978-1-61284-798-6/11/$26.00 ©2011 IEEE 5C5-1

UAS INSERTION INTO COMMERCIAL AIRSPACE: EUROPE AND US STANDARDS PERSPECTIVE

Eric A. Euteneuer, Honeywell, Minneapolis, MN Dr. George Papageorgiou, Honeywell, Toulouse, France

Abstract

The use of Unmanned Aircraft Systems (UAS) by the military has greatly increased over the last decade. This experience has led to the development of Concept of Operations (CONOPS) and technology in line with defense and security missions. It is evident that UASs are becoming the military's choice for dull, dirty and dangerous missions. Prompted by the military’s experience, civil agencies have identified a large variety of missions that potentially could be performed by UASs with clear benefits.

UAS are already in production and use today. However, they are limited in their use within civil airspace due to the lack of mature sense-and-avoid technology and undefined methods for proving safety. These key requirements will not only enable military, civil, and eventually commercial objectives, they will have a direct impact in initial and operating costs. Therefore, to unlock the potential of UASs, it is required to develop affordable UAS solutions that can be safely and transparently integrated into non-segregated airspace.

It is important to realize that unmanned civil aviation is a revolution, not an evolution, as CONOPS and the necessary technology for flight in non-segregated airspace are not mature, and standards (MASPS or MOPS) do not exist. Therefore, UAS integration into non-segregated airspace will require the simultaneous development of CONOPS, technology and standards, and the involvement of all UAS stakeholders, that is, end users, industry, regulators, Air Navigation Service Providers (ANSPs), etc. from all over the world.

Two groups are leading the development of standards for safe and transparent UAS integration into non-segregated airspace: EUROCAE WG-73 in Europe; and RTCA SC-203 in the US. WG-73 will propose its developed standards to EASA and the European National Aviation Authorities (NAAs), whereas SC-203 will propose its developed standards to the FAA. Therefore, if WG-73 and SC-203 are not aligned, there is a risk that different standards are

developed on either side of the Atlantic. Consequently, UAS solutions will not be interoperable, and standards will be costlier and take longer to develop. This is not in the interest of any of the UAS stakeholders and especially the end users.

This paper will compare the activities of WG-73 and SC-203. The focus will be on Sense-And-Avoid (SAA) activities for Beyond Visual Line-Of-Sight (BVLOS) operations and will expand on the ATM environment, markets/CONOPS, and UAS safety objectives.

1: UAS Insertion Problems (ATM Environment)

In this section, we will discuss the UAS insertion problems that are currently being solved by WG-73 and SC-203. To put these UAS insertion problems into context, we will first describe the ATM environments in Europe and the US.

In its Annex 11 on ATS, ICAO established seven airspace classes. The airspace classes are denoted A through to G, and the services provided and flight requirements vary from one airspace class to another. Figure 1 summarizes the seven ICAO ATS airspace classes. It is interesting to note that all traffic in airspace classes A through to D is known to ATC (IFR and VFR traffic), since ATC clearance is required in order to enter these airspace classes. Furthermore, in airspace classes A through to C, separation of IFR traffic from all other traffic is provided by ATC (IFR and VFR). Thus, IFR traffic in airspace classes A through to C is only responsible for collision avoidance, since separation is provided by ATC. Finally, airspace classes A through to E are referred to as controlled airspace, since separation services are provided in these airspace classes, whereas airspace classes F and G are referred to as uncontrolled airspace, since no separation services are provided. Commercial Aviation typically operates in controlled airspace, whereas General Aviation typically operates in uncontrolled airspace.

