GPS/GLONASS Multi-Constellation SBAS Trial and Preliminary Results ION GNSS 2012 Nashville, TN Sept. 17-21, 2012 GPS/GLONASS Multi-Constellation SBAS Trial and Preliminary Results for East-Asia Region Takeyasu Sakai, H. Yamada, and K. Hoshinoo, Electronic Navigation Research Institute

Introduction Combined use of GPS and GLONASS with SBAS augmentation: ION GNSS Sept. 2012 - 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 other 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.

Concept of SBAS Geostationary Satellites GPS Augmentation Signal ION GNSS Sept. 2012 - Slide 2 Concept of SBAS Geostationary Satellites GPS Ground Monitor Stations Users Uplink Augmentation Signal Ranging Accuracy Integrity

Additional Constellation ION GNSS Sept. 2012 - Slide 3 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.

in the Current SBAS Standard ION GNSS Sept. 2012 - Slide 4 Part 1 GLONASS in the Current SBAS Standard

The SBAS standard in the Annex to the Civil Aviation Convention ION GNSS Sept. 2012 - Slide 5 Current SBAS Standard Already have definition of GLONASS: The SBAS standard is documented by ICAO (International Civil Aviation Organization); 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: FDMA signals; Reference time and coordination system; PRN mask numbers; IOD along with corrections; and Satellite position computation. The SBAS standard in the Annex to the Civil Aviation Convention

FDMA Signals FCN (Frequency Channel Number): ION GNSS Sept. 2012 - Slide 6 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)

Time and Coordinate Systems ION GNSS Sept. 2012 - 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 GNSS Sept. 2012 - Slide 8 PRN Masks 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 !!! Solution: Dynamic PRN Mask Actually, PRN mask can change; Controlled by IODP (Issue of Data, PRN Mask); RTCA MOPS states this occurs “infrequently” while SBAS SARPS does not. 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 GNSS Sept. 2012 - 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 GNSS Sept. 2012 - 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 GNSS Sept. 2012 - 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

GLONASS Ephemeris Item Bits Range Resolution Contents tb 7 15-1425 min ION GNSS Sept. 2012 - Slide 12 GLONASS Ephemeris Item Bits Range Resolution Contents tb 7 15-1425 min 15 min Epoch time tn 22 2-9 s 2-30 s Clock correction (const) gn 11 2-30 s/s 2-40 s/s Clock correction (1st order) x 27 27000 km 2-11 km Position X in ECEF y Position Y in ECEF z Position Z in ECEF vx 24 4.3 km/s 2-20 km/s Velocity X in ECEF vy Velocity Y in ECEF vz Velocity Z in ECEF 5 6.2 mm/s2 2-30 km/s2 Acceleration X in ECEF (only perturbation) Acceleration Y in ECEF (only perturbation) Acceleration Z in ECEF (only perturbation) Total 208 x y z ..

Implementation and Experiment ION GNSS Sept. 2012 - Slide 13 Part 2 Implementation and Experiment

Software Implementation ION GNSS Sept. 2012 - Slide 14 Software Implementation ENRI’s software SBAS simulator: ‘RTWAD’ runs on usual PC and Linux Workstations; Generates SBAS message stream: one message per second; Run modes: Offline operation mode: for preliminary investigation using RINEX files; Realtime operation mode: verification of actual performance with realtime raw data. Needs user-domain receiver software to evaluate performance. Upgrade for GLONASS (and QZSS): Input module: RINEX observation and navigation files containing GLONASS; Implemented GLONASS extension of SBAS standard; User-domain receiver software is also upgraded to be GLONASS-capable; QZSS is also supported as it is taken into account like GPS. Software SBAS Simulator (RTWAD) User-Domain Receiver Software Network GPS observables Position Error SBAS Message Stream Position Output User-side observations Reference station observations

Dynamic PRN Mask Dynamic PRN mask: Transition of PRN mask: ION GNSS Sept. 2012 - Slide 15 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 GNSS Sept. 2012 - Slide 16 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 GNSS Sept. 2012 - Slide 17 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: RINEX 2.12 observation and navigation files. For our experiment: 8 sites for reference stations; Reference Station (1) to (8) 3 sites for evaluation. User Station (a) to (c) Period: 12/7/18 – 12/7/20 (3 days).

PRN Mask Transition QZSS GLONASS GPS ION GNSS Sept. 2012 - Slide 18 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 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. GLONASS GPS

Elevation Angle GPS GLONASS QZSS PRN Mask Transition 5 deg @ Tokyo ION GNSS Sept. 2012 - Slide 19 Elevation Angle GPS GLONASS QZSS PRN Mask Transition 5 deg @ Tokyo 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 GNSS Sept. 2012 - Slide 20 # of Satellites vs. Mask Angle 17 SVs 9.8 SVs 7.4 SVs @ User (b) 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 17 satellites at 5 deg and 9.8 satellites even at 30 deg.

Availability vs. Mask Angle ION GNSS Sept. 2012 - Slide 21 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.

DOP vs. Mask Angle HDOP = 2.3 @ User (b) ION GNSS Sept. 2012 - Slide 22 DOP vs. Mask Angle HDOP = 2.3 @ User (b) GLONASS-only users suffer poor geometries; Multi-constellation with QZSS offers HDOP of 2.3 even for 40 deg mask.

User Position Error: Mask 5deg ION GNSS Sept. 2012 - Slide 23 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 ION GNSS Sept. 2012 - Slide 24 User Position Error: Mask 30deg GPS+GLO+QZS: 0.372m RMS of horizontal error at user location (b); Multi-constellation offers a good availability even for 30 deg mask.

RMS Error vs. Mask: User (a) ION GNSS Sept. 2012 - Slide 25 RMS Error vs. Mask: User (a) 0.528m @ User (a) Northernmost user location; Multi-constellation provides robust position information through mask angle of 5 to 40 deg.

RMS Error vs. Mask: User (b) ION GNSS Sept. 2012 - Slide 26 RMS Error vs. Mask: User (b) 0.602m @ User (b) User location near the centroid of reference station network; For vertical direction, 10 deg mask shows the best accuracy except GLONASS only case.

RMS Error vs. Mask: User (c) ION GNSS Sept. 2012 - Slide 27 RMS Error vs. Mask: User (c) 0.588m @ User (c) Southernmost user location; There is little dependency upon user location; possibly because ionosphere condition is quiet for the period of this experiment.

Vertical Protection Level ION GNSS Sept. 2012 - Slide 28 Vertical Protection Level Reduce GPS only GPS+GLO+QZS 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; Means improvement of availability of navigation.

Conclusion Combined use of GPS and GLONASS with SBAS: ION GNSS Sept. 2012 - Slide 29 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: Support of realtime operation mode; Realtime operation test broadcasting augmentation information for both GPS and GLONASS on QZSS L1-SAIF augmentation channel; Use GLONASS observables in generation of ionospheric correction; Mixed use of different types of receiver for reference stations; Further extension to support Galileo.