ION ITM 2013 ION ITM Jan 2013 Slide

ION ITM- 2013 ION ITM Jan. 2013 Slide San Diego, CA Jan. 28 -30, 2013 QZSS L 1 -SAIF Supporting GPS/GLONASS Multi-Constellation Augmentation T. Sakai, H. Yamada, K. Hoshinoo, and K. Ito Electronic Navigation Research Institute, Japan

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, L 1 -SAIF and LEX; – The first QZS satellite was successfully launched on Sept. 11, 2010. • L 1 -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 L 1 single frequency. • ENRI has been developing L 1 -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.

ION ITM Jan. 2013 - Slide 2 QZSS Concept GPS/GEO • • • Footprint of QZSS orbit; Centered at 135 E; Eccentricity 0. 075, Inclination 43 deg. QZS • 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.

ION ITM Jan. 2013 - Slide 3 L 1 -SAIF Signal QZS satellites Ranging Function GPS Constellation Error Correction Integrity Function • Three functions by a single signal: ranging, error correction (Target accuracy: 1 m), and integrity; • User receivers can receive both GPS and L 1 -SAIF signals with a single antenna and RF front-end; • Message-oriented information transmission: flexible contents. SAIF： Submeter-class Augmentation with Integrity Function Ranging Signal User GPS Receivers

ION ITM Jan. 2013 - Slide 4 L 1 -SAIF Signal • QZSS broadcasts wide-area augmentation signal: – Called L 1 -SAIF (Submeter-class Augmentation with Integrity Function); – Designed and developed by ENRI. • L 1 -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 L 1 C/A based on SBAS techniques: – Broadcast on L 1 freq. with RHCP; Common antenna and RF front-end; w Modulated by BPSK with C/A code (PRN 183); w 250 bps data rate with 1/2 FEC; Message structure is identical with SBAS; w Differences: Large Doppler and additional messages. – Specification of L 1 -SAIF: See IS-QZSS document (Available at JAXA HP).

ION ITM Jan. 2013 - Slide 5 L 1 -SAIF Corrections • 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); • L 1 -SAIF provides corrections only; No L 1 -SAIF ranging. Horizontal Error Vertical Error Standalone RMS GPS Max 1. 45 m 2. 92 m 6. 02 m 8. 45 m RMS 0. 29 m 0. 39 m Max 1. 56 m 2. 57 m System Standalone GPS Augmented by L 1 -SAIF Augmentation to GPS Only Note: Results shown here were obtained with surveygrade antenna and receiver in open sky condition.

ION ITM Jan. 2013 - Slide 6 GLONASS Support: Motivation QZSS L 1 -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 L 1 -SAIF.

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 400 ns in July 2012; – SBAS broadcasts the difference by Message Type 12; w GLONASS-M satellites are transmitting the difference as parameter t. GPS in almanac (non-immediate) data: t. GPS = t. GPS − t. GLONASS. • 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:

ION ITM Jan. 2013 - Slide 8 PRN Masks • PRN Mask: – SBAS/L 1 -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/L 1 -SAIF signal; But, 32 GPS + 24 GLONASS = 56 !!! 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 Spare • 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.

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 Time LTC IOD=a LTC IOD=b

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: w 5 MSB of IOD is validity interval, V; w 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. Previous Ephemeris Next Ephemeris Time LTC IOD=V 1|L 1 Ephemeris Validity Interval V 1 LTC IOD=V 2|L 2 Ephemeris Validity Interval V 2 L 2

ION ITM Jan. 2013 - Slide 11 Satellite Position • GLONASS ephemeris data: – GLONASS transmits ephemeris information as position, velocity, and acceleration in ECEF; w Navigation-grade ephemeris is provided in 208 bits for a single GLONASS SV; w 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. w These terms can be computed for the specific position and velocity of SV; w GLONASS ICD A. 3. 1. 2 gives the equations below (with some corrections). Perturbation terms in ephemeris

ION ITM Jan. 2013 - Slide 12 Upgrade of L 1 SMS • L 1 -SAIF Master Station (L 1 SMS): – Generates the L 1 -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. QZS GPS L 1 SA L 2 SA GEONET GSI L 1 C/ A, Measured Data L 2 P al n g Si IF Closed A -S Loop L 1 SMS ENRI L 1 -SAIF Message nd ba K- GLONASS L 1 C/A , L 2 P – – QZSS MCS JAXA

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 180 s is given to users to securely catch the new PRN mask. Transition time 180 s tcutover PRN Mask (IODP=i) FC FC LTC FC Cutover PRN Mask (IODP=i+1) FC LTC Corrections before cutover FC FC LTC FC Corrections after cutover FC

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. True Time GLONASS System Time t t. GLONASS GPS System Time Receiver clock for GPS satellites t. R ^ B GLONASS Dt. GPS Time offset broadcast to users t. GPS Receiver Time -da. GLONASS ^ B GPS Receiver clock for GPS satellites

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: w 6 sites for reference stations; Reference Station (a) to (f) w 11 sites for evaluation. User Station (1) to (11) • Period: 2013/1/6 01: 00 to 2013/1/9 23: 00 (94 hours).

ION ITM Jan. 2013 - Slide 16 PRN Mask Transition QZSS GLONASS GPS • 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.

ION ITM Jan. 2013 - Slide 17 Elevation Angle GPS GLONASS QZSS 5 deg PRN Mask Transition @ 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.

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.

ION ITM Jan. 2013 - Slide 19 User Position Error: Mask 5 deg • GPS+GLO+QZS: 0. 310 m RMS of horizontal error at user location (7); • Looks some limited improvement by using multi-constellation.

ION ITM Jan. 2013 - Slide 20 User Position Error: Mask 30 deg • GPS+GLO+QZS: 0. 335 m RMS of horizontal error at user location (7); • Multi-constellation offers a good availability even for 30 deg mask.

ION ITM Jan. 2013 - Slide 21 Error vs. User Location: 5 deg 0. 421 m 0. 283 m North South • Expect horizontal accuracy of 0. 3 to 0. 5 m with L 1 -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.

ION ITM Jan. 2013 - Slide 22 Error vs. User Location: 30 deg 0. 425 m North South • The horizontal accuracy is still within a range between 0. 3 and 0. 5 m for the multi-constellation configuration; • The accuracy degrades to 1 or 2. 5 m for GPS-only single-constellation configuration.

ION ITM Jan. 2013 - Slide 23 Conclusion • ENRI has been developing L 1 -SAIF signal: – Signal design: GPS/SBAS-like L 1 C/A code (PRN 183); – Planned as an augmentation to mobile users. • GPS/GLONASS/QZSS multi-constellation support: – L 1 -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 L 1 -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.