Requirements, Issues, and Challenges for Senseand Avoid in Unmanned Aircraft Systems

Xavier Prats,∗ Luis Delgado,† Jorge Ramírez,‡ Pablo Royo,§ and Enric Pastor¶

Technical University of Catalonia, 08860 Castelldefels, Spain

DOI: 10.2514/1.C031606

The sense and avoid capability is one of the greatest challenges that has to be addressed to safely integrate

unmanned aircraft systems into civil and nonsegregated airspace. This paper gives a review of existing regulations,

recommended practices, and standards in sense and avoid for unmanned aircraft systems. Gaps and issues are

identified, as are the different factors that are likely to affect actual sense and avoid requirements. It is found that the

operational environment (flight altitude, meteorological conditions, and class of airspace) plays an important role

when determining the type of flying hazards that the unmanned aircraft system might encounter. In addition, the

automation level and the data-link architecture of the unmanned aircraft system are key factors that will definitely

determine the sense and avoid system requirements. Tactical unmanned aircraft, performing similar missions to

general aviation, are found to be the most challenging systems from an sense and avoid point of view, and further

research and development efforts are still needed before their seamless integration into nonsegregated airspace.

I. Introduction

IN CIVIL aviation, the onboard flight crew are responsible fordetecting and identifying threatening objects or terrain, performing

actions to safely separate their own aircraft from such objects, andexecuting avoidance maneuvers as a last-resort mechanism. Thisprocess is usually referred to as the see-and-avoid capability.Unmanned aircraft systems (UASs) donot have theflight crewonboardand, therefore, the see-and-avoid capability is essentially lost. YetUAScan be equipped with several sensors and systems that can replace thisextremely important functionality, and themore appropriate term senseand avoid (S&A) is used for UAS. In [1], S&A is defined as “theprocess of determining the presence of potential collision threats, andmaneuvering clear of them; the automated equivalent to the phrase “seeand avoid“ for the pilot of a manned aircraft.”

Behind this apparently simple definition, several issues still lingerwhen trying to apply current regulations (developed for mannedaviation) to UAS. Significant operational differences exist betweenUASandmanned aircraft, which have to be analyzed beforeUAScansafely be integrated into civil and nonsegregated airspace [2–5]. Inthis context, an excellent and comprehensive review on existingmanned and unmanned regulations worldwide, along with valuablethoughts and recommendations on this UAS integration, is given in[6]. UAS operations in civil airspace are required to provide at leastthe same level of safety as that of manned aviation. Even if severaldemonstration initiatives have already been conducted [7–9], at

present it is still unclear how to demonstrate or achieve thisequivalence. Independent of the methodology, this minimum safetyrate will need to be translated into system reliability requirements inthe near future. For example, in [10], a midair collision riskassessment is presented, aiming at estimating the number of expectedcollisions per hour of flight. This study would be useful whenestablishing the minimum performance requirements of S&Asystems in different scenarios (see also [6]).

S&A forUAS is an ongoing subject of discussion and debate in theprincipal regulatory and aviation safety agencies (for instance, see[11–13]) and the subject of study for the principal standardizationbodies [1,14,15]. Based on the published information so far, thispaper highlights the possible requirements that might be demandedof an S&A system, classifies them as a function of the UAScharacteristics, and identifies the different factors that may affect theactual implementation of these requirements. Thence, the purpose ofthis paper is twofold. First, to give a review of existing regulations,recommended practices, standards, and relevant developments intechnology for S&A, while identifying gaps and issues. Second,because a global S&A solution for UAS does not exist, to stress thefact that its actual implementation will depend on several factors,which are identified and discussed in this paper.

The paper is organized as follows: The next section presents thegeneral requirements expected of an S&A system and the particularconditions affecting these requirements are identified in Sec. III.Section IV then discusses how S&A requirements may change as afunction of certainUAScategories, andfinally the paper is concludedin Sec. V.

II. General Requirements for Unmanned AircraftSystem Sense and Avoid

In civil aviation, several mechanisms are present to minimize theprobability of collision with other aircraft, objects, or terrain.Generally speaking, they are categorized into two mainfunctionalities: separation assurance and collision avoidance.Separation assurance aims at keeping minimum distances betweenthe aircraft and potential intruders, considering lateral and verticalplanes. A loss of separation is considered a serious issue, and ideallyit would never occur. Nevertheless, as a last-resort maneuver, acollision-avoidance functionality can prevent an imminent collisionin case of loss of separation.

In this section, a short summary of the principal characteristics ofthese functionalities and then systems involved is given. We thendiscuss the possible requirements for an S&A functionality for UAS,pointing out the main issues that are still open in the regulatory

Received 29 July 2011; revision received 3 December 2011; accepted forpublication 6 December 2011. Copyright © 2011 by Xavier Prats iMenéndez. Published by the American Institute of Aeronautics andAstronautics, Inc., with permission. Copies of this paper may be made forpersonal or internal use, on condition that the copier pay the $10.00 per-copyfee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers,MA 01923; include the code 0021-8669/12 and $10.00 in correspondencewith the CCC.

∗Assistant Professor, Telecommunication and Aerospace EngineeringSchool of Castelldefels, Office C3-104, Avinguda Esteve Terradas, 5;xavier.prats@upc.edu. Member AIAA.

†Assistant Professor, Telecommunication and Aerospace EngineeringSchool of Castelldefels, Office C3-120, Avinguda Esteve Terradas, 5;luis.delgado@upc.edu. Member AIAA.

‡Lecturer, Telecommunication and Aerospace Engineering School ofCastelldefels, Office C3-119, Avinguda Esteve Terradas, 5; jorge.ramirez@upc.edu.

§Lecturer, Computer Architecture Department, Office C4-010, AvingudaEsteve Terradas, 7. Member AIAA.

¶Associate Professor, Computer Architecture Department, Office C4-002,Avinguda Esteve Terradas, 7; enric@ac.upc.edu. Member AIAA.

JOURNAL OF AIRCRAFT

Vol. 49, No. 3, May–June 2012

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context. Some final requirements have not yet been adopted by anyregulation, but some organizations have already issued documentsand published preliminary reports. This section summarizes theinformation available at the time of writing.

A. Separation Assurance and Collision Avoidance Functionalities

Minimum separation values depend on several factors such as theairspace class, the flight rules, the flight phase, the air traffic control(ATC) surveillance means (if any), the performance of the onboardnavigation systems, etc. Roughly speaking, lateral minimumseparation between aircraft can range from3 nmiles in terminal areaswithATC radar separation services to up to 60 nmiles for two aircraftat the same altitude on a North Atlantic track. Yet, in noncontrolledairspace, there is no exact value of minimum separation, and aircraftmust remain well clear of each other.

