vs. Mask Angle 100% Availability @ User (b) The number of epochs with position solution decreases with regard to increase of mask angle; Multi-constellation with QZSS achieves 100% availability even for 40 deg mask.

User Position Error: Mask 5deg IWG/25 June 2013 - Slide 15 User Position Error: Mask 5deg GPS+GLO+QZS: 0.310m RMS of horizontal error at user location (b); Looks some improvement by using multi-constellation.

User Position Error: Mask 30deg IWG/25 June 2013 - Slide 16 User Position Error: Mask 30deg GPS+GLO+QZS: 0.372m RMS of horizontal error at user location (b); Multi-constellation offers good accuracy even for 30 deg mask.

RMS Error vs. Mask Angle 0.602m @ User (b) IWG/25 June 2013 - Slide 17 RMS Error vs. Mask Angle 0.602m @ User (b) User location near the centroid of reference station network; The accuracy degrades but is maintained to 0.6m for horizontal even for 40deg mask angle by using GLONASS and QZSS as well as GPS.

Vertical Protection Level IWG/25 June 2013 - Slide 18 Vertical Protection Level Reduce GPS only GPS+GLO+QZS @ User (b) Protection levels mean the confidence limit at 99.99999% confidential level; In these chart, unsafe condition exists if there are plots at the right of the diagonal line; GLONASS reduces VPL; This means improvement of availability of navigation.

Conclusion Combined use of GPS and GLONASS with SBAS: IWG/25 June 2013 - Slide 19 Conclusion Combined use of GPS and GLONASS with SBAS: Multi-constellation SBAS, capable of augmenting both GPS and GLONASS, and additionally QZSS, is implemented and tested successfully; Potential problems and solutions on realizing a multi-constellation SBAS based on the current standard were investigated; It is confirmed that the performance of SBAS-aided navigation is certainly improved by adding GLONASS, especially when satellite visibility is limited; Adding GLONASS also reduces protection levels and thus improves availability of navigation. Ongoing and future works: Realtime operation test to broadcast multi-constellation augmentation information via QZSS L1-SAIF augmentation channel; Preliminary tests have been conducted often in this year successfully; Using GLONASS observables in generation of ionospheric correction; Mixed use of different types of receiver for reference/user stations; Further extension to support Galileo.