5C5-2

Figure 1. ICAO ATS Airspace Classes

Initially, both WG-73 and SC-203 were trying to solve UAS insertion into all airspace classes. This proved to be a difficult problem, and more recently (over the last 2-3 years), a more tractable step-wise approach to UAS insertion has been adopted by both standardization groups. Figure 2 summarizes the first UAS insertion steps of WG-73 and SC-203, together with the ATM environments in Europe and the US. WG-73 is solving UAS insertion into airspace classes A, B and C (for IFR traffic en-route; terminal area operations are currently not considered). This is essentially a cooperative environment since Mode-S (ELS) transponders are mandated in airspace classes A, B and C. Thus, the thinking within WG-73 is that

UAS insertion into airspace classes A, B and C can be based on a cooperative SAA system that only provides collision avoidance. On the other hand, SC-203 is solving insertion into airspace classes A, E and G. This is a low traffic density environment since towered airports in the US are surrounded by airspace classes B, C and D. Since Mode-C transponders are only mandated in airspace class A and part of airspace class E, UAS insertion into airspace classes A, E and G will have to include a non-cooperative SAA system that provides both collision avoidance and self-separation. It is evident that the US is solving a more challenging first step that includes the European first step.

5C5-3

Figure 2. UAS ATM Insertion Scope

Both Europe and the US are in the process of modernizing ATM through SESAR and Next Gen respectively. Part of this modernization involves the deployment of ADS-B out, see Figure 2. In the US, the ADS-B out mandate has been published, will come into effect in 2020 (forward-fit and retro-fit) and applies to the same airspace where Mode-C transponders are mandated today. In Europe, the ADS-B out mandate is still a draft, is currently planned to come into effect in 2017 (forward-fit and retro-fit) and will probably apply to the same airspace where Mode-S (ELS) transponders are mandated today. It is important to realize that the ATM environment can have a significant impact on UAS insertion. ADS-B out has the potential to reduce the Size Weight and Power of the SAA system, and eliminate the need for non-cooperative intruder detection, if it was mandated in all airspace. Such a mandate could potentially save billions in development costs expected for non-cooperative surveillance. [1] However, given the uncertainty around ADS-B out in Europe, WG-73 is focused on fast insertion based on Mode-S, whereas SC-203 is considering both Mode-C and ADS-B out so that

standards are compatible in today’s and tomorrow’s airspace.

In summary, for its first UAS insertion step, WG-73 is exploiting ATC separation, Mode-S (the current ATM environment) and segregated airports, whereas SC-203 is exploiting low traffic density airspace, Mode-C and ADS-B out (the current and future ATM environments), and non-towered airports (airspace class G airports). In the following sections, we will explore what these differences mean in terms of UAS missions, SAA system high level requirements and technology, and UAS deployment timelines.

2: UAS Market Enabled (Operations) In this section, we will explore the UAS market

that could potentially be unlocked in the 2015-2020 timeframe by WG-73 and SC-203, if the groups’ schedules do not slip (see Section 6). The size of the UAS market will be defined by the intersection of UAS missions, regulations and solutions, i.e. the intersection of end users (demand), regulators (safe operations) and industry (supply). To enable a UAS

5C5-4

market, all three are required. In what follows, we will consider UAS missions and regulations. Before discussing the intersection of UAS missions and regulations, we will provide an overview of how each country structures its airspace as a function of flight level/ altitude.

Each country develops its own airspace classification from the seven ICAO ATS airspace classes. For example, the British and American airspace classifications are summarized in [2] and [3] respectively, and the UK does not use airspace class B, whereas the US does not use airspace class F. Each country then uses its own classification to structure the airspace above its territories, i.e. what airspace class at what flight level/ altitude and what airspace class surrounding its airports. For Europe this classification can be found in [4] and for the US in the FAA document referenced above. For en-route airspace, and for the larger European countries and

the US, this is summarized in Figure 3. As is evident, in Europe, there is variation from country to country, and EUROCONTROL is working to harmonize all airspace above Europe in order to facilitate cross-border flights. The dashed red lines in Figure 3 denote the airspace that is currently being addressed by WG-73 and SC-203. WG-73 is addressing upper-altitude airspace, whereas SC-203 is addressing all-altitude airspace. Consequently, the UAS missions that could potentially be enabled by WG-73 in the 2015-2020 timeframe are upper airspace missions and transition through non-segregated airspace (to get from one segregated area to another), whereas SC-203 could potentially enable all airspace missions. Since WG-73 is only working en-route operations (terminal area operations are currently not considered), a UAS would have to take-off, climb (cork-screw up), descend (cork-screw down) and land in segregated airspace.

