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SBAS IWG/25 St. Petersburg, Russia June 25-27, 2013

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: GPS/GLONASS-capable receivers are now widely available;

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SBAS IWG/25 St. Petersburg, Russia June 25-27, 2013

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

  2. 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.

  3. Motivation SBAS GEO Augmentation Additional Constellation = GLONASS GPS Constellation • 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.

  4. 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

  5. (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)

  6. (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 ?

  7. (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

  8. Previous Ephemeris IODE=a Next Ephemeris IODE=b Time LTC IOD=b LTC IOD=a LTC IOD=a LTC IOD=a LTC IOD=b (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.

  9. Previous Ephemeris Next Ephemeris Time Ephemeris Validity Interval LTC IOD=V2|L2 Ephemeris Validity Interval LTC IOD=V1|L1 V2 V1 L1 L2 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.

  10. Perturbation terms in ephemeris (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).

  11. 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

  12. 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

  13. 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.

  14. # 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.

  15. 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.

  16. 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.

  17. 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.

  18. 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.

  19. 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.

  20. 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.

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