4th AOR W/S on GNSS Kuala Lumpur, Malaysia Dec. 8-10, 2012 Multi-GNSS Augmentation by L1-SAIF Signal: Preliminary Results Takeyasu Sakai Electronic Navigation Research Institute
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. • 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: • Signal design: SBAS-like message stream on L1 C/A code (PRN 183); • Possibility of Multi-GNSS augmentation: combined use of GPS and other constellations would improve the availability of position solutions. • Especially where visibility is limited. • Upgraded L1-SAIF experimental facility and conducted a Multi-GNSS trial.
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 at 135E; • Eccentricity 0.075, Inclination 43deg.
QZSS L1-SAIF Signal • QZSS broadcasts wide-area augmentation signal: • Called L1-SAIF (Submeter-class Augmentation with Integrity Function); • Augmentation signal for mobile users 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 the SBAS specifications: • 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 from SBAS: PRN, large Doppler, and some additional messages. • Developed easily if one has the experience to develop SBAS-capable receiver; • Specification of L1-SAIF: See IS-QZSS document (Available at JAXA HP).
Ranging Function QZS satellites GPS Constellation Error Correction Ranging Signal Integrity Function L1-SAIF Signal Functions 3 Functions by L1-SAIF • 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/L1-SAIF Receivers SAIF： Submeter-class Augmentation with Integrity Function
GPS Satellites QZS Ranging Signal L1-SAIF Signal K-band Uplink Ranging Signal Measure- ments L1-SAIF Message GEONET L1SMS QZSS MCS GSI Server (Tokyo) ENRI (Tokyo) JAXA TKSC (Tsukuba) ENRI L1-SAIF Master Station • L1-SAIF Master Station (L1SMS): • Generates L1-SAIF message stream in realtime and transmits it to QZSS MCS developed by and installed at JAXA; • Installed at ENRI, Tokyo; 90km from JAXA Tsukuba Space Center; • Dual frequency GPS measurements at some locations in Japan necessary to generate L1-SAIF messages are sent from GEONET in realtime.
System Horizontal Error Vertical Error Standalone GPS L1-SAIF Augmentation Standalone GPS RMS 1.45 m 2.92 m Max 6.02 m 8.45 m w/ L1-SAIF RMS 0.29 m 0.39 m Max 1.56 m 2.57 m L1-SAIF Correction: GPS only GPS Only Result 6 reference stations User location for this test L1-SAIF expe- rimental area • Example of user position error at Site 940058 (Takayama); • Realtime operation with MSAS-like 6 reference stations in Japan; • Period: 19-23 Jan. 2008 (5 days). Note: Results shown here were obtained with geodetic-grade antenna and receivers at open sky condition.
QZS Augmentation Additional Constellation = GLONASS GPS constellation Users Adding GLONASS: Motivation • 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.
GLONASS: Differences from GPS • FDMA signals: • Change carrier frequency settings with regard to ranging sources. • Reference time and coordinates: • Time: broadcast time offset information by an L1-SAIF message; Avoids increase of unknowns in user receivers; • Coordinates: convert PZ-90.02 to WGS-84. • PRN numbers and insufficient capacity of mask pattern: • Assign PRN numbers of 38 to 61 as GLONASS slot numbers of 1 to 24; • Introduce dynamic PRN mask solution to broadcast augmentation information supporting more than 51 ranging sources, reflecting the actual visibility. • Missing IOD (Issue of Data): • IOD is used to identify ephemeris information in order to match ephemerides between L1-SAIF Master Station and users; Currently using IODE for GPS; • Identify ephemeris information based on the time of broadcast. • Satellite position computation: based on PVA as described in GLONASS ICD.
User-side observations Network GPS Observables (RINEX) User-Domain Receiver Software Position Error L1-SAIF Master Station (L1SMS) Software Reference station observations Position Output SBAS Message Stream Upgrade for GLONASS Software Implementation • ENRI’s L1-SAIF Master Station (L1SMS) Software: • Generates L1-SAIF 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 of L1SMS for supporting GLONASS and QZSS: • Input module: RINEX observation and navigation files containing GLONASS; • Implemented GLONASS extension as explained before, for offline mode of L1SMS; • User-domain receiver software is also upgraded to be GLONASS-capable; • QZSS is also supported as it is taken into account like GPS.
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 • Showing satellite PRN identifier being augmented at each epoch; • 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. • IODP (issue of Data, PRN Mask) indicates change of PRN mask. GLONASS GPS
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 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.
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 • 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 • 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) 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) 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) 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.
Availability vs. Mask Angle 100% Availability @ User (b) • The number of epochs with valid position solution decreases with regard to increase of mask angle; • Multi-constellation with QZSS achieves 100% availability even for 40 deg mask.
Conclusion • Combined use of GPS and GLONASS with L1-SAIF: • Potential problems and solutions on realizing a Multi-GNSS L1-SAIF, capable of augmenting GPS, GLONASS, and QZSS simultaneously were investigated; • L1-SAIF Master Station is upgraded for supporting Multi-GNSS augmentation and tested successfully; Currently only for offline mode using RINEX files; • It is confirmed that the performance of L1-SAIF augmentation is certainly improved by adding GLONASS and QZSS, especially when satellite visibility is limited. • Ongoing and future works: • Support of realtime operation mode; • Broadcast of augmentation information for both GPS and GLONASS on QZS-1 real signal; Plan fromthe second half of this month; • Use of GLONASS/QZSS observables in generation of ionospheric correction; • Further extension to support Galileo. • For further information, contact to email@example.com