Figure 3. Airspace Structure and Enabled Commercial Missions

5C5-5

Both standardization groups address the needs of the military that wants to be able to transition through non-segregated airspace (to get from one segregated area to another) and perform upper altitude ISR missions. However, there is a difference in the size of the non-military UAS market that is being addressed by the two groups. The non-military missions reported in Figure 3 are taken from Deliverable 1.2 of the INOUI project. The INOUI project was funded by the European Commission and explored UAS integration into the SES, see [5]. This is not an exhaustive list but certainly gives an indication of the plethora of non-military missions, most of which are ISR missions. Nuclear contamination, volcanic ash cloud and wildfire monitoring; border, infrastructure and event surveillance; and pipeline inspection are examples of non-military missions that could be successfully performed with a UAS in a green and cost effective manner. It is clear that SC-203 is trying to enable a larger non-military UAS market in the 2015-2020 timeframe than WG-73, as the non-military missions that need access to lower airspace are not addressed by WG-73.

3: Sense and Avoid Airspace safety includes many aspects which

contribute to safe operations at various points in a conflict timeline. These include strategic conflict management, separation provisions, and collision avoidance. Strategic conflict management includes airspace organization and management, demand and capacity balancing, and traffic synchronization. Separation provisions are executed by either the airspace user or a separation service provider such as ATC. Collision avoidance is always the responsibility of the airspace user. Sense and Avoid (SAA) includes aspects of and integration with separation provisions and has a responsibility to perform collision avoidance and is meant to fulfill the “see and avoid” aspect of 14 CFR 91.113, Right-of-Way Rules: Except Water Operations.

To better define the scope of SAA, SC-203 and WG-73 have adopted the SAA definitions and functions of the FAA SAA Workshop [6]. These definitions define SAA to encompass two high level functions: Self Separation (SS) and Collision Avoidance (CA). Self-Separation is intended to resolve any conflict early, so that a UAS remains “well clear” of other aircraft and avoids the need for

last-minute collision avoidance maneuvers. Collision Avoidance is a more drastic maneuver that is that is designed to prevent the aircraft from penetrating the NMAC volume. Figure 4 depicts the thresholds of each function.

Figure 4. Notional UAS Separation Concepts

The Self-Separation Threshold is the boundary around the UAS at which the self-separation function declares that action is needed to preclude a threat aircraft from penetrating the Collision Avoidance Threshold, thereby maintaining “well clear”. The Collision Avoidance Threshold is the boundary around the UAS at which the CA function declares that action is necessary to avoid a collision and prevent the threat from penetrating the fixed Collision Volume. Figure 4 depicts these boundaries which are notional and are not defined by fixed volumes but rather a time-to-collision, or tau-like value, that is dependent on the aircraft’s state, intruder geometry, UAS performance, and location and intent of other vehicles in the NAS. This boundary may be different for each intruder. The UA must be able to perform both functions against both cooperative and non-cooperative traffic.

While it is desired that any SAA system include both SS and CA capabilities, this may not always be necessary. Where ATC is providing air traffic separation services, SS may be disabled so that the SAA system doesn’t interfere with ATC defined separation distances.

While both SC-203 and WG-73 have recognized the importance of SAA and are trying to develop

5C5-6

standards to enable UAS operations in non-segregated airspace, each group is tackling the problem differently. WG-73 is focusing on cooperative airspace to take advantage of the separation services provided by ATC and existing equipage mandates, but this requires operations in potentially high-density airspace (A, B, C) and/or limits mission capabilities via segregated airspace for terminal operations and transitions through commercial airspace. SC-203 is focusing on low-density airspace (A, E, & G), but requires the UA to perform both SS and CA functions on cooperative and non-cooperative traffic. Both methodologies have their pro’s and con’s, but these differences in focus could lead to diverging separation and safety standards that could hinder the implementation of UAS globally.