If separation is lost, a collision avoidance functionality can avoid acollision. In some cases, this functionality is achieved cooperativelybetween the two aircraft involved. This means that the conflictingaircraft use common systems and procedures that have been designedto jointly detect an imminent collision with enough time to react andavoid it. Not all aircraft are equipped with these systems, however,and neither are other flying objects, such as birds, or terrain. Thus,whenever visibility conditions permit, every pilot inmanned aviationis expected to see and avoid these hazards. This means that, in theseconditions, the flight crew is ultimately responsible for ensuringaircraft safety by preventing and avoiding collisions.

Figure 1 depicts the different existing mechanisms in separation(self-separation as well as air traffic management and procedures)and collision avoidance (noncooperative and cooperative), which aresummarized as follows:

1) Noncooperative collision avoidance is the lowest levelmechanism to prevent an imminent collisionwith any type of aircraft,obstacle or terrain. In manned aviation, this relies entirely on theability of crew members to see and avoid. Conversely, for UAS thisfunctionality must be assumed by an S&A system (or group ofsystems).

2) Cooperative collision avoidance includes all the systems andprocedures between cooperative aircraft that can avoid imminentcollisions. The standard for these airborne collision avoidancesystems (ACAS) is specified by ICAO in [16]. ACAS are based onsecondary surveillance radar (SSR) transponder signals receivedonboard and operating independently from ground-based equip-ment. They provide advice to the pilot on potential conflictingaircraft that must also be equipped with SSR transponders.

The traffic collision avoidance system (TCAS) is a particularACAS implementation widely used in commercial aviation. ACAS/TCAS-I only provide traffic alerts (TAs) when a collision threat isdetected. In addition to TAs, ACAS/TCAS-II provide the pilot withResolution Advisories (RA), proposing an avoidance maneuver inthe vertical plane. Future ACAS versions will also incorporatehorizontal maneuvers in the resolution advisories [16,17].

3) Self-separation mechanisms are the lowest layer that canguarantee a minimum safe separation distance. In manned aviation,

see-and-avoid mechanisms are again widely used for this purpose,especially in noncontrolled airspace under visual meteorologicalconditions. Besides this, self separation can be significantlyimproved with different kinds of airborne separation assistancesystems (ASAS), which consist of highly automated systems thatpresent the pilot with information to enhance their situationalawareness. Moreover, ASAS can even provide a set of explicitsolutions to guarantee separation with other aircraft while reducingthe workload of the crew.

The majority of ASAS applications are based on the automaticdependent surveillance (ADS) concept (where each aircraft transmitsits position and receives the positions transmitted by other aircraft orvehicles using the same system) and some sort of cockpit displaytraffic information. Thus, these kinds of applications are expected todramatically enhance the situational awareness and, consequently,safety levels in noncontrolled airspace, although they are also aimedat delegating separation tasks from controllers to pilots in somecontrolled airspace [18].

4) Air traffic management (ATM) consists of a wide set ofmechanisms and services aimed at providing the maximum capacityto airspace and airports to accommodate demand while ensuring thehigh levels of safety. ATM can be divided into three categories:airspace management, air traffic flow management, and air trafficservices (ATS). The latter includes alert services, flight informationservices, and air traffic control (ATC). The availability of theseservices dependsmainly on the flight rules∗∗ and class of airspace theaircraft is in.

5) Operational procedures are the outermost layer in assuringseparation with other aircraft (along with known obstacles andterrain). Here, we find not only navigation procedures but alsoaircraft operating procedures.

Of all these layers, the noncooperative collision avoidancefunction is the most challenging one to implement in UAS. Theremaining layers are, to some extent, more likely to be easilyintegrated into UAS with the currently available technology andregulations. Thus, S&A is one of the main issues that must beaddressed before integrating UAS into civil and nonsegregatedairspace.

The expected requirements for the collision avoidancefunctionality can be summarized as follows:

1) Detect and avoid midair collisions with other flying trafficaccording to the right-of-way rules.

2) Detect and avoid other flying objects (such as birds).3) Detect and avoid ground vehicles (when maneuvering on

ground).4) Detect and avoid terrain and other obstacles (such as buildings

or power lines).5) Avoid hazardous weather.It is worth noting here the difference between an aircraft

functionality (such as separation assurance or collision avoidance)and the different mechanisms, systems, or group of systems that aredesigned (and certified) to provide the aircraft with such afunctionality. In this paper, we use the term S&A system to refer tothe generic system able to provide the collision avoidancefunctionality. Therefore, by S&A system, we can refer not only to asingle set of sensors or systems but to a wide range of devices andoperational procedures. This could include systems speciallydesigned to achieve the collision avoidance functionality or otherUAS systems that, even if they are not designed specifically forcollision avoidance purposes, have an impact on this functionality(such as the data-link system of the UAS). Of all these requirements,detecting and avoiding midair collisions with other traffic is the oneusually focused on when considering S&A technologies [12], and it

Fig. 1 Separation and collision avoidance mechanisms.

∗∗A civil aircraft can fly under visual flight rules (VFR) or underinstrumental flight rules (IFR). VFR operations are based on visual cues thatthe pilot takes from outside the cockpit, not only for aviating the aircraft butalso for navigating and avoiding collisions with other aircraft, obstacles andterrain. Conversely, IFR flights rely on several flight instruments for aviatingand navigating the aircraft and, usually, separation is assured by an ATCservice

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is the most challenging requirement (especially for noncooperativetraffic).

Other requirements, such as weather and terrain avoidance, can beperformed onboard the unmanned aircraft (UA) by using the sametechnologies already available for manned aircraft, with minormodifications in system operations, such as the terrain avoidancewarning system or the ground proximity warning system. Thesesystems are advisory in nature, however, and as we have alreadyobserved, with the resolution advisories generated by ACAS-IIsystems, the onboard pilot is ultimately responsible for executing anavoidance maneuver. Therefore, existing standards will also need tobe updated to address remote pilot operations and/or UAS autom-ation to avoid terrain [15].

Even if an S&A system must operate autonomously andindependently of the ATM system or any other means of UASseparation provision [13], it is clear that this system may alsocontribute to achieving the separation assurance functionality (andvice versa) by maintaining spacings or sequencing the aircraft, asdone visually in manned aviation.

