# Presentation: 205147

## SQM:SBAS Workshop

**SQM:****SBAS Workshop**

**ZETA****  ****    ****ASSOCIATES**

- 21 June 2005

## Overview

**WAAS IOC Approach**
**Signal Deformation Background**
**WAAS FLP Approach**

## 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 **

## 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**

## Example Signal Deformations and Resulting Correlation Functions

## 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 **

## Comparison of “Full” and “Most Likely” Threat Regions

## 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)**

## 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**

## 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 **

## 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**

## 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**

## 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**

## 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**