Open issues of SAA include the quantification of separation thresholds (possibly including a separate Well Clear Threshold) and seamless ATM integration with regards to SS.

4: Safety Objectives and Analysis To fly in non-segregated airspace, all UAS must

be shown to be airworthy to conduct flight operations. In the US, UAS should be maintained and conform to the same airworthiness standards as defined for the 14 CFR parts under which UAS are intended to be operated. However, compliance with the “see and avoid” aspect of 14 CFR 91.113, Right-of-Way Rules: Except Water Operations, is a primary compliance issue in UAS operational approvals.

The FAA UAPO has released guidance documentation [7] directing Certificate of Waiver or Authorization (COA) applicants to provide system safety studies that support an “extremely improbable” determination for any UAS “see and avoid” strategy in lieu of visual observers. Unfortunately, there is no FAA established methodology or guidance for conducting UAS SAA safety analysis. As the number of COA applications continues to increase, a

common approach is required to reduce burden on both the approval agencies such as the FAA and with the entities tasked with preparing the data. Therefore the development of a quantifiable and repeatable methodology for performing UAS SAA safety analysis is essential.

Similarly, the early safety objective identified in EUROCAE WG-73 have focused on AMC 2x.1309 airworthiness standards [8] which determine the potential risk of harm to third parties (rate of catastrophes per flight hour). Under this methodology, SAA system would be held to the same standards as other flight critical systems such as flight controls or other safety systems such as TCAS. The regulations basically say that any failure conditions that would prevent continued safe flight and landing must be extremely improbable.

The FAA SAA Workshop Final Report [6] identified the three most common methods used for safety analysis: (1) Target Level of Safety (TLS) or Safety Target, (2) Relative Risk, and (3) Comparison against a reference system. Appendix 4 of the FAA SAA Workshop Final Report: Comparison of the Safety Methodology Approaches captures the outcome of the assessments determined by the workshop participants. The group concluded that the TLS approach is the most likely to succeed. This approach is a comprehensive analysis that is traceable and quantifies the total risk of the system. The TLS approach is also the most directly relevant to the FAA’s SMS process which will most likely be consulted for UAS certification. RTCA SC-203 has adopted this methodology for assessing UAS safety, though no specific target numbers have been defined to date. With SC-203’s adoption of the TLS safety objective, WG-73 has considered moving towards this safety objective as well.

Table 1 provides a brief description of the safety objectives that have been considered and which objectives are in favor in the standards groups.

5C5-7

Table 1. Safety Objectives

WG-73 SC-203 Safety Objective Note

�� 1309/airworthiness approach � Rate of catastrophes/ hr

�� �� Safety Target – TLS Approach � Models end-to-end system performance

� Rate of MAC/FH

Relative Risk / ACAS Approach � Safety of operations with SAA vs. safety of operations without SAA

� Risk ratio

Comparative Assessment/ELOS Approach

� New system is significantly similar to an existing system

� Performance comparison

.

4.1: TLS Allocations TLS is typically defined as the maximum-

acceptable probability of a mid-air collision per flight-hour (MAC/FH). The TLS accounts for all of the elements that combine to give a total probability of collision. These include factors such as rate of encounter which is a function of airspace class and strategic conflict management, rate of ATC errors, probability of SAA failures, and the statistical probability of a collision given a violation of the NMAC volume of 500 ft horizontally and +/- 100 ft vertically.