As mentioned previously, the particular mechanisms available foreach layer depicted in Fig. 1 depend on several factors, such as thetype of aircraft, airspace, meteorological conditions, flight rules, etc.For example, in noncontrolled airspace, the ATM layer is hardlypresent. In instrument meteorological conditions (IMC), the abilityto see and avoid will be drastically reduced for manned aircraft; self-separation mechanisms will undoubtedly be different depending onwhether ADS is available for all the aircraft. The ultimate goal ofseparation and collision-avoidance functionalities is to guaranteethat the probability of collision is below a certain target level forsafety. Thus, a tradeoff must exist between the layers depicted inFig. 1 and, if one protection layer is enhanced, performancerequirements of another layer could be reduced. For example,manned aircraft in IMC have little chance of being able to performsee-and-avoid functions, but they are likely to operate in anenvironment where the ATM and procedural layers are thick enoughto compensate for this lack of performance in noncooperativecollision avoidance. This could also be valid for UAS, by enhancingthe performance of self-separation mechanisms and, as aconsequence, reducing the requirements for S&A systems.

Furthermore, other considerations specific to UAS operationsexist: the automation level of the UAS (autonomous, automated, orremotely controlled), for instance, the type of communications relaywith the control station, or even the presence of UAS operators in theairfield of operations. Moreover, flights over populated areas alsoraise safety issues as minimum safety figures are usually derivedfrom the number of fatalities that an accident may cause [6,19].

B. Sense Requirements

Sense functionalities, widely speaking, include the detection of allthe external hazards that might affect a givenflight. Some basic senseparameters that will have to be considered in the design phase of anS&A system are 1) the detection range of hazardous objects, whichmust allow the following avoidance maneuver to be executed withsufficient time to result in theminimum requiredmiss distance; 2) thefield of regard, defined as the area capable of being perceived ormonitored by a sensor and which must demonstrate that the S&Asystem meets the right-of-way basic rules; and 3) other parameterssuch as measurement accuracy, reliability, and update rate [15].

Concerning the field of regard, right-of-way rules state that pilotsmust avoid all objects, with the exception of overtaking traffic.According to [20], a horizontal azimuth angle of �110 deg off theaircraft nose is recommended for visual scanning inmanned aviation.The same values are expected to be applied to UA sense systems.Furthermore, [1] proposes an angle of elevation of�15 deg for UASsense systems.

The sense system must detect cooperative and noncooperativetraffic and accommodate UAS operations in different flight modesand airspace classes. The system might rely, however, in part onhuman intervention and the communications latency is an importantfactor to be assessed [14]. The detection of a collision threat must be

at a minimum range, allowing a resolution maneuver that keeps bothaircraft well clear. This minimum detection distance will greatlydepend on the performance of both aircraft (such as the cruise speed,turn rate, and climb or descent rates) as well as in the definition of thewell clear term, which is discussed in the next section. Furthermore,this detection should be possible in all weather conditions the UAmay encounter and even in the case of loss of direct command,control, or communications with the command ground station. Thus,it is of paramount importance to consider all these factors whendesigning the sense subsystem for the UAS.

The on-time detection of hazardous flying objects is a verychallenging feature for the sense subsystem. Different techniquescan be used to fulfill this objective, and they are the subject ofintensive research. In [10], a classification of technologies thatwould be able to detect traffic is proposed, resulting in eight differentcategories, including radar sensors (for instance, see [21–24]),optical sensors along with image processing algorithms (forinstance, see [25–28]), or even external visual surveillance means(either by ground observers or chase planes). Furthermore, multi-sensor-based systems are also being developed, combining severaltechnologies that aim to detect all kinds of traffic, in all weatherconditions [29–31].

Sensor technologies aim to meet or even exceed the performanceof current human visual traffic detection. In 2003, NASA equipped aUAwith a radar system able to detect noncooperative targets and atraffic advisory system to detect cooperative ones. With this UA andsome surrounding traffic, flight tests were carried out [32,33].Human visual detection performances were also analyzed to becompared with the UAS ones. As reported in [10], only the trafficadvisory systemwas sufficient for all encounter scenarios.Moreover,further research showed that the human eye was inadequate to detectand prevent collisions in several situations and even limited sensorsperform better than the human eye [32].

The difficulty of detecting other flying objects will depend on thenature of these objects themselves. Nevertheless, not all possiblehazardous objects are present in all situations and therefore, if UASoperations are restricted to certain types of conditions (such asaltitudes or airspace classes), the sense requirements need to beadapted to the type of objects that might be encountered in thosesituations. In this context, a definition of the attributes of thesepotential threats becomes extremely important when developing asense system. In the work undertaken in [13], an exhaustive analysiscategorizes all possible flying objects that may represent a threat ofcollision. Seventeen different categories are proposed, ranging fromradio-controlled aircraft, parachutists, kites, and fauna to all typesand sizes of aircraft. For each type of object, it is noted in whichconditions these objects are unlikely to be encountered, for exampleabove certain altitudes, in certain weather conditions, or in differentairspace classes.

Based on the object categorization already proposed in [13], asightly new taxonomy is proposed in this paper and summarized inTable 1. Here, all flying objects have been grouped into two maincategories: objects that, in the worst case, might need optical meansto be detected and objects that can always be detected by nonopticaltechniques. Optical techniques are those based on visible and/ornear-visible (ultraviolet/infrared) signals, such as conventionalvideo, lidar, or thermal images. These techniques, in general,perform very badly in IMC conditions. On the other hand, nonopticaltechniques are based on radio-frequency electromagnetic signals andare affected, to a limited extent, by bad weather. These techniquesmay include primary and/or secondary radar technology, eitherground-based or onboard systems, formed by either standalone ormultilateration architectures. Depending on aircraft skin andillumination conditions, very light aircraft can be detected by radar ata greater distance than an optical sensor. Moreover, some aircraftappearing in the optical category (such as hot air balloons or verylight aircraft) could eventually be equipped with SSR transponders.Yet this radar detection or level of cooperativeness cannot beguaranteed in all cases, and consequently we have considered theseaircraft in the category of aircraft that require optical means to bedetected (in the worst case scenario).

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C. Avoidance Requirements

After a collision threat has been sensed, the appropriate avoidancemaneuver must be identified and executed, taking into account that itmust be compatible with the performance and flight envelope of theUA. These avoidance or resolution maneuvers may include one ormore of the following changes in the flight trajectory: airspeed,altitude, or heading. If, as a consequence of an avoidance maneuver,the flight path deviates from an ATC clearance or instruction, it mustbe reported to the ATC as soon as possible. Moreover, after theconflict is solved, subsequent maneuvers must return the aircraft tothe original flight plan or to a newly assigned flight path, while beingcompliant with the right-of-way rules. Over the last decades, a largenumber of methods have been proposed to automate air trafficconflict detection and resolution [34], and extensive safety studiesare needed before these systems can be certified for UAS [35,36].

