Page 1 SQM: SBAS Workshop ZETA ASSOCIATES 21 June 2005

Page 2 ZETA ASSOCIATES Overview WAAS IOC Approach —Signal Deformation Background WAAS FLP Approach

Page 3 ZETA ASSOCIATES 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

Page 4 ZETA ASSOCIATES 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 ( , f d,  ) —Model adopted by ICAO in ~1999

Page 5 ZETA ASSOCIATES Example Signal Deformations and Resulting Correlation Functions Threat Model A: Digital Failure (  ) Threat Model B: Analog Failure (f d,  ) Threat Model C: Combination of A & B

Page 6 ZETA ASSOCIATES 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

Page 7 ZETA ASSOCIATES Comparison of “Full” and “Most Likely” Threat Regions

Page 8 ZETA ASSOCIATES 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)

Page 9 ZETA ASSOCIATES 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 (  UDRE and  UIVE are well overbounded quantities) —Performance ‘cost’ with MERR is floor values for GIVE and UDRE

Page 10 ZETA ASSOCIATES 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 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

Page 11 ZETA ASSOCIATES 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

Page 12 ZETA ASSOCIATES 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

Page 13 ZETA ASSOCIATES WAAS FLP Signal 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

Page 14 ZETA ASSOCIATES 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