While there is great debate about how to define overall UAS TLS numbers, it is thought that there will be at least two TLS numbers essentially based on airspace where transponders are required. In the US national airspace, transponders are required in Class A, B, C, E above 10,000 ft MSL, E above A, B, and C airspace, and the Mode C Veil. Transponders are not required in Class D, G, and E below 10,000 ft MSL (excluding E above A, B, C, and Mode C Veil). The “Manual on Airspace Planning Methodology for the Determination of Separation Minima” recommend that the value of 5x10-9 MAC/FH be used as TLS in transponder-required airspace. A conservative value of TLS based statistics of no-transponder airspace is 1x10-7 MAC/FH. There are others that may value a lowest-common-denominator situation, defining TLS of 10-9 against a non-cooperative intruder in uncontrolled airspace, but this would imply that the system performs significantly better against cooperative intruders in Class A

airspace and place an undue burden on the system. Still others may propose more than two TLS values which may coordinate with airspace class or even altitude bands within an airspace class. For the purpose of this paper, we will assume there are two TLS values that correspond to mandatory verses non-mandatory transponder equipped airspace, 5x10-9 MAC/FH and 1x10-7 MAC/FH respectively.

For demonstration purposes, assume statistics can determine rate of encounters for the respective transponder equipage airspace and the rate of ATC errors that induce an encounter. Numbers shown in Figures 5 and 6 for Rate of Encounter and multiplier based on ATC error rates are based on discussions at WG-73 and SC-203 plenaries and [9-11]. The probability of a mid-air collision (MAC) given a near mid-air collision (NMAC) is defined in [12]. Performance of the SAA system can now be computed given the TLS values and other contributing MAC factors and are consistent with [13 and 14]. Figure 5 and Figure 6 show these SAA allocations for transponder and no-transponder airspace respectively.

Please be advised that the numbers represented here are for example purposes only and have not been defined or approved of by any standardization body such as WG-73 or SC-203.

There are also additional methodologies to allocating or computing TLS numbers. This high-level fault tree/allocation serves as an example.

5C5-8

Figure 5. Example TLS Allocation for Classes A, B, C, and Subset of E

Figure 6. Example TLS Allocation for Classes D, a Subset of E, F, and G

In both cases, the resulting allocation for probability of failure of the SAA system is 10-3 NMAC’s per encounter. This is the result of the differing statistics of rate of encounters in the different airspaces and the differing UAS TLS values.

Note that this allocation method is only valid so long as the terms of the allocation are sufficiently independent. For example, if ATC/Separation Assurance or SAA maneuvering (correctly or incorrectly) results in higher rates of encounters, the systems/functions identified in the allocations need to be broken down further until there is no strong dependency between any of the terms in the TLS allocation process.

5: SAA System Technology Using the TLS approach, it was shown that both

the WG-73 and SC-203 UAS insertion problems could lead to the same SAA system high level requirement, that is, a probability of SAA system

failure of the order of 10-3 NMAC/encounter. So, exploiting either ATC separation (the WG-73 first step) or low traffic density airspace (the SC-203 first step) leads to the same high level SAA TLS performance requirement. In this section, using simplified analysis, we will flow down the SAA system high level requirement to non-cooperative intruder detection, a SAA system sub-function. The reason for focusing on non-cooperative intruder detection is because it is the most technologically challenging and expensive part of SAA. Questions like “Is it needed in airspace classes A, B and C?” and “Are radars the only solution or would EO/ IR cameras be sufficient for VMC operations?” continue to be the subject of much debate within both standardization groups.

Consider Figure 7. The left of Figure 4 lists the SAA system sub-functions proposed in the FAA sponsored SAA workshop final report [6]. Similar sub-functions (but not identical) have been proposed in the NATO Industrial Advisory Group SG-134 main report [15]. The sub-functions are: detect intruders; track intruders; evaluate collision potential; prioritize collision threats; declare action is required; determine an avoidance maneuver; command maneuver; and execute maneuver. We will focus on a missed detection of the detect intruders sub-function and ignore failures of the other sub-functions, e.g. a fight controls malfunction in the execute maneuver sub-function. Furthermore, we will assume that all missed detections lead to a NMAC. Note that since we are interested in trends and not absolute values, the (three) simplifying assumptions do not impact our analysis.