The most basic requirement for the avoidance maneuver is toperform it in such a way that the distance from the intruder aircraft orobject is equal to or greater than a minimum required miss distance.Current manned regulations state that the aircraft must remain wellclear of the intruder but no explicit distances are given; for instance,see the American Federal Aviation Regulation (FAR) Sec. 91.113[37]. Yet it is generally and implicitly understood that the minimummiss distance should be at least 500 ft in all directions [38]. However,as reported in [39], the industry itself regards 500 ft of lateralseparation as the worst-case minimum distance for S&A. Quotingthis document, “the application of 500 ft horizontal separation couldgenerate a heightened sense of collision risk [and therefore, it isproposed] an increase in horizontal separation to 0.5 nmiles, [which]would reduce this perception and also the collision risk itself. [. . .]These minima would only apply away from aerodromes”.

Although, despite the above considerations, in [12] the term wellclear is also used when referring to separation functionalities and notcollision avoidance ones; it is defined as the state in which twoaircraft are separated in such away that they do not initiate a collisionavoidance maneuver. Therefore, according to this last definition, thiswell clear boundary would vary as a function of the UA and intruderperformance, conflict geometry, closure rates, and relativeaccelerations. However, this boundary is still a subject of ongoingresearch. For example, in [40], a new approach is proposed to treatwell clear as a separation standard, thus posing it as a relative statebetween aircraft where the risk of collision first reaches anunacceptable level. In this way, an analytically derived boundary forwell clear is obtained.

Nevertheless, besides the actual value of this minimum missdistance or boundary, special consideration should be given to

collaborative aircraft equipped with ACAS, because the avoidancesubsystem safety analysis must show compatibility with themaneuvers executed by existing ACAS-II systems. Nowadays, allturbine-engined airplanes of a maximum certified takeoff mass inexcess of 5700 kg, or authorized to carry more than 19 passengersmust be equipped with ACAS-II [41]. In this context, coordinatedmaneuvers can range from complex full four-dimensionalcoordinated maneuvers to only basic heading or altitude changesin the horizontal and vertical planes, respectively. ICAO alsorecommends equipping all aircraft with such a system. Flight crewprocedures for the operation of ACAS are found in [42], whileprocedures regarding the provision of related ATS are describedin [43].

The Minimum Operational Performance Standards (MOPS) forTCAS-II are found in the Radio Technical Commission forAeronautics (RTCA) document Do-185A [44] and could beapplicable, to some extent, to UAS. For instance, and as [15] alreadypoints out, TCAS-II assumes typical transport category aircraftperformance for collision avoidance and resolution advisories (RA)algorithms, whereas many UAS may not be capable of the sameperformance characteristics. Moreover, RA are executed by pilots inmanned aircraft and, if RA are executed autonomously by a UAsystem, this increases the safety requirements on the system.Conversely, if a UA operator executes RA, issues of data link latencyand reliability must be addressed.

Unusual UAS performance (if compared to transport categoryaircraft) must also be assessed from the ATC point of view, becausecurrent ATC practices and training are based on current existingaircraft. Therefore, UA performancewill have to be included in ATChandbooks to be able to accommodate UA and provide safe andefficient separation and traffic information services. It should benoted that predicted UAS S&A systems will also support self-separation functions and, consequently, some responsibility couldeventually be shifted from ATC to the UAS (as is also likely inmanned aviation in the near future [18]). On a research level, severalinitiatives are being investigated, aimed at considering theparticularities of UAS when designing collision avoidancealgorithms (for instance, see [45–50]).

Finally, avoidance means must also be designed to comply withvisibility and cloud clearance criteria (with specific requirementsdepending mainly on the airspace class) for the UA to be seen byother aircraft and therefore comply with the flight rules. BesidesS&A requirements, severe weather could result in damage to theUAS and affect its airworthiness and must therefore be detected andavoided.

Table 1 Flying objects taxonomy (based on [13])

ID Characteristics

Optical sensor

F Fauna: birds the size of a goose or larger, which do not generally fly in IMC nor above 1000 ft above ground level (AGL). However, migrating birds canbe encountered at higher altitudes, typically between 5000 and 7000 ft AGL and often at specific seasonal periods and in specific locations. Generally,the greater the height above the ground, the less likely it is that birds will be encountered.

K Kites and tethered balloons: both the object itself and the cable connecting them to the ground. In general, operations above 400 ft should be notified byNOTAM.

B Hot air balloons: do not operate in IMC.P Parachutists: do not operate in IMC. Their activity is usually notified by NOTAM or known by the ATS.A Unpowered air sports: such as hang gliders, paragliders, etc. Do not operate in IMC.R Radio controlled model aircraft operated by hobbyists: generally operated in VMC below 400 ft AGL and within LOS of the operator (typically 500 m).

Operation above 400 ft should also be notified by NOTAM, in general.G Gliders: do not operate in IMC.S Powered air sports: such as very light aircraft, ultralights, motor gliders, motor paragliders, etc. Do not operate in IMC.U UA not equipped with cooperative means to be detected and/or sufficiently small to make difficult their detection with nonoptical devices.

Nonoptical sensor

D Dirigible airships.H Helicopters: considering both civil and military.L Light aircraft: such as nonpressurized general aviation.Q Pressurized general aviation, passenger aircraft, and cargo aircraft.M Military fighters and high performance jets.V UA equipped with means that help their detection with nonoptical devices (like ADS–B or SSR transponders).

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III. Particular Considerations for Unmanned AircraftSystem Sense and Avoid Systems

So far, we have seen that S&A design parameters should take intoconsideration several factors, like weather, terrain, and the variety offlying objects that the UA might encounter. Moreover, we have seenthat traffic-avoidance design criteria will mainly depend on theminimum miss distance with the intruder aircraft, actual aircraftperformance and limitations, a correct interpretation and implement-ation of the right-of-way rules, the collision avoidance capabilities ofthe intruder, and the compatibility with ATC clearances, if present.Therefore, it is clear that basic design parameters for a UAS S&Asystem can change dramatically as a function of several particu-larities of the actual UAS implementation.

The authors believe that the flight scenario, the UAScommunication data-link architecture, and the UAS level ofautomation are the three most relevant factors that may influence thefinal requirements for a safe design of an S&A system and these arediscussed in this section.

