US Radiocommunications SectorFact Sheet

Working Party: WP 5B Document No: USWP5B20-15

Ref: Annex 16 to Document 5B/411-E Date: 2 February 2018

Document Title: Working Document Towards a Preliminary Draft Revised Report ITU-R M.2204-0

“Characteristics and spectrum considerations for sense and avoid systems use on unmanned aircraft systems”

Author(s)/Contributors(s):

Don NellisFederal Aviation Administration800 Independence Ave., S.W.Washington, DC 20591

Michael NealeACES Corporation for the FAA

Fabrice KunziGeneral Atomics

Phone/Email:

Phone: (202) 267-9779e-mail: Donald.Nellis@faa.gov

Phone: (858) 705-8978e-mail: michael.neale@ACES-INC.COM

Phone: (858) 753-8312e-mail: Fabrice.Kunzi@ga-asi.com

Purpose/Objective: The purpose of this contribution is to provide additional updates in support of a revision to ITU-R Report M.2204-0.

Abstract: This contribution will propose to additional modifications to ITU-R Report M.2204-0 to build on the information provided at the November 2017 meeting of WP-5B. These additions are intended to update information contained in this 2010 Report and to incorporate additional Radionavigation frequency allocations that could be used to support UAS sense and avoid systems. The US will be requesting to upgrade the status of this document to a Preliminary Draft Revised Report.

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IntroductionReport ITU-R M.2204-0 was published in November 2010 in support of WRC-12 Agenda Item 1.3 efforts to identify the requirements of Unmanned Aircraft Systems (UAS). Since the report was published, not only have the requirements of UAS evolved but, changes have been made to the Radio Regulations at WRC-12 and WRC-15 that affect the frequency bands identified in this report.

ProposalThe United States of America proposes to continue the efforts to revise Report ITU-R M.2204-0. This effort includes updated information on other users of the frequency bands identified in this report. The intent of this effort is to provide more accurate information to better assist developers of unmanned aircraft sense and avoid systems.

Attachment

Note: Proposed changes to Annex 16 of 5B/411-E are highlighted in Turquoise.

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Radiocommunication Study Groups

Source: Document 5B/TEMP/171

Subject: Revision of Report ITU-R M.2204-0

Document 5B/XXX-E2 February 2018English only

United States of America

WORKING DOCUMENT TOWARDS A PRELIMINARY DRAFT REVISED REPORT ITU-R M.2204-0

Characteristics and spectrum considerations for senseand avoid systems use on unmanned aircraft systems

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ATTACHMENT

WORKING DOCUMENT TOWARDS A PRELIMINARY DRAFT REVISED REPORT ITU-R M.2204-1

Characteristics and spectrum considerations for senseand avoid systems use on unmanned aircraft systems

(2010-201X)

TABLE OF CONTENTS

1 Introduction....................................................................................................... 4

2 General descriptions and terminology.............................................................. 6

2.1 Terminology..................................................................................................... 6

2.2 Airspace............................................................................................................ 6

2.3 Applicability of sense and avoid to overall collision avoidance approach....... 7

2.4 Existing aeronautical radionavigation allocations............................................ 7

2.5 Existing radionavigation allocations................................................................. 8

3 Technical considerations S&Asense and avoid................................................ 8

3.1 Aircraft-based S&Asense and avoid................................................................. 8

3.2 Other technical issues....................................................................................... 13

4 Spectrum considerations for UASunmanned aircraft sense and avoid system. 14

4.1 Aeronautical radionavigation spectrum currently allocated for airborne radars 14

4.2 Aeronautical radionavigation spectrum currently allocated for ground radars 15

4.3 Other aeronautical radionavigation spectrum................................................... 15

4.4 Radionavigation spectrum................................................................................ 16

5 Summary........................................................................................................... 17

s

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ObjectiveNumerous unmanned aircraft (UA) applications have been demonstrated or are planned that will dramatically increase the numbers of UA worldwide. With integration of UA into non-segregated airspace very close, it is essential that adequate spectrum be found to support UA operations including the spectrum requirements for UA sense and avoid (S&A) systems.Summary of RevisionThis revision provides changes to

ScopeUnmanned aircraft (UA) applications have been expanding throughout the world and will continue to increase the numbers of UA worldwide. With integration of UA into non-segregated airspace underway, it is essential that spectrum to support UA sense and avoid (S&A)1 operations be clearly identified. This Report identifies spectrum available for UA S&A operations and provides guidance for each of the frequency bands identified.