In the right part of Figure 7, the detect intruders sub-function has been expanded to show the cooperative and non-cooperative sensors. The SAA system high level requirement is flowed down to the detect intruders sub-function as a requirement of 10-3 missed detections/encounter. This requirement is further flowed down to the non-cooperative sensor by assuming a cooperative sensor performance of 10-2 missed detections/encounter. The assumed cooperative sensor performance is conservative (10-3 could be achievable) and has been taken from the TCAS MOPS [16]. The graph shows the required non-cooperative sensor performance as a function of the percentage of non-cooperative intruders. The required performance ranges between 10-1 missed detections/encounter in a cooperative environment to

5C5-9

10-3 missed detections/encounter in a 100% non-cooperative environment. Note that in a cooperative environment, improving cooperative sensor

performance would eventually eliminate the need for a non-cooperative sensor.

Figure 7. Example of Subsystem Performance Allocation Based on SAA TLS

Thus, as expected, the required non-cooperative sensor performance depends on the percentage of non-cooperative intruders (i.e. the airspace class in which the UAS is flying) and the cooperative sensor performance. If the regulators mandate the highest performance non-cooperative sensor, i.e. 10-3 in this example, this would lead to an expensive SAA system that is an overdesign for some airspace classes (see Figure 7). Rather than mandate the sensor type and performance, we recommend that the regulators mandate the SAA system high level requirements and accept different SAA system solutions for different operational environments, e.g. different solutions for different airspace classes and different meteorological conditions (just like for manned aviation). If not, the non-military UAS market, that is purely cost driven, will never take off. Furthermore, such an approach would naturally account for any technological breakthroughs in

sensors and future upgrades in the ATM environment, e.g. an ADS-B out mandate for all airspace or a ground-based SAA extension of ATC.

6: Timelines and Roadmaps When developing UAS standards, coordination

needs to happen not just between SC-203 and WG-73, but with manned aviation standards development as well, particularly SESAR and NextGen. EU SESAR and US NextGen programs are comprehensive overhauls of the airspace to make air travel more convenient and dependable, while ensuring safety, security, and convenience in the presence of expected growths in travel volumes. In a broad sense, UAS standards development needs to be aware of the expected changes, but more specifically, some of the technologies being developed for UAS are also being developed for manned aviation within

5C5-10

SESAR and NextGen such as aircraft separation standards and collision avoidance advances.

Figure 8 depicts a simplified roadmap of UAS and manned aviation standards and technology development. Currently, there is some level of coordination between SC-203 and WG-73 MASPS development; agreement to use ED78A/DO-260 was a good start. Possibly problematic is the EU focus on

cooperative surveillance solutions and UAS Collision Avoidance with reliance on ATC to solve separation problems verses the US focus on both Self Separation and Collision Avoidance to both cooperative and non-cooperative traffic while taking advantage of low-density airspace. In both situations, the reduction in scope could lead to diverging standards or technology solutions.

Figure 8. UAS and Manned Aviation Standards and Technology Development Roadmap

There is also loose correlation between UAS and manned aviation technology development and enablers within SESAR and NextGen [17]. Both programs are tackling Self Separation and advances to current collision avoidance technologies such as TCAS. However, there needs to be a stronger correlation and cooperation between all the standards developing bodies, particularly with manned aviation, lest different or incompatible standards are developed. If this were to occur, industry investment in UAS technology would wane and the inclusion of

UAS into national airspace would slip further into the future.

Not featured here are similar differences in the EDA’s approach to military SAA development via the MIDCAS program verses the US DOD’s SAA Roadmap and DOD SAA R&D spending which echo the differences between WG-73 and SC-203, respectively.

5C5-11

Conclusions It is the authors’ point-of-view that current UAS

standards and technology development in the EU and US are currently aligned, but could quickly become incompatible. The timelines of WG-73 and SC-203 are correlated, but WG-73 is really considering a subset of the SC-203 scenario by focusing on cooperative traffic and collision avoidance and pushing the responsibility of separation onto ATS. SC-203 and US developments are taking advantage of low-density airspace, but are tackling the full spectrum of the problem: both cooperative and non-cooperative traffic with full SAA functionality (Self Separation and Collision Avoidance). Major hurdles exist in both standards bodies, particularly the definitions of the Self Separation and Collision Avoidance thresholds, “well clear,” and definitive safety objectives. If care is not taken and different thresholds or safety objectives are chosen, the timeline for broad UAS implementation will likely move further into the future and industry investment is likely to wane.