A. Influence of the Flight Scenario

Civil aviation airspace is classified in seven different classes (A, B,C, D, E, F, andG). As a function of this airspace class, different levelsof ATS are given to aircraft, ranging from full separation assurance(classes A and B) to noncontrolled airspace (classes F and G). In theremaining classes (C, D, and E), the ATC provides different levels ofseparation and/or traffic information in function of the flight rules.For example, in class E airspace, IFR flights are only separated fromother IFR flights, whereas VFR flights receive only trafficinformation advisories. The International Civil Aviation Organ-ization (ICAO) airspace class definition is found in [51], but eachstate can adopt a subset of these classes and introduce changes in theirdefinition.

The type of airspacewhere UAS operations will be carried out willmainly determine the level of cooperativeness of the other traffic andthe availability of ATC for assuring separation or enhancing thesituational awareness with respect to other traffic. On the other hand,some systems operate at specific altitude ranges and, if the UA is notflying there, it is not likely to encounter them. In a similar way, the

weather conditions affect the type of hazardous objects that might beencountered, as in IMC some aircraft are not present (such as glidersor balloons).

1. Airspace Class

It is possible to aggregate the types of airspace into two maincategories, as is proposed in [13]: those airspace types in which allaircraft are cooperative (known environment) and the remaining onesin which some noncooperative aircraft may be present (unknownenvironment). Even if the specific requirements for each airspaceclass may differ slightly from one country to another, in airspaceclasses A to D, it is generally a requirement to operate with an SSRtransponder. Moreover, it is also quite usual to mandate the use ofSSR transponders above a certain altitude, regardless of the airspaceclass. For example, in the U.S., transponders are required in airspaceclasses A, B, C, and E above FL100, although some aircraft areexempt (see FAR Sec. 91.215 [52]). The RTCA MOPS fortransponders are found in [53], which specifies some requirementsfor the flight crew control and for the monitoring of the operation ofthe transponder. These requirements could be sufficient forapplication in a UAS.

The signal emitted by aircraft SSR transponders is received bySSRs, and the derived aircraft positions are enhanced with theinformation encoded in those signals and displayed on the ATCscreens. Thus, a Mode A transponder transmits just a five-digitidentifier, whereas a Mode C transponder also transmits thebarometric altitude of the aircraft. Newer Mode S transponders [54]have the capability of transmitting even more information, such asthe position of the aircraft. In this context, automatic dependentsurveillance–broadcast (ADS–B) applications allow other aircraft,ATC, and ground vehicles to send and receive surveillanceinformation of surrounding traffic. In this way, the flight crew’ssituational awareness is significantly improved and some separationresponsibilities can eventually be shifted from the ATC to the flightcrew [18]. However, ADS–B is not currently mandated, and differentdata-link technologies (other than Mode S transponders) exist andare standardized in [55,56], while the data-link independent standardfor ADS–B systems is published in [57].

Fig. 2 Flying objects as a function of the airspace class and weather conditions.

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Furthermore, SSR transponders are also the base technology forcurrent implementations of the TCAS. TCAS-equipped or, moregenerally, ACAS-equipped aircraft are easier to detect and, if the UAis equippedwith such a system, the generated TAswould increase thesituational awareness of the UAS flight crew and improve the sensecapabilities. The TCAS-I standard is published in [58], while relaxedrequirements can be found in [59], when only the traffic advisoryairborne equipment is implemented. Implementing only the trafficadvisory reduces the cost of such a system with respect to TCAS-Iand could be useful in certain UAS implementations. As mentionedbefore, TCAS-II implements resolution advisories too, but the waythese will derive into effective avoidance maneuvers for UAS stillremains an open issue.

2. Meteorological Conditions and Flight Altitude

The dependencies onmeteorological and flight altitude conditionsare analyzed in [13] and summarized in Fig. 2, where the types ofobjects that are likely to be found in the air (according to thetaxonomy of Table 1) are given as a function of the meteorologicalconditions and flying altitude. Known and unknown trafficenvironments are also differentiated in Fig. 2 and, because SSRtransponders are generally mandated for aircraft flying betweenFL100 and FL600, a known environment has been supposed in thisaltitude range. Notice that the carrying SSR transponders aboveFL100 requirement does not apply to all countries and/or aircrafttypes. For example, gliders without this equipment are likely to befound above FL100 as parachutists usually jump from FL140.However, these kinds of activities take place invery specific locationsand are usually known beforehand by the ATS and/or published inNotices to Airmen (NOTAMs). Thus, Fig. 2 shows the types ofobjects that are most likely to be found in a general case, providingthat some exceptions andminor particularitiesmay exist as a functionof specific national regulations.

The objects that must be detected optically are the mostchallenging ones for S&A systems. Yet, as is observed in Fig. 2, thepossibility of encountering them decreases as altitude increases.Above FL100, only gliders or parachutists might be found and, asexplained before, their operations will probably be coordinated withthe ATS and/or published in NOTAMs. It is worth mentioning that,when flying over FL600, we are again in an unknown trafficenvironment, because the airspace class is G. This part of the airspaceis not really used by civilian manned aircraft, although HALE UA(which are expected to be detected by electromagnetic means) andsome high-performance military aircraft might be present.

Below FL100, all kinds of objects that must be detected opticallycan be found in an unknown environment and in VMC conditions. InIMC, where the optical sensors are less effective, the possibility offinding any of these objects is reduced to tethered balloons and othernoncooperative UA. Commercial aviation and military fighters areusually not present in unknown environments. Finally,whenflying ina known environment below FL100, all objects that can be detectedwith nonoptical means are likely to be encountered in all weatherconditions and altitudes; fauna and potential parachutists are the onlykind of objects that require optical detection that might be present inVMC conditions.

3. Visual Line-of-Sight Operations

Aparticular but relevant operation scenario is when the flight crewalways has direct visual contact with the UA. These visual line-of-sight operations could even justify the absence of onboard means tosense the traffic. In this case, it is necessary not only to be able to seethe UA but also the surrounding airspace where threatening trafficmay exist. For greater distances, the visual performance of the flightcrew, along with the reduction of the spatial situational awareness,will not be able to fulfill the sense requirements, and other supportsystems will be required. Moreover, the orography is an importantfactor to consider when operating in visual line-of-sight conditions,because some traffic could be hidden behind the terrain.

When an aircraft is beyond line of sight, the responsibility forsensing cannot be assumed exclusively by the flight crew. It is worth

mentioning that these kinds of operations do not always imply alocation far from the ground control station. For example, the flightcrew could be located on one side of a building, controlling aUA thatis performing a perimeter surveillance of the same building. Becausethere is not a straight, visual, obstacle-free line between the UA andthe flight crew for the entire mission, this operation would beconsidered beyond visual line of sight.

