L1/L5 SBAS MOPS to Support Multiple Constellations

1. L1/L5 SBAS MOPS to Support Multiple Constellations Todd Walter, Juan Blanch, and Per Enge Stanford University http://waas.stanford.edu

2. Background • At IWG 20 in Madrid it was agreed that the L5 MOPS should support 4 constellations with 90 active corrections • At IWG 21 in Stanford it was agreed that there was interest in providing a better level of service than LPV-200 (e.g. CAT-II) • Details of service are uncertain

3. Discussion • At IWG 24 in Toulouse some concerns were raised over Stanford’s proposed L5 MOPS (called ICD2) • Longer interval between clock updates may require larger bounding term • Use of change indicators rather than full DFRE’s • Ability to alert more than 7 SV’ due to single event • Effect of message loss for this alert message • Delayed broadcast of DFREI raises effective value (called border effect) • Alternate proposal (called ICD1) followed L1 MOPS more closely • Requires two MT6’s to support more than 51 total corrections

4. Proposed ICD2 Components • Expanded PRN mask • Removal of FCs • Alert message with DFRECIs • Single satellite correction message • Smaller quantization errors • Updatable DFRE table • Support for more SBAS orbit types

5. Expanded PRN Mask • 37 PRNs per constellation • GPS, GLONASS, Galileo, Compass • 39 PRNs available for SBAS GEOs • 23 spare PRNs for QZSS, IRNSS etc. • More possible if fewer than 37 given to each constellation • Fully compatible with existing L1 MOPS and other proposed L5 changes (ICD1 & ICD2)

6. Need for 63 GPS PRNs? • GPS ICD has 63 defined PRNs • The definition table for PRN codes also has 210 slots and assigns the first 63 to GPS • SBAS MOPS (229D) already assigns slots 38-61 to GLONASS • No need to match tables 1-to-1 • Can have a mapping from one to the other • Do we need to support 63 GPS PRNs? • Will other constellations want the same? • Could no longer fit SBAS PRN mask into a single message • Unlikely to have more than 37 active PRNs

7. Removal of Fast Corrections • Fast correction originally intended to support Selective Availability (SA) • No longer needed as satellite atomic clocks are extremely predictable over time periods of interest (< 400 secs) • Velocity << 1 cm/sec • Acceleration is immeasurably small over a few minutes • More details later in presentation

8. New Alert Message • Allows more than 51 corrections • By sending DFRE Change Indicators instead of full DFREIs • Alternate methods to achieve > 51: • Longer updates between SV corrections • Increase bit rate (e.g. use Q. channel) • Not require DFRE update every 6 sec • ICD1 achieves this goal with two type 6 messages • Extends LTC update interval to 240 sec

9. Single Satellite Correction Message • MTs 25 & 28 each provide updates for two satellites • Loss of either message affects both SVs • Instead make one message contain the LTC and covariance for one SV • Cleaner connection between the two • Save some of the SV ID bits • Still an improvement even if fast correction not eliminated • Compatible with ICD1 & ICD2

10. Smaller Quantization • 12.5 cm resolution is a noticeable contributor to WAAS inaccuracy • Clock dynamic range originally set due to SA – can be much smaller • Reducing dynamic range and quantization error improves accuracy for both ICD1 and ICD2 • For ICD1 would also recommend eliminating RRC by differencing adjacent fast corrections

11. Updatable DFRE Table • Broadcasting the DFRE table allows the DFRE quantization to be optimized to the service • Ensure use of all DFREIs • Minimize quantization penalty • User applies maximum value if they have not yet received the table • Changes would be very rare and tightly controlled (perhaps MT0 in between) • Compatible with ICD1 & ICD2

12. SBAS SV Orbit Messages • Specifies augmented Keplerian elements • Allows for more than just geostationary and near-geostationary orbits • Better accuracy • Better long-term performance for almanacs • Retain existing GEO messages • This is an additional/alternate message • Compatible with ICD1 or ICD2

13. SV Clock Errors & Rate Data from March 24-30, 2013

14. SV Clock Error Acceleration Data from March 24-30, 2013

15. Linear Behavior of SV Clocks Data from March 24, 2013

16. Linear Prediction Clock Data MOPS Corrections (12.5 cm resolution) Extrapolated Fit ICD2 Correction (3 cm resolution) Linear Fit