KeywordsUnmanned Aircraft

Related ITU Recommendations and ReportsRecommendationsITU-R F.1242: Radio-frequency channel arrangements for digital radio systems operating in the

range 1 350 MHz to 1 530 MHzITU-R S.1340: Sharing between feeder links for the mobile-satellite service and the

aeronautical radionavigation service in the Earth-to-space direction in the band 15.4-15.7 GHz

ITU-R S.1341: Sharing between feeder links for the mobile-satellite service and the aeronautical radionavigation service in the space-to-Earth direction in the band 15.4-15.7 GHz and the protection of the radio astronomy service in the band 15.35-15.4 GHz

ITU-R S.1426: Aggregate power flux-density limits, at the FSS satellite orbit for radio local area network (RLAN) transmitters operating in the 5 150-5 250 MHz band sharing frequencies with the FSS (RR No. S5.447A)

ITU-R S.1427: Methodology and criterion to assess interference from terrestrial wireless access system/radio local area network transmitters to non-geostationary-satellite orbit mobile-satellite service feeder links in the band 5 150-5 250 MHz

ITU-R M.1454: E.i.r.p. density limit and operational restrictions for RLANS or other wireless access transmitters in order to ensure the protection of feeder links of non-geostationary systems in the mobile-satellite service in the frequency band 5 150-5 250 MHz

ITU-R M.1463: Characteristics of and protection criteria for radars operating in the radiodetermination service in the frequency band 1 215-1 400 MHz

1 Within the International Civil Aviation Organization (ICAO) the S&A is refered to as “Detect and Avoid (DAA)”.

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ITU-R M.1464: Characteristics of radiolocation radars, and characteristics and protection criteria for sharing studies for aeronautical radionavigation and meteorological radars in the radiodetermination service operating in the frequency band 2 700-2 900 MHz

ITU-R M.1466: Characteristics of and protection criteria for radars operating in the radionavigation service in the frequency band 31.8-33.4 GHz

ITU-R M.1584: Methodology for computation of separation distances between earth stations of the radionavigation-satellite service (Earth-to-space) and radars of the radiolocation service and the aeronautical radionavigation service in the frequency band 1 300-1 350 MHz

ITU-R M.1638: Characteristics of and protection criteria for sharing studies for radiolocation (except ground based meteorological radars) and aeronautical radionavigation radars operating in the frequency bands between 5 250 and 5 850 MHz

ITU-R M.1730: Characteristics of and protection criteria for the radiolocation service in the frequency band 15.4-17.3 GHz

ITU-R M.1796: Characteristics of and protection criteria for terrestrial radars operating in the radiodetermination service in the frequency band 8 500-10 680 MHz

ITU-R M.1827: Guideline on technical and operational requirements for stations of the aeronautical mobile (R) service limited to surface application at airports in the frequency band 5 091-5 150 MHz

ITU-R M.1904: Characteristics, performance requirements and protection criteria for receiving stations of the radionavigation-satellite service (space-to-space) operating in the frequency bands 1 164-1 215 MHz, 1 215-1 300 MHz and 1 559-1 610 MHz

ITU-R M.2007: Characteristics of and protection criteria for radars operating in the aeronautical radionavigation service in the frequency band 5 150-5 250 MHz

ITU-R M.2008: Characteristics and protection criteria for radars operating in the aeronautical radionavigation service in the frequency band 13.25-13.40 GHz

ITU-R M.2059: Operational and technical characteristics and protection criteria of radio altimeters utilizing the band 4 200-4 400 MHz

ITU-R M.2085: Technical conditions for the use of wireless avionics intra-communication systems operating in the aeronautical mobile (R) service in the frequency band 4 200-4 400 MHz

Abbreviations/GlossaryABS&A Aircraft-based sense and avoidACAS Airborne collision avoidance systemADS-B Automatic dependent surveillance broadcastARNS Aeronautical radionavigation serviceATC Air traffic controlCNPC Control and non-payload communicationsGBS&A Ground-based sense and avoidNMAC Near-miss aircraft collisionRF Radio frequencyS&A Sense and avoidSWaP Size, weight and power

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TSO Technical standard orderUA Unmanned aircraftUACS Unmanned aircraft control stationUAS Unmanned aircraft systemWRC World Radiocommunication Conference

1 IntroductionThough UA have traditionally been used in segregated airspace where separation from other air traffic can be assured, administrations expect broad deployment of UA in non-segregated airspace alongside manned aircraft in the future. Current and future unmanned aircraft system (UAS) operations may Unmanned aircraft system (UAS) operations include scientific research, search and rescue operations, hurricane and tornado tracking, volcanic activity monitoring and measurement, mapping, forest fire suppression, weather modification (e.g. cloud seeding), surveillance, communications relays, agricultural applications, environmental monitoring, emergency management, and law enforcement applications. Thus, sSignificant growth is forecast in the UAS sector of aviation and the projected growth is shown in Figure. 1.

FIGURE 1

Cumulative total of unmanned aircraft systemsUAS available for operation

1.000

2.000

3.000

4.000

5.000

6.000

2008 2010 2012 2014 2016 2018 2020 2022 2024 2026 2028 2030

Total government

Total commercial

Year

Cum

ulat

ive

tota

l of U

AS

avai

labl

e fo

r ope

ratio

n

0

For an air vehicle to operate in non-segregated airspace there is a requirement to see and avoid other aircraft, properly act and respond to certain weather conditions, and remain well clear of obstacles as outlined in the International Civil Aviation Organization’s (ICAO’s) Annex 2, “Rules of the Air”. Without the pilot onboard the aircraft, this requirement must be fulfilled by an electronics means.