B. Influence of the Unmanned Aircraft System Architecture

Nobody questions the presence of a human controlling the UAS,but the distance between UAS crew and the aircraft could result inreal-time and information issues that may threaten certain S&Aarchitectures or solutions. Thus, the data-link particularities, namelythe latency in the communications and the available bandwidth,could seriously influence the performance of the onboard systems.Because in most UAS architectures the data-link system might benecessary to achieve the collision avoidance function, its criticalitylevelwould definitely influence the S&A system architecture [60]. If,for instance, the data link is not considered as a critical system, anautonomous S&A capability will be needed onboard in the event of aloss of radio link. On the other hand, if the data link is conceived as acritical component (and part of the S&A system), S&A capabilitiescould perhaps be shifted to theUASground station. Therefore, losingthe radio link should be regarded with the same criticality level aslosing an autonomous S&A capability.

Moreover, the degree of automation of the UAS is also a key factorin the S&Acapability; because highly automated platformswill needmore reliable sense subsystems than those with a high contributionfrom the human crew.

1. Unmanned Aircraft System Data-Link Communications Relay

Communication latencies include communication delays, S&Ascan rates, onboard and ground processing times of the differentalgorithms involved, pilot-in-the-loop reaction times, andcoordination with ATC. Two main categories exist when talkingabout command, control, and telemetry communications: those thatare in line of sight (LOS)with theUAS and those beyond line of sight(BLOS). LOS communications are defined as those that do notrequire any communications relay between the UA and the groundstation. It is worth mentioning that LOS communications can besuccessfully achieved in certain beyond-visual-LOS operations.

LOS communications could vary from a few meters to tens ofnauticalmiles. Small latencies in LOS communications could allow adirect video link from the UA to the flight crew. This video link andthe assumption of responsibility of the flight crewwhen assessing thesituation, elaborating the evasive actions and performing them,simplify the whole S&A system. However, the data link itself willremain as a critical component and the exigencies over the real-timeperformances would be very demanding. Furthermore, a highbandwidth, which might not be available, would be required totransmit, in real time, the high-resolution images that will be neededto achieve the same level of safety as manned flights.

Different communication strategies are expected for BLOScommunications: 1) direct radio-frequency (RF) communications,2) terrestrial networks (TN), and 3) satellite communications (SC).

Even if the UA is beyond visual LOS with the ground crew, directRF communications can be successfully implemented in someconditions. These architectures provide fast links, and latency is notdeemed a big issue for S&A applications. However, the frequencyused for these communications might suffer from insufficientbandwidth, especially if video, aiming at implementing the sensefunction at the ground station, is transmitted.

The architecture of terrestrial networks shifts part of thecommunications to a service provider that already owns a dedicatednetwork over a certain area. With this architecture, the bandwidthlimitations depend on the actual means of the provider, and S&Avideo architecture will be conditioned to this available bandwidth,the latency of the data transmission, and the stability of the communi-cations. Thus, depending on the quality of service guaranteed by theprovider, a strategy for S&A including video transmission could be

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considered or discarded as a function of the real-time performanceand integrity of the network.

Satellite communications introduce large latencies because of thelong distances that communication signals must travel. These delaysmean that the S&A functionalitymust be performed entirely onboardthe UA.

2. Unmanned Aircraft System Level of Automation

The level of autonomy envisaged for the UAS could notablysimplify the requirements for the S&A system. During the phase ofthe allocation of aircraft functions to systems [61], the abstractfunctionality of the aircraft is divided between the flight crew and theautomated systems supporting each of the functionalities. As afunction of the level of responsibility assumed by the flight crew, wecan establish four levels of automation.

Radio-Controlled (RC) Aircraft: In this situation, the pilot directlycommands the attitude of the aircraft by directly controlling theflightsurfaces through a dedicated radio-control system. As the aircraftitself has no means to autonomously modify its trajectory, allavoidance maneuvers will be performed exclusively by the humanpilot and therefore all sense informationmust be available to this pilotin real time.

Pilot in Line (PiL): This architecture allows the aircraft to followprogrammed flight plans but allows the pilot to take control of theaircraft and aviate it at any time. Because the flight crew retains thecapability of assuming control of the trajectory, the responsibility ofthe S&A functions can be shared by systems and humans.

Pilot on Line (PoL): Here, the pilot has the capability to takecontrol of the UA but only to navigate it, the aviation function beingperformed autonomously. The difference between allowing theflightcrew to directly aviate or to navigate the aircraft may seem subtle, butit has an enormous implication on the S&Asystems.Aviate is definedasmodifying the trajectory of the aircraft by controlling its attitude inreal time. Whereas navigate means to give guidance inputs to theaircraft (in the form of headings or even waypoints), aiming atmodifying the flight path as well. Therefore, if the time scale for piloton-line operations is significantly longer than in previous categories,it is likely that S&A systems will be built onboard, and the UAwillassume the entire responsibility for sensing and performingavoidance maneuvers.

Full Autonomous (FA) UAS: Full autonomy is understood as thecapability of the UA to achieve its entire mission, without anyintervention from the human flight crew.With this architecture, S&Amust be performed exclusively by onboard means. However, thesekinds of operations have yet to be contemplated by any regulatorybody in the world.

IV. Sense and Avoid Requirements as a Functionof the Unmanned Aircraft System Type

There are several ways to categorize UAS, which can be groupedas a function of one or several of their particularities, such as theweight or performance of the UA, level of autonomy, altitude ofoperation, communications data link, or the type of operations ormissions carried out. For example, the UK Civil Aviation Authoritydivides the UAS according to the UAweight [62]. The first categoryis called small aircraft, which includes aircraft weights lower than20 kg. The next category is the light UAV and includes aircraft

between 20 and 150 kg. The last category is just called UAV andincludes aircraft of 150 kg or more.

Another typical categorization directly relates aircraft size (orweight) to the type of expected mission and altitude. For instance, in[10], five different categories are proposed after analyzing thecorrelation between UA weight and typical operating areas andaltitudes. Table 2 displays this UAS categorization, along with someadditional high-level particularities commonly found in eachcategory. This is the categorization that has been chosen in this paperto discuss potential S&A requirements as a function of the UAS type.