Affordability is also an issue that should not be readily dismissed in the development of standards. While airspace safety is of the utmost of importance, the safety objectives must not be defined such that it stifles airspace access by making suitable UAS system unaffordable. Additional effort must also be made of exploit and align with manned aviation ATM modernization technologies such as SESAR and NextGen where technologies such as Self Separation are also being developed.

An opportunity of closer Europe/US and UAS/manned-aviation alignment exists. If we don’t seize it, industrial investment will decline, implementation timelines will move further into the future, and solutions will become less affordable resulting in a smaller market

Disclaimer This paper represents the opinions and

observations of the authors. Reference to TLS numbers are approximations made by the authors given current statistics and proposals in standards discussions. UAS TLS numbers have not been finalized by any standards group that the authors are aware of.

Additionally, UAS standards are ongoing and in a state of flux. It is possible that decisions could be made before the date of publication that could change this analysis.

References [1] Lacher, Zeitlin, et. al., 2010, “Airspace Integration Alternatives for Unmanned Aircraft”, Presented at AUVSI’s Unmanned Systems Asia-Pacific 2010, MITRE

[2] http://www.caa.co.uk/docs/64/200890108ATSAirspaceClassificationV3.pdf

[3] FAA, Instrument Flying Handbook, Chapter 8, http://www.faa.gov/library/manuals/aviation/instrument_flying_handbook/media/faa-h-8083-15a%20-%20chapter%2008.pdf

[4] Eurocontrol, http://www.eurocontrol.int/airspace/gallery/content/public/Classification%20mid%20april%202007.pdf

[5]Innovative Operational UAS Integration, INOUI, http://www.inoui.isdefe.es/

[6] FAA Sponsored “Sense and Avoid” Workshop, 2009, “Sensor and Avoid (SAA) for Unmanned Aircraft Systems (UAS)”,

[7] FAA UAPO AIR-160, 2008, “Interim Operational Approval Guidance Unmanned Aircraft Systems Operations in the U.S. National Airspace System”, http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/systemops/aaim/organizations/uas/coa/faq/media/uas_guidance08-01.pdf

[8] Kallevig, Tore B., 2011, EUROCAE WG-73 on Unmanned Aircraft Systems”, Blyenburgh & Co ©, http://www.uvs-international.org/index.php?option=com_docman&task=doc_view&gid=1742&Itemid=25

[9] Baily, Schrooeder, Pounds, 2005, “The Air Traffic Control Operational Errors Severity Index: an Initial Evaluation”, DOT/FAA/AM-05/5, http://www.faa.gov/library/reports/medical/oamtechreports/2000s/media/0505.pdf

[10] ESARR 4 – Risk Assessment and Mitigation in ATM, http://www.eurocontrol.int/src/public/standard_page/esarr4.html

5C5-12

[11] Tomasello, Filippo, 2010, “Safety Requirements for UAS Detect and Avoid (D&A)” presentation, EASA

[12] RTCA DO 298, 2005, “Safety Analysis of Proposed Change to TCAS RA Reversal Logic”

[13] Graham and Orr, 1970, “Separation of Air Traffic by Visual Means: An Estimate of the Effectiveness of the See-and-Avoid Doctrine”

[14] Andrews, J.W., 1991, “Unalerted Air-to-Air Visual Acquisition”, MIT/Lincoln Laboratory

[15] NATO NIAG SG-134, 2010, “Sense and Avoid Annexes – Detailed Teams Reports”

[16] RTCA DO 185A, 1997, “Minimum Operational Performance Standards for Traffic Alert and Collision Avoidance System II (TCAS II) Airborne Equipment”, RTCA

[17] FAA, 2011, “NextGen Implementation Plan”, Appendix A

30th Digital Avionics Systems Conference

October 16-20, 2011