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1. Page 1 ZETAASSOCIATES SQM:SBAS Workshop 21 June 2005

2. Overview • WAAS IOC Approach • Signal Deformation Background • WAAS FLP Approach

3. WAAS IOC Signal Deformation Approach • WAAS IOC signal deformation detection utilizes real-time and offline monitors • Real-time monitor protects against “most likely” signal deformation threat while offline monitor protects against full ICAO threat model • Integrity case for this threat utilized the maximum error range residual (MERR) approach and a priori event occurrence

4. Signal Deformation Background • Differential GPS processing from March 1993 demonstrated that inclusion of SV-19 significantly degraded position accuracy • Investigations into SV-19 performance concluded the satellite suffered a failure such that it deformed the transmitted signal • Signal deformation resulted in non-common range errors across different receiver types • Research into GPS signal generation failure modes resulted in the definition of the full ICAO signal deformation model • Signal generation failure modes represented by only three parameters (D, fd, s) • Model adopted by ICAO in ~1999

5. Threat Model A: Digital Failure (D) Threat Model B: Analog Failure (fd, s) Threat Model C: Combination of A & B Example Signal Deformations and Resulting Correlation Functions

6. WAAS IOC Real-time Signal Deformation Monitor • Original WAAS design for signal deformation was specific to SV-19 fault observed in 1993 • Little detail for threat available • Position error reported in the 3 to 8m range as function of the specific correlator spacing used to determine pseudorange • WAAS monitor relied on MEDLL, narrow and wide correlator pseudorange processing • Definition/acceptance of full ICAO model resulted in expanded WAAS IOC threat • “Most Likely” threat used in IOC WAAS was a subset of the full threat model but included the most similar waveforms to the SV-19 failure

7. Comparison of “Full” and “Most Likely” Threat Regions

8. Monitor Details • IOC signal deformation detection utilizes the code-carrier coherence monitor (see reference list) • Multipath deviations (code-carrier corrected for dual freq iono) averaged across the network • Correlator data not available • Monitor performance and noise characteristics validated using 8 days of data with prototype algorithm • Detection with this monitor results in the satellite being set to DU for 9 hours (Remainder of satellite pass)

9. Integrity Case • Acceptance by the WIPP of a reduced threat was deemed sufficient provided later stages of WAAS would add real-time monitoring of the full ICAO threat model • Acceptance was also premised on presence of a robust offline monitoring capability • Utilization of a MERR approach and a priori event occurrence were carefully evaluated by the WIPP and deemed acceptable for the signal deformation threat • MERR concept takes advantage of system margin for nominal operation (sUDRE and sUIVE are well overbounded quantities) • Performance ‘cost’ with MERR is floor values for GIVE and UDRE

10. Rationale for A Priori:Signal Deformation Threat • The occurrence of a signal deformation is random • The probability of satellite deformation failure is small (analysis used 10-4 but case could be made for even smaller event occurrence) • The exposure time to a signal deformation must not be infinite • When such a failure is detected the deformation is treated very conservatively by setting the satellite to DU

11. Rationale for MERR:Signal Deformation Threat • Large errors are detected readily with high probability using real-time monitor • Small errors that are not detected with high probability are also small enough that they will not have significant impact on the User • The probability of satellite deformation failure is in itself small • Offline monitoring will detect the failure and thus limit exposure to that threat

12. WAAS IOC Offline Signal Deformation Monitor • Offline signal deformation monitor instituted to limit exposure time to undetected satellite failure with the real-time monitor • Offline processing is conducted using data from six to eight geographically diverse stations • Receivers at these stations output multipath correlator measurements for each satellite tracked • Current processing in transition to 24/7 operation with analysis/reporting occurring during regular business hours • GPS satellite failure detected/confirmed with this monitoring would result in FAA notification to the GPS controlling authority

13. WAAS FLPSignal Deformation Approach • Full ICAO Threat Model will be monitored with real-time process • New WAAS reference receivers (G-II) with correlator output functionality begin installation this summer • Current offline processing is serving as prototype for eventual online monitor • FLP signal deformation monitor will significantly reduce the need for offline monitoring • Probably quarterly checks to ensure the noise characteristics used in design validation are still representative

14. Summary • Signal deformation threat in IOC WAAS was mitigated with hybrid of real-time and offline monitoring • Robust offline monitoring was leveraged heavily in the integrity case for this threat • Led to acceptance of most likely threat (subset of full ICAO model), MERR approach and use of a priori • References • P. Shloss, R. Phelts, T. Walter, P. Enge, “A Simple Method of Signal Quality Monitoring for WAAS LNAV/VNAV” ION GPS 2001, September 2001 • K. Shallberg, P. Shloss, E. Altshuler, L. Tahmazyan, “WAAS Measurement Processing, Reducing the Effects of Multipath” ION GPS 2001, September 2001