It should be noted that, even if some objects are not likely to befound above certain altitudes (such as fauna or light aircraft), theUAS may encounter them during climb and descent phases. In thiscontext, some possible solutions allowing a less demanding S&Asystem would be to temporarily segregate some airspace to fit theseclimb and descent phases; use chase aircraft following theUA duringthese phases; climb/descend in a controlled airspace class with acompletely known environment for the ATC, while avoidingoverflying populated areas to reduce the risk of a crash due to apotential collision with fauna; etc. In this paper, we focus on theenvironment where the UAS might perform its normal operations.For example, consider a high-altitude, long-endurance (HALE)UASthat is expected to fly nonstop for weeks or even months. The S&Asystem could be designed to fulfill the requirements at a high altitudeand, for instance, detect only cooperative traffic. Chase planes ortemporal airspace segregation could be used for climb/descentoperations, in this way ensuring collision avoidance during thesephases.

Figure 3 identifies, for eachUAS type, the possible flight scenarioswhere their missions could be carried out. Recalling Table 1 and

Table 2 UAS classification as a function of the UA weight [10]

Type Weight, lb Operating scenario Typical operating altitude Typical Airspace Typical operatingcruise speed, kt

Endurance

Micro <2 Local Up to 500 ft G MinutesMini 2–30 Local 100–10,000 ft G 30–90 Several hoursTactical 30–1000 Regional 1400 ft–FL180 B, C, D, E 80–110 5–10 hMALEa 1,000–30,000 Continental FL180–FL600 A 100–200 10 h to daysHALEb 1,000–30,000 Intercontinental Above FL600 G 20–400 10 h to days

aMedium-altitude, long-endurance.bHigh-altitude, long-endurance.

Fig. 3 Typical flight scenarios for each type of UAS.

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Fig. 2, we can see the type of objects that are more likely to beencountered for each UAS type during their normal operations (i.e.,excluding climbs/descents to the mission altitudes, transit routes,etc.). Moreover, we can seewhether the UAS S&A equipment mightinclude some optical or nonoptical sense devices to detect intruders.Besides the type of intruders to be detected, further S&Arequirements will depend on the specific UAS architecture. Table 3displays with a checkmark the type of data link and automation levelthat is most likely to be found for each of the UAS types considered.

The micro-UAS category is a particular case regarding the S&Arequirements. This type of UAS is expected to be operated in VMCconditions, below FL100, and by using LOS communications. Inthese conditions, it is still possible the UAmight encounter gliders orhot air balloons.Yet the probability of collisionwhileflying inLOS isgreatly reduced, and the exact location of the mission usuallydetermines the real risk of encounters. Micro UAS are perhaps thesoleUAS category able tofly fully autonomously. However, this kindof aircraft is mostly expected to fly in an intruder-free environment:either indoors or outdoors at very low altitudes (well below 500 ft).Therefore, for this UAS category, an S&A systemwould not even benecessary in most cases.

All of the remaining categories require a pilot in command. UAflying at high altitudes and BLOS are likely to have communicationdelays that would impede a full remote S&A system. Therefore, thegreater the delay, the higher the automation that would be needed, asseen in Table 3.

Furthermore, an interesting paradox is also found when looking atall the previous tables. Smaller (and therefore simpler) UA, such asmini and tactical UA, are expected to operate in an environmentwhere optical and nonoptical sensors would be required. Conversely,because they are expected to fly at high altitudes, bigger UA areexpected to find only traffic that is detectable with nonopticalsensors, mostly in a known traffic environment for the ATC and withno concerns about collisions with fauna or terrain. However, sensingrequirements, in terms of detection range andmeasurement accuracy,might be hard to satisfy, because collision speeds can be relativelyhigh in midair collisions. It seems that sense functionalities will bemore challenging for small UA, which would have to integrate thesecapabilities in small systems and probably with lower budgets.Avoidance capabilities of big UAwould be similar to current ACAS-equipped aircraft, due to their similar flight performance. Neverthe-less, HALE and medium-altitude, long-endurance (MALE)platforms are subject to higher latencies in their data-link communi-cations, which will pose major issues, resulting in a more complexand demanding avoidance subsystem if compared with smaller UA.

On the other hand, smaller UA will definitely operate in verychallenging environments, with the presence of noncooperativeaircraft, fauna, and ground obstacles. For micro and mini UA, somecollisions might not be a real threat to humans, because theseplatforms are very light and have low kinetic energy. Conversely,tactical UA, performing similar missions to general aviation, arefound to be themost challenging systems from anS&Apoint of view,because they are expected to fly in awide and heterogeneous range ofscenarios, quite probably with BLOS communications and highautomation levels. Thus, further research and development efforts arestill needed before their seamless integration into nonsegregatedairspace.

To sum up, very high sense challenges, along with possible loweravoid challenges, are expected for small UA, and vice versa forheavier platforms. Figure 4 summarizes this relationship.

Even if the previous UAS categorization seems to be the mostappropriate way to assess S&A requirements, it is also useful togroup the UAS in different categories. Besides the generalconclusions discussed above, some particularities may also apply,depending on other characteristics of the UA.

A. Sense and Avoid Particularities

Besides UA mass and operating altitude, other importantparticularities of the UASwill potentially influence the possible finalS&A requirements. In this section, we discuss the influence of theUA flight performance, the type of UAS mission, and theiroperational behavior.

1. Unmanned Aircraft Flight Performance

A categorization based on flight performance characteristics isalready proposed by the RTCA in their document Do-320 [14].There, the following UAS categories are proposed: 1) turbojet fixed-wing (e.g., Global Hawk, N-UCAS); 2) turboprop fixed-wing (e.g.,Predator B); 3) reciprocating/electric engine fixed-wing (e.g.,Predator A, Shadow 200); 4) vertical takeoff and landing (VTOL,e.g., Firescout, RMAX Type II); 5) airship (e.g., SA 60 LAA).

Aircraft performance has a direct impact on the reaction timesduring an avoidance maneuver or while planning separationstrategies. It has an impact not only on the avoidance requirements,but on the sense part as well, because intruders would probably haveto be detected at longer ranges. Regarding TCAS/ACAS systems,low-performance UA will not be compliant with the avoidancemaneuvers defined in the current implementation of these systems,and further research and development in this area will be needed.Furthermore, aircraft performance also has an impact on ATC, ascontrollers typically group aircraft according to their flightperformance to manage flows while maintaining separation.Therefore, newprocedures and handbooks forATCofficerswill haveto be assessed.

