# Presentation: 460654

## GPS FAILURE MODES AND EFFECTS ANALYSIS FOR GPS BASED FLIGHT OPERATIONS          Dr. David Walsh			Dr. Washington OchiengDr. Rigas Ioannides		Dr. Shaojun FengCAA ISN				Centre for Transport StudiesUniversity of Leeds			Imperial College London

**Notes:**
Developed by CAA ISN, Imperial College, SRG

## Aim of Research

To enable optimised GPS-based air traffic service operations by answering the following question:

What is the impact of GPS performance/failures?

Accurate and simple answer required

- To enable optimised GPS-based air traffic service operations by answering the following question:
- What is the impact of GPS performance/failures?
- Accurate and simple answer required

**Notes:**
‘Normal’ performance 
- quantified SIS performance which does not exceed performance specification, includes unusual events

‘Failures’ 
- performance outside specification

Capture all events that affect GPS performance and determine their impact

## Why is GPS a challenge?

- Complexity
- satellites, modelling, signal generation, signal path effects, receivers, rx algorithms
- anomalies or failures can occur at any stage
- GPS failure modes are different
- Positioning performance varies with position of aircraft and satellites, and so with time (space time points)
- It is used by multiple aircraft over wide areas
- Institutional control

## Issues for developing a validation tool

- Many space time points, different failure types, different satellites failing
- Complex process to assess the performance of the system
- Random simulations required
- Geometry, bias magnitude and random nominal range errors affect the system
- Infeasible sample size (Space Time points X random simulations X all satellites X number of failure types)
- Leads to assumptions and compromises
- Uses Max slope concept
- Only the MDD satellite is tested this can be overconservative or too optimistic
- Clock drift error assumption
- Not giving the ability of system’s assessment for different failure types
- Predict RAIM availability
- Screens ‘poor’ satellite geometries, binary information about integrity not actual value of PMD, not complete information (RNP)

## Requirements in Performance Analyses 

- Complete end-to-end process
- GPS failure modes –fully captured and characterised
- Receiver effects must be included
- All space/time geometries must be included
- Avoid doing thousands of random simulations for each geometry and for each different failure type

## Requirements in Performance Analyses (2) 

- Provide RNP values
- Incorrect satellite exclusions should be considered
- If FDE algorithms become standard
- All satellites should be assessed
- That would enable us to find which satellite if failed will have the greatest impact

## Key points in the proposed method  

- Model probabilities (PMD, PFA) and accuracy with respect to geometry
- No need for random simulations
- Generic functions, can be used for any track
- Satellite outages captured
- Eliminates need for compromising on the number of geometry points and number of satellites to be assessed
- Direct link to RNP parameters
- Need to involve other geometry descriptors not slope
- Range of biases
- Different geometries are affected by different bias values
- Can assess the performance of the system for any bias

## Failure Modes and Effects Analysis (FMEA)

- Analyse failure modes to determine effects
- Classify failure modes according to severity (FMEA)
- Classify failure modes according to criticality (FMECA)
- criticality = severity * probability of occurrence
- The US FAA/DoT Changed approach in 2002
- Parallel GPS FMEA currently underway (Leader: Karen Van Dyke)

**Notes:**
Solution

## UK CAA GPS FMEA

- Failure modes
- All GPS components, Receiver, algorithms and augmentations
- Extensive Failure Mode Register managed in a standard format
- Identify effects and determine severity/criticality
- GPS performance analysis
- Input to safety analysis by the CAA SRG and others
- Example SV clock failures

**PRN 23 failure – 1.1.04**

**PRN 27 failure – 29.8.04**

## ISSUES

- Flightoperation

- Collect all space/time geometries

- Modelprobabilities

- CalculateRNP

- SafetyAnalysis

- _ISSUES_

- _SOLUTION_

- End to end process

- End to end process

- Capture failures

- Failures captured

- Assess all space/time points

- All space/time pointscaptured

- Include incorrect exclusions

- Includes incorrect exclusions

- RAIM limitations: ‘slope’

- Link to flight operations

- Link to flight procedure

- Link to flight operation

- Realistic assessment in time

- Failure model

- Probabilities modelled,not simulated

- Uses new parameters not‘slope’ to determine probabilities

- RAIM limitations: failure types

- Assume Gaussian distribution

- RNP calculation usesfailure type

- GPS PerformanceAnalysis

- Over-bound

## Gatwick Approach (1)

| London Gatwick RNAV (GNSS) RWY26 |  |
| --- | --- |
| Duration: 5.7 minutes | Accuracy: 220m |
| Average speed: 167 knots | Integrity Risk: 1 x 10-7 per hour |
| Initial approach height: 6000ft | Availability: 99% |
| Length: 16 NM | Continuity: 1 x 10-5 per hour |
| RNP0.3 |  |
| GPS nominal constellation (SPS2001) | Range standard deviation = 12.5m |

## Gatwick Approach (2)

- Failure mode: SIS failure - satellite clock error (most common)
- Approach details input into simulator
- Collect Space/Time Geometries: multi-approaches/sidereal days
- Pmd, Pfa modelling
- Calculate RNP+ outputs
- Accuracy
- Integrity
- Availability
- Continuity
- RAIM outage

**Example results**

## Output (1) – Example horizontal position error distribution

- Normal distribution random error on all satellites (=12.5m)
- Modelled bias applied at critical points on satellite with maximum slope.  Accuracy (95%) = 23.6m

- Number of samples

- Horizontal error in metres

**Notes:**
12 points 
Sample size 86400*12=1,036,800
Samples with Critical Bias:  1618
Histogram enlarge the tail part

## Output (2)

| Integrity |  |  |
| --- | --- | --- |
| Integrity Risk | 2.2 x 10-8
 | RNP Requirement:1 x 10-7 per hour |

| Continuity |  |  |
| --- | --- | --- |
| Interruptions over sample size | 1 x 10-5 per hour | RNP Requirement:1 x 10-5 per hour |

| Availability |  |  |
| --- | --- | --- |
| Availability (%age) | RAIM availability 
= 99.989%
System availability = 99.77% | RNP requirement: >99% |
| Max. continuous period of unavailability | 20 seconds
 |  |

## Conclusions

- _End-to-end GPS FMEA process_ developed
- Directly applicable to a _specific flight operation_
- No assumptions or compromises of RAIM analyses
- _RNP+ output_ for each Approach
- Process can be used for _any GPS-based flight operation_
- Intention to use our FMEA tool in validation of overseas territories approaches

## Acknowledgement

- The UK CAA Safety Regulation Group (SRG) for supporting the research