The Wide Area Augmentation System (WAAS) Todd Walter Stanford University Todd Walter Stanford University

2 Conclusions  WAAS is used to provide aircraft navigation from enroute through vertically guided approach  Integrity was and is the key challenge  Important to understand what can go wrong and how to protect users  Careful analysis of feasible threats  New civil frequencies and additional constellations will further improve performance

3 Outline  The Wide-Area Augmentation System  Integrity Analyses  Comparison with Terrestrial Navigational Aids  Future Directions

4 WAAS

Wide Area Augmentation System (WAAS) – Program Status 5 Federal Aviation Administration WAAS Architecture 38 Reference Stations 3 Master Stations 4 Ground Earth Stations 2 Geostationary Satellite Links 2 Operational Control Centers Courtesy:

Wide Area Augmentation System (WAAS) – Program Status 6 Federal Aviation Administration Geostationary Satellites (GEO) Provides Dual Coverage Over United States Telesat 107  W PanAmSat 133  W Courtesy:

Wide Area Augmentation System (WAAS) – Program Status 7 Federal Aviation Administration

8 Error Sources  Satellite errors  Ephemeris  Clock  Signal  Propagation errors  Ionosphere  Troposphere  Local Errors  Multipath  Receiver Noise

9 Master Station Schematic

10 Complicated Schematic

11 GPS Performance (Usually) On a good day, the red circle encloses 95% of the GPS position fixes.

12 Major GPS Faults About Twice a Year Example: Ephemeris Failure on April 10, 2007 On a bad day, the GPS errors can be much worse. WAAS & GBAS eliminate these large errors.

13 Integrity Approach  Aviation integrity operates on a guilty until proven innocent principle  Error bound is the maximum possible value given the measurements  This is unlike conventional systems that describe the most likely errors  Protection level is a % bound on worst reasonable conditions  Very different from 95% achieved accuracy

14 Failure of Thin Shell Model Quiet DayDisturbed Day

15 Undersampled Condition Courtesy: Seebany Datta-Barua

16 11/20/ :00:00 GMT

Wide Area Augmentation System (WAAS) – Program Status 17 Federal Aviation Administration Localizer Performance Vertical (LPV) Coverage Courtesy:

Wide Area Augmentation System (WAAS) – Program Status 18 Federal Aviation Administration WAAS RNP 0.3 Current Coverage Courtesy:

19 WAAS LPV and LPV-200 Vertical Position Error Distributions July 2003 to June 2006 Courtesy: FAA Technical Center 3 years 20 WRSs 1 Hz data

20 Navigational Aids  Instrument Landing System (ILS)  Glideslope antenna for vertical  Localizer for horizontal

21 ILS Installations: Each Runway End Requires At Least Two Transmitters 1318 ILS’s nationwide

22 Airports with WAAS LPV No GPS Equipment Required at Airport 50 Pieces of WAAS Equipment Serve the Continent As of November, WAAS-based LPV’s ~1000 for non ILS runways

23 Localizer Approaches at Moffett Field Courtesy: Sharon Houck

24 Utility of Protected Accuracy from WAAS WAAS (& GBAS) tunnels: Do not flare like ILS Do not have beam bends Are programmable Are adaptable Localizer performance with vertical guidance (LPV) Safer than lateral nav. (non-precision approach) Same decision ht. as Cat I GBAS for Cat. II & III

25 Current WAAS Performance

26 Future L1/L5 Performance

27 L1-only Threats  Reference station multipath, noise, cycle slips, and other receiver errors – WRE bias, CNMP  Satellite clock/ephemeris – UDRE/MT28  Erroneous ionospheric delay estimates - GIVE  Erroneous WRE clock estimates, interfrequency bias estimates – RDM  L1 code/carrier incoherence – CCC  L1 signal deformation – SQM  Multiple convolved threats – UPM, convolution analysis  Antenna biases, GEO biases – specific analyses

28 L1/L5 Threats  Reference station multipath, noise, cycle slips, and other receiver errors – WRE bias, CNMP – unchanged  Satellite clock/ephemeris – UDRE/MT28 – need to remove L1/L2 bias from Fast Correction, could remove uncertainty from UDRE  Erroneous ionospheric delay estimates - GIVE – uneeded  Erroneous WRE clock estimates, interfrequency bias estimates – RDM – IFBs uneeded, WRE clocks may be handled by UDRE/UPM

29 L1/L5 Threats cont.  L1/L5 code/carrier incoherence – CCC – new/updated monitor, MERR reduced without GIVE while threat is potentially increased  L1/L5 signal deformation – SQM – new/updated monitor, MERR reduced while threat is increased. User space will need to be greatly restricted  Multiple convolved threats – UPM, convolution analysis – new/updated monitor specific to L1/L5 user  Antenna biases, GEO biases – specific analyses – new/updated analysis, effects are greater

30 CCC Threats  Iono-free ccc metric expected to have 2.6 times as much noise as L1-only version  Trip thresholds may need to be increased  Could lead to UDRE bumps or trips  MERR substantially reduced without GIVE  Need much reduced thresholds and  test  L1 only monitor has margin thanks to GIVE  Need to collect data to see real  test  Need L5 data, but L2C might be OK  High risk factor for not being able to match L1 UDREs

31 Frequency Dependencies L1 errorL5 errorRSSSum of absolutes Iono error Clock / ephemeris or tropo L1 Only Iono- free

32 SDM Threats  Bias threats on L1 have 2.26 times greater influence on iono-free users  L5 biases also must be added  MERR substantially reduced without GIVE  Nominal and threat biases potentially 3.5 times those on L1 only  Bias terms in VPL potentially very beneficial to iono-free user  Need to further restrict user space  L1 and L5 may need to be a very tight box around monitor choice  Difficult to get international buy-in

33 Conclusions  WAAS is used to provide aircraft navigation from enroute through vertically guided approach  Integrity was and is the key challenge  Important to understand what can go wrong and how to protect users  Careful analysis of feasible threats  New civil frequencies and additional constellations will further improve performance

34 Potential of L1/L5 GPS/Galileo Performance