2. Unmanned Aircraft System Missions

With regard to UAS missions, there is no doubt that nowadays ahugemarket is emerging from the potential applications and servicesthat can be offered by UA in the civil domain. UAS can perform awide variety of functions, andmany of their missions are described inthe literature; for instance, see [14,15,63,64]. According to thesereferences, civilian applications can be grouped into four categories:

Communications Applications: Telecommunication relayservices, cell phone transmissions, and broadband communicationsare just a few communications applications. Typically, theseapplications will be performed at very high altitudes, with very stableUA platforms, and at specific and known locations, or at very

Table 3 Typical data link and automation level

solutions for each UAS type

Data link Automation level

BLOS

LOS RF TN SC RC PiL PoL FA

Micro ✓ ✓ ✓ ✓ ✓ ✓

Mini ✓ ✓ ✓ ✓ ✓ ✓

Tactical ✓ ✓ ✓ ✓

MALE ✓ ✓ ✓ ✓

HALE ✓ ✓

Fig. 4 Relation of the sense vs avoid challenges as a function of the UAS

type.

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low altitudes, with ad-hoc networks formed by small and low-flying UA.

Environmental Applications: With the UAS capability for remotesensing, applications like atmospheric research, oceanographicobservations, weather forecasting, or precision farming can be moreeffective. These applications will generally scan a specific area,which can be very small, such as a farmyard, or can cover areas ofhundreds of square kilometers. Typically, aircraft trajectories will bestable and predictable along a predefined scan pattern, which will beknown beforehand.

Monitoring Applications: Forest fire detection, internationalborder patrol, fish farm monitoring, or high-voltage power linemonitoring are among the most important missions in this category.Compared to previous applications, these missions will mainlydescribe lineal or perimetral trajectories rather than scans over areas.In general, routine trajectories with some degree of predictability intheir behavior will be found.

Emergency Applications: This group includes firefighting, search-and-rescue missions, oil slick observation, flood watch, hurricanewatch, and volcanomonitoring, among others. Theflown trajectorieswill be the most unpredictable ones if compared to previousapplications, because theywill be highly dependent on the changes inthe situation and the type of mission demanding the UAS support.

Emergency and monitoring applications are the most challengingones from an S&A point of view, because the UAwould be requiredto fly over a wide range of flight scenarios and altitudes, which, forthe most part, would be hard to predict beforehand. Yet, in someemergency applications, it is likely that dedicated ATC may supportUAS operations. Moreover, in the case of an emergency, airspacecould be segregated temporarily (for instance, over awild forest fire),and therefore the S&A requirements would be reduced. On the otherhand, it is hard to assume that monitoring applications will benefitfrom dedicated ATC and/or airspace restrictions and, therefore, theS&A requirements will be more demanding. For communicationsand environmental operations, the predictability of the trajectorieshas a positive impact on the S&A requirements, because somemitigation measurements could be applied. In addition, thesemissions will be performed over specific locations, well known inadvance, and the possibility of temporarily segregating the airspacecould also be an option to reduce S&A requirements.

3. Unmanned Aircraft System Operational Behavior

In [14], the RTCA gives another interesting categorization relatedto operational behaviors of the UA once airborne. Three differentflight profiles are presented that represent generic operational UAbehaviors. These are 1) point-to-point UAS operations, consisting ofdirect flights that do not include aerial works, route deviations, ordelays (for example, transport of passengers and/or cargo);2) planned aerial works, such as orbiting, surveillance, or trackingmissions using predefined waypoints; and 3) unplanned aerial worksinvolving UAS whose intended flight path cannot be publishedbeforehand because the UAS is adapting the flight plan continuouslyas a function of the needs of the mission.

Nowadays, in manned aviation, point-to-point operations areflown by commercial aviation aircraft, and nonoptical S&A systemsalready exist, such as the TCAS. On the other hand, aerial works areperformed by general aviation aircraft, and separation is assured bysee-and-avoid procedures. In the case of the UAS, it is clear thatunplanned aerial works will present the most challenging environ-ments regarding S&A requirements. State of the art separation andcollision avoidance systems are, in general, devoted to solvingstraight point-to-point conflicting trajectories. However, some UASmissions will involve complex trajectories, such as scans orconditional behavior, and an accurate and reliable trajectory predic-tion tool will be a key component for the S&A system. Thesenonconventional trajectories could trigger TCAS/ACAS false alarmsto surrounding traffic, and this issue will have to be assessed, too. Inthis context, the exchange of flight intent information among theaircraft [65] and/or ADS–B-derived applications will probably helpto overcome these limitations [50,66].

V. Conclusions

At present, no regulation exists concerning sense and avoid (S&A)for unmanned aircraft systems (UAS). Nevertheless, this is a subjectof great interest to regulatory bodies, and currently one of the majorobstacles to be overcome before access to civil and nonsegregatedairspace can be granted to UAS. Certain standards already exist forsystems used in manned aviation, and in some cases these systemrequirements can easily be translated into UAS requirements. Yetseveral issues still linger when trying to apply to UAS requirementsthat were developed with the premise that the flight crew are on theplane.

As discussed in this paper, S&A design parameters should takeinto consideration several factors, such as weather, terrain, and thevariety of flying objects that the unmanned aircraft (UA) mightencounter. In addition, traffic avoidance design criteria will mainlydepend on the minimum miss distance with the intruder aircraft,actual aircraft performance and limitations, a correct interpretationand implementation of the right-of-way rules, the collision avoidancecapabilities of the intruder, and the compatibility with ATCclearances, if present.

Even basic design parameters for a UAS S&A system candramatically change as a function of several particularities of theactual UAS implementation. This paper highlights the flightscenario, the UAS communication data-link architecture, and theUAS level of automation as the three most relevant factors that mayinfluence the final requirements for the safe design of an S&Asystem. Even for the same UA platform, the risk of fatalities in someoperational scenarios (for example, in line of sight and away frompopulated areas) would be well within the required safety levels.Because the UAS spectrum covers a great variety of UA sizes,airframe designs, capabilities, and mission particularities, theparticular requirements for their S&A system will be stronglyaffected by all these variables. Therefore, the presence or absence ofsome S&A functionalities, the sensor requirements, and theavoidance performance would be justified by an appropriate and ad-hoc safety case study in accordance with the UAS concept ofoperations.

It iswidely agreed that UAS integration in civil airspace should notincur any cost to current airspace users, but it is also true that the UASshould pose no greater risk to people or property than other currentairspace users, and therefore requirements and regulations will beexpected to adapt to the specificities and particularities of thispromising and exciting new chapter in the history of aviation.

Acknowledgments

Thiswork has been partially funded by theMinistry of Science andEducation of Spain under contract CICYT TIN 2010-18989. Thiswork has also been cofinanced by the European Organization for theSafety of Air Navigation (EUROCONTROL) under its Co-operativeActions of R&D in EUROCONTROL Innovation (CARE-INO III)program. However, the content of the work does not necessarilyreflect the official position of EUROCONTROL.

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