Development of Aviation Safety Margin for Aeronautical ...



|[pic] | |ACP-WG-F29/WP-09 |

| |International Civil Aviation Organization |2013-09-01 |

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AERONAUTICAL COMMUNICATIONS PANEL (ACP)

29th MEETING OF WORKING GROUP F

Nairobi, Kenya 3-12 September 2013

|Agenda Item X: |Interference from non-aeronautical sources |

Working Paper on the Development of Aviation Safety Margin for Aeronautical Services

(Prepared and presented by Andrew Roy)

|SUMMARY |

|This paper discusses the concept of the aviation safety margin in aeronautical radiocommunications and |

|radionavigation safety systems, suggesting further development for ICAO. |

|ACTION |

|It is proposed that the working group take this information into account during its deliberations. |

Introduction

The current implementation and use of an Aviation Safety Margin (ASM)[1] has highlighted several variations when applying an appropriate safety margin to sharing studies with aeronautical safety systems. Although an established concept, the different possible approaches have led to confusion and opposition to its implementation. Therefore, the concept should be more clearly defined by ICAO in its function and application.

The debate at WG-F28 on ASMs prompted several questions[2] that need to be addressed in creating a more comprehensive framework for ASM implementation. A key concern is attempting to put a value on the unknown to achieve the required availability for an aviation safety system. However, in attempting in answer this, the debate also raised additional questions on the function and application of an ASM.

Aim

To define the conditions and intent of the ASM safety margin concept.

Implementing a safety margin

The concept of a safety margin is an abstract one, as ITU-R Recommendation M.1903 implies in its definition[3]. In attempting to reduce failure levels to an order of 1x10-7 or less for aviation RF communications[4], all aspects of an RF link must be taken into account. This requires quantified data and modeling to apportion any adverse impact to a theoretical RF link model. However, certain aspects such as accounting for all RF emissions, unexpected propagation effects, modulation effects, malfunctioning equipment, and unit-to-unit performance variations are not currently defined by accurate values or models, yet must be accounted for to achieve the availability requirements.

Furthermore, the total availability of a system is not isolated to the RF environment, external processes and structures provide additional assurance to aviation systems to meet regulatory requirements. This includes system safety cases, redundant systems, human factors, recovery procedures, etc. These factors need to be quantified in an appropriate manner that is suitable for an RF regulatory environment such at the ITU-R.

The end result is an attempt to approximate the unknown and include it in a form understandable to a standard RF modeling process, as noted by the ICAO Spectrum Handbook[5]. Precise calculation of this qualitative data into an appropriate single value is not feasible without introducing even larger error margins at the expense of significant work, hence the standard aviation practice to apply at least a 6dB safety margin in the absence of other data.

There have been several suggestions of using measured data to provide a suitable value to the safety margin. Yet the practical implementation to comprehensively survey and baseline an aviation environment to provide assurance up to the maximum 1x10-7 failure rate may not be fully achievable with the required accuracy. Static ground measurements will not accurately represent the potential interference from multiple ground sources seen by an airborne receiver, as already recognized in the ITU-R[6]. Existing or new RF surveys may partially support a value and should be investigated, but such work would still not fully incorporate all these aspects.

Relationship to the protection margin

Several instances of an ASM in existing documentation seemed to incorporate, or even blur with, the protection margin. This is incorrect given the protection ratio’s definition and intent[7]. A safety margin should be an extension to the protection margin, separating the factors into known elements that can be accurately represented in the protection margin, and the unknowns that should be approximated into the safety margin. Such a method provides clarification for the requirements of safety services under Article 4.10, incorporating a logical explanation of what has been accounted for and why.

In defining the safety margin as an extension to the protection ratio, it should also take the same form as the stated protection margin. This could include carrier power increase, reduction in interference noise power, rise over thermal percentage, etc.

Defining an appropriate value

The aim of any protection criteria should be to resolve as many factors as possible into the protection margin, reducing the magnitude of a safety margin where feasible. Therefore, if all aspects could be resolved, then the safety margin could be theoretically reduced to zero. However, with aviation receivers operating in LOS above multiple sources, combined with an extremely high availability requirement, this does not seem feasible.

In discussing the above, several references have been made to logically explaining the factors that are incorporated into the safety margin. This creates a second issue to its implementation; what is an appropriate level of assurance to support a quantified set of unknowns, and where should the documentation used in the justification be developed. Although supporting data such as RF surveys and system reliability information may provide some evidence, appropriately weighting these factors and incorporating other unknowns will require a decision framework or criteria to approve.

Consideration should also be given to possible different classifications of a safety margin. Aviation operates in a unique environment, with airborne receivers operating in LOS of multiple ground emitters[8]. However, not all aeronautical receivers operate in the air, and other safety services also have high availability requirements. Development of a comprehensive safety margin concept should be mindful of any precedent set for other areas.

Conclusion

In applying an ASM to protect a system, an attempt must be made to quantify unknowns and other qualitative information into a form that is compatible with a competitive international RF regulatory environment. Combined with the unique elements of operating safely in an aviation environment, requires a logical and amenable to analysis of potential factors supported by measured data where possible.

However, if the credibility of a safety margin is to be maintained, then a greater effort should be made to ensure consistency in the approach and explaining these variables in the context of the system to justify a safety margin. ICAO should take a leading role in this regard, creating recommendations that are compatible with international regulatory forums such as the ITU-R.

Action for the meeting

It is proposed that the working group:

Take this information into account during its deliberations.

• Consider the elements in Annex 1 in the development of the ASM concept within ICAO.