17. Prediction Accuracy

18. PseudorangeOverbound • L1 MOPS with current quantization and 6 sec FC updates requires a PR overbound of > 12.5 cm after 6 sec • ICD1 without quantization error and 6 sec FC requires a PR overbound of > 7 cm after 6 sec • ICD2 requires a PR overbound of > 12.2 cm after 36 sec • Could set acceleration bound to zero and include small linear growth term • Slower clock broadcast for 1 constellation should lead to less than than a 1 m difference in VPL

19. Border Effect • For one constellation, no more than 3 DFREIs require update within a 36 second interval >99.9% of the time • For two constellations, no more than 5 DFREIs require update within a 60 second interval >99.9% of the time • Nominally, there should be no border effect for one or two constellations • Even for three or four constellations, the border effect should only delay a small number of DFREIs by order 6-18 seconds

20. Nominal DFREI Changes per Time Interval Predicted WAAS DFREI changes due to geometry for 31 SV GPS constellation excluding changes to NM

21. Change Indicator Interpretation #1 • A no change (NC) indication points back to the full DFREI in the satellite correction • Changes may need to be indicated until 2 or more correction message broadcasts • Or could use degradation parameters 2 3 4 I 21 7 8 9 10 I 6 12 13 14 15 I 16 17 18 20 11 19 1 I 5 22 24 25 I 27 28 29 26 I 1 2 3 30 5 I 6 7 8 9 10 4 I 1 23 I Integrity message Satellite correction DFREI = 6 NC #1 DFREI #1 = 7 DFREI #1 = 7 DFREI #1 = 7 DFREI #1 = 7 DFREI #1 = 7 DFREI #1 = 7 NC #1 or DFREI #1 = 7

22. Change Indicator Interpretation #2 • A no change indication points to the most recently received DFREI in the last 18 sec • Three successive DFREI updates are sufficient to fully update DFREI • Unable to determine DFREI if three prior integrity messages are lost 2 3 4 I 21 7 8 9 10 I 6 12 13 14 15 I 16 17 18 20 11 19 1 I 5 22 24 25 I 27 28 29 26 I 1 2 3 30 5 I 6 7 8 9 10 4 I 1 23 I Integrity message Satellite correction DFREI = 6 NC #1 DFREI #1 = 7 DFREI #1 = 7 DFREI #1 = 7 DFREI #1 = 7 NC #1 NC #1 NC #1

23. Interpretation #2 • If a user has not received a full DFREI update for a satellite in the last 18 seconds • The satellite must be set to NM unless it is already DNU • It remains in that state until a full DFREI value is obtained • unless the DFRECI sets it to NM or DNU • The loss of three successive integrity messages could cause all satellites to go NM • Three independent message outages is very rare (< 10-9) • A single outage would need to last 13 seconds or longer • Should also be rare • Recovery may be slow as will need to wait for satellite corrections to be broadcast • Up to 120 seconds for all satellites • However, the border effect on availability should be negligible even for four constellations

24. Discussion Point #1 • The decision to support more than 51 active corrections can be revisited • Did not want the MOPS to constrain future more demanding operations • Additional constellations support lower protection levels • Requirements for such operations have not yet been determined

25. 99.5% VPL 1 Constellation 2 Constellations 3 Constellations 4 Constellations

26. 99.5% HPL 1 Constellation 2 Constellations 3 Constellations 4 Constellations

27. HPL & VPL as a Function of Number of Satellites 101 SVs 74 SVs 47 SVs 24 SVs

28. Discussion Point #2 • Fast Corrections are primarily only meaningful for a single constellation • For two constellations they are slowed to 33 second intervals or longer • For three or more constellations they are on par with the long term corrections and limit ability to fit within bandwidth • However legacy ground systems are set up to generate Fast Corrections • May be lower initial cost to retain them

29. Recommendations • The remaining ICD2 components can be implemented with either approach • Recommend adopting: • Expanded PRN mask • Combining Type 25 and Type 28 info into a single satellite correction message • Reducing quantization errors • And eliminating RRC from adjacent FCs • Updatable DFRE table • Support for more SBAS orbit types as an additional message