# Presentation: 103846

## Ionospheric Integrity Lessons from the WIPP

- Todd Walter
- Stanford University
- http://waas.stanford.edu

## History

- Ionospheric Storms and Disturbances Originally Tested Via Scenarios
*Simulated disturbances added to simulated ionosphere*
*Generally, large geographic features were placed near center of network*
- Ionospheric Algorithm Originally Based on JPL GIM Code
*Tuned to work on scenarios*
*Live data from WRSs not yet available*

## Ionospheric Models

- Provides Truth
*Good for initial algorithm validation*
- Very Smooth
*Average TEC*
*Loses small-scale variations*
*Spatial gradients smoothed as well*

- Useful Tool Before Data Was Available
- However Does Not Faithfully Represent Real-World Instantaneous Ionosphere

## Example Scenario

- From “Ionospheric Specification for the Wide Area Augmentation System (WAAS) Simulation Studies” by Steve Chavin, ION GPS-96

- dTEC/dt = 0.74 TECU/min
- Gradient = 0.085 TECU/km
- Shell Height = 360 km

## Problem

- Algorithm Tuned to Work on Scenarios
*All passed easily*
- Did not Work as Well on Real Data
*Required extensive retuning*
- Simulated Ionosphere Did Not Faithfully Reproduce Real Ionosphere
*Real disturbances worse than predicted*
*Real slant-to-vertical errors better than predicted*
- Failing Scenario Could Prove Loss of Integrity
- However, Passing All Scenarios Would Not Demonstrate Positive Integrity
*Worst-case scenario is algorithm dependent*
*Does not demonstrate probability of missed detection requirement is met*

## National Satellite Test-Bed

- Early Prototyping
- Dual-Frequency Survey Receivers
- Single Threaded
- Initiated in 1993
- Full Deployment Started in 1996

- Prototyping Occurred During Solar Minimum
*No significant ionospheric disturbances observed*
- Caused Us to Become Overconfident
- Performance Dominated by Receiver Artifacts
*Reasonability checks instituted to mitigate these errors*
- Too aggressive, would remove much of solar max observed behavior

## 11 Year Solar Cycle

- Solar activity changes dramatically over an 11 year solar cycle
- Ionosphere at the peak is much worse than at minimum
- Most disturbances at peak and declining phase

- NSTB

- WIPP

## WIPP

- At the End of 1999 FAA Certification Required a Change in the Safety Analysis
*Level D code not considered reliable*
*Threat models required for all monitors*
*Rigorous accounting for monitor observability*
- Certification of Ionospheric Algorithms Left to Ionospheric Experts
- Experts Created Threat Models From Data
- Reliable threat not hypothetical
- Must protect against worst observed conditions
- Must overbound historical observations
- Must have a demonstratable probability of missed detection

## Supertruth Data

- 25 WRS - 3 Threads Each - Carrier Leveled - Biases Removed - Voting to Remove Artifacts
- Clean Reliable Data Collected at the Peak of the Solar Cycle
- Contained Worst Observed Gradients (Temporal and Spatial over CONUS)
- Most Severely Disturbed Days Formed the Basis for Threat Model
- Ionospheric Disturbances Are Deterministic, but Sampled Randomly
*Worst cases are sampled over time*
- Will appear in the data as they move w.r.t IPPs
- Apply Data Deprivation to Model  Effects of Poor Observability

## Ionospheric Measurements

## Storm Example

## Differences in Vertical Delay

- Difference in Vertical Delay vs. IPP Separation Distance for Two Days:

- Quiet Day
-  July 2nd 2000

- Disturbed  Day
-  July 15th 2000

## CONUS Ionosphere Threat

- Not Well-Modeled by Local Planar Fit
*Ionosphere well-sampled**1*
*Ionosphere poorly sampled**2*
- Ionosphere Changes Over the Lifetime of the Correction
- User Interpolation Introduces Error
- 1. “Robust Detection of Ionospheric Irregularities,” Walter et al. ION GPS 2000
- 2. “The WAAS Ionospheric Threat Model,” Sparks et al. Beacon Symposium 2001

## Well-Sampled Ionosphere

- Chi-Square Metric Acts as “Storm-Detector”
*Test using small decorrelation value*
- Nominally ~ 35 cm one-sigma
*Passing test accepts larger value*
- Typically ~ 85 cm one-sigma
- Analytic Approach
*Does not require data except as validation*
*Fully specified before the first storm data of April 2000*

## Under-Sampled Ionosphere

- Purely an Empirical Threat Model
- Worst Storm Data Used
- IPPs Removed From Estimation to Simulate Poor Sampling
*Three quadrant removal schemes*
*Storm detector used on remaining data*
- Threat Based on Worst Deviation for Given Set of Metrics
- Threat Region Is 5x5 Degree Cell Centered on IGP

## CORS Data

## Goal of Sigma Undersampled

- To protect against an unfortunate sampling of the ionosphere such that we fail to detect an existing disturbance
- Presumes the ionosphere is non-uniform near the IGP, i.e. it is divided into at least two states: a quiet one that is sampled, and a disturbed one that is not

## Goal of Data Deprivation

- To divide the IPPs into two groups: one that samples a relatively quiet ionosphere and does not trip the chi-square, the other that samples ionosphere not well-modeled from the quiet points
*Data deprivation is used to simulate conditions that were not actually experienced but may reasonably be experienced in the future*
*It allows us to investigate threats that occurred in well observed regions as though they had occurred in poorly observed regions*
*Want to have the quiet IPP distribution match those that may occur on operational system*

## Storm Days

- Over the last 5 years, ~100 active days have been identified
- ~50 affected or would have affected WAAS performance
- ~45 supertruth files generated
*Working on files for the lesser days*
- 16 days affect our empirical threat model (serious effect)
- All supertruth files available to international SBAS community

## Temporal Threat Model

- Also an Empirical Threat Model
- Worst Storm Data Used
*Storm detector used*
- Look to See Largest Change With Respect to Planar Estimate
- Overbound of Worst Rate of Change Ever Observed
- Correlation with Spatial Gradient
*Errors are currently double counted*

## Temporal Threat Model

- No Storm Detector
*> 3 m / min*
*> 6 m /2 min*
*> 7 m over 5 min*

- With Storm Detector
*< 0.5 m / min*
*< 1.25 m /2 min*
*< 2.5 m over 10 min*

## Post IOC Storms

- October 29-31 and November 20, 2003 Were Some of the Worst Storms Ever Observed
- Conservatism in GIVE Calculation Protected Users
*No HMI observed at any location*
- None even close
- However Worse Than Predicted
*Still much uncertainty in ionosphere*

## Conclusions

- FAA Certification Required All Users Bounded Under All Conditions
*Ionospheric deviations are deterministic*
*Ionospheric deviations are observable*
- Threat Models Essential for Limiting Ionospheric Behavior
- Each Monitor Must Account for the Limits of Its Observability
- Approach is Very Conservative
*We are still learning!*