Annex 1 – Aviation Safety Margin Concept

Proposed Definition

(Modified from ITU-R Rec M.1903)

The aviation safety margin accounts for the risk of loss of life due to radio-frequency interference that is real but not quantifiable in the system protection ratio. To support safety-of-life applications and achieve the availability required by ICAO, all sources of interference and system disruption must be accounted.

Summary points on application of an ASM

• Additional safety margin accounts for unknown aspects that cannot be included in RF propagation or sharing models.

• The safety margin is applied as an extension to the system protection margin.

• Applied value takes the same form as the protection margin.

• Aviation expert qualitative assessment should decide on the value, and should be supported by measured data where feasible.

Suggested for incorporation into the ICAO frequency management handbook

Aviation safety factor (existing text)

9.2.22 Some aeronautical applications (for example, precision approach and landing) are regarded as having high criticality in safety terms thereby meriting an additional safety factor. The analysis of aeronautical applications would consider the total operational situation, which would then be narrowed down to the element involving unacceptable interference. From there, an additional protection factor of not less than 6 dB would be applied to increase the operational assurances to the required level.

Aviation safety margin (proposed)

9.2.22 Aeronautical safety applications are required to have a high availability rate to ensure continued operation through worst case interference. To ensure all factors are accounted for, the analysis of aeronautical applications should consider the total operational situation, narrowing down the elements involving unacceptable interference. These should be accommodated for in any theoretical study by applying an additional margin to increase the operational assurances to the required level. Until established on the basis of further study, an additional safety margin of not less than 6 dB should be applied.

The nature of interference and its detection (existing text)

9.4.1 Interference from any source, or of any type, is recognizable by a change in the receiver output signal. Quantification is not necessary for it to be harmful, and many types of interference, e.g. pulse, are not easily quantified but nevertheless are indisputably harmful. In this regard, particular care is necessary with systems in which the output is neither aural nor visual, such as modern digital systems or systems where the output is used to operate control systems, and detection may go unnoticed for some time. The task of assessing the threat posed by other signals so as to make a decision of acceptability, for example in allocation sharing, must however have a basis which is logical and amenable to analysis.

9.4.2 For the theoretical assessment of compatible sharing with other radio services (a situation becoming more common), or where the threat is unwanted emissions from a known non-aviation system, a quantitative criterion has to be stated and used as a reference for decision making. For this purpose, a maximum interference threshold limit is normally chosen which has been selected on the basis of acceptable degradation, taking into account all other environmental conditions. In the absence of other data, the usual planning ratio for wanted-to-unwanted signals within the aviation service should be enhanced to give a margin for uncertainties which cannot be quantified. An increase of not less than 6 dB is often taken to be appropriate for this safety factor.

9.4.3 At higher frequencies in the GHz ranges, and for wide-band low signal services, a more appropriate criterion is the acceptable increase in the noise floor, or the noise temperature, of the receiving system. Antenna gains or losses are a necessary inclusion to replicate real-life conditions. The final approach and landing phase is accepted as being the most important of the safety-critical services. The model described below is recommended for this analysis.

The nature of interference and its detection (proposed text)

9.4.1 Harmful interference to aeronautical safety systems is not always quantifiable given the dynamic environment that airborne radiocommunications systems operate in. These effects can have a detrimental effect on the performance of a radiocommunications link, reducing the availability of safety systems below the required level set by ICAO. However, for the theoretical assessment of compatible sharing with other radio services, or where the threat is unwanted emissions from a known non-aviation system, a quantitative margin has to be stated and used as a reference for decision making.

9.4.2 ICAO recommends that protection criteria for aviation safety services should be enhanced to give a safety margin for uncertainties which cannot be quantified, but will create harmful interference. In defining the safety margin as an extension to the protection ratio, it should also take the same form as the stated protection margin. This could include carrier power increase, reduction in interference noise power, rise over thermal percentage, etc.

Further questions:

• What factors should be taken into account in deciding on a safety margin?

• What level of assurance is required to accurately represent the unknown factors being incorporated into a safety margin value?

• What existing sources of information may support an analysis of an airborne RF environment?

• Should a more general safety case process be incorporated into ICAO process to support the safety margin?

• Is a default value considered a viable policy?

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[1] WP22 - Working Paper on the Use of the Aviation Safety Margin for Aeronautical Services. WG-F Meeting 28 (03/13).

[2] Report ACP WG-F/28. WG-F Meeting 28 (03/13).

[3] ‘An aviation safety margin (ASM) is used to account for risk of loss of life due to radio-frequency interference that is real but not quantifiable. To support safety-of-life applications, all interference sources must be accounted for.’ Recommendation ITU-R M.1903 (01/2012). Characteristics and protection criteria for receiving earth stations in the radionavigation-satellite service (space-to-Earth) and receivers in the aeronautical radionavigation service operating in the band 1 559-1 610 MHz

[4] ICAO Document 9869 – ICAO Manual on Required Communication Performance.

[5] ‘In the absence of other data, the usual planning ratio for wanted-to-unwanted signals within the aviation service should be enhanced to give a margin for uncertainties which cannot be quantified.’ - ICAO Document 9718 - Handbook on Radio Frequency Spectrum Requirements for Civil Aviation (Draft Rev 6).

[6] Recommendation ITU-R SM.1535 (07/2001). The protection of safety services from unwanted emissions

[7] Protection ratio - The minimum value of the wanted-to-unwanted signal ratio, usually expressed in decibels, at the receiver input, determined under specified conditions such that a specified reception quality of the wanted signal is achieved at the receiver output. ITU Radio Regulations (2012). Volume 1, Article 1.170

[8] Report ITU-R SM.2057 (2005). Studies related to the impact of devices using ultra-wideband technology on radiocommunication services.

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