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Two primary architectures are currently under considerations by various standardization bodies: sensor systems are under development to allow a UAS to meet this requirement. The first class comprises sensor(s) or electronic system(s) on the air vehicle and is called aircraft-based sense and avoid (ABAirborne S&A2). The second class involves sensor(s) or electronic system(s) monitoring the air space from the ground and is referred to as gGround-bBased sense and avoid (GBS&A3). In the Airborne S&A architecture all sensors necessary for the detection of other aircraft are onboard the aircraft, where the Ground Based S&A architecture uses ground-based sensors to detect and track aircraft in the airspace of interest. It is anticipated that eEquipage of The decision of whether to equip the UA with an ABS&A system or to rely on the use of a ground based GBS&A is would be dependent on the operation in question and considers elements such as airspace class4, type of aircraft and available size, weight and power (SWaP) of airspace the UA operates.

As shown in Fig. 1, the number of commercial and government UAS is rapidly expanding. UAS densities vs. altitude and size are shown in Tables 1 and 2. The goal of aAirspace access for appropriately equipped UA systems is to achieved by demonstrating that the change of introducing a UA into the airspace in questions does not degrade the existing level of safety of that airspace providing a level of safety equal to that of an aircraft with a pilot in the cockpit. If UAS operateions in non-segregated civil airspace, they must be integrated safely and adhere to current operational rules that provide an acceptable level of safety similar to that of a conventional manned aircraft. Thus it is envisioned that UAS will require an sense and avoid (S&A) system that can maintain simultaneous tracks of nearby aircraft, terrain, weather, and obstacles to replace current functionality and actions performed by the pilot on manned aircraft.5

TABLE 1

Unmanned aircraft systemUAS densities vs. altitude based on Figure. 1 projections

UA density UA/10 000 km²At surface (3 UA at an airport)

2.3950-FL50 (1 500 m) 4.017

FL50-FL195 (1 500-6 000 m) 1.560> FL 195 (> 6 000 m) 0.644

Total density 8.616

2 Also known as Airborne Detect and Avoid (ABDAA).3 Also known as Ground Based Detect and Avoid (GBDAA).4 The world’s navigable airspace is divided into three-dimensional segments, each of which is assigned to a specific class. Most nations adhere to the classification specified by the International Civil Aviation Organization (ICAO)  in which classes are fundamentally defined in terms of flight rules and interactions between aircraft and Air Traffic Control (ATC). Individual States may also designate Special Use Airspace, which places further rules on air navigation for reasons of national security or safety.5 While most discussions around S&A focus on avoiding other aircraft, the pilot in a manned aircraft is also responsible for avoiding additional hazards such as weather, terrain, wind shear, etc. Current S&A standardization efforts have limited their scope to only include detecting and avoiding other aircraft.

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TABLE 2

UAS Unmanned aircraft system densities vs. size based on Figure. 1 projections

UA categories Per 10 000 km2 Per spot-beam* In regional-coverage beam**

Large 0.440 21 341Medium 1.950 93 1.515

Small 8.031 385 0Total 10.421 501 1.856* 480 000 km2

** 7 800 000 km2

S&A systems make use of the radiofrequency (RF) spectrum in two separate ways: the sensors (primarily radars) used for the detection of intruding aircraft and the data link between the aircraft and the ground station. While the S&A system is not the primary reason for the need of a data link with unmanned aircraft, it introduces additional requirements to downlink the tracks of intruding aircraft to the ground station for display to the pilot. As such, Since the S&A systems will beare used to ensure the safety of life and property thus , a radiofrequency (RF) based S&A system is one of these technologies. Tthese RF systems will need to be designated are considered a safety service and operate in an aeronautical radionavigation service (ARNS) allocation.

2 General descriptions and terminologyUnmanned aircraft are powered, aircraft that do not carry a human pilot, use aerodynamic forces to provide vehicle lift, and may fly semi-autonomously or autonomously, or be piloted remotely. The current state-of-the-art in UAS design and operation is leading has led to the rapid development of UAS applications to fill many diverse requirements. UAS applications that have been demonstrated or planned come from such areas as include agriculture, communications relays, aerial photography, mapping, emergency management, scientific research, environmental management, and law enforcement. The safe operation of UAS outside segregated in civil airspace requires addressing the same issues as manned aircraft, namely integration into the air traffic control (ATC) system. Because the pilot is no longer aboard, a method of replacing the pilot’s “see and avoid” responsibilities as well as procedures, are required (see ICAO’s Annex 2 “Rules of the Air”). While existing onboard aircraft systems mayhave been adapted or incorporatedmodified to accommodate the S&A requirements for cooperative targets, it is likely that new technologies as well as additional systems will be required are needed for detecting and acting on non-cooperative targets.

2.1 Terminology

Unmanned aircraft (UA): Designates all types of aircraft remotely controlled.

Unmanned aircraft control station (UACS): Facilities from which a UA is controlled remotely.

Control and non-payload communications (CNPC): The radio links, used to exchange information between the UA and UACS, that ensure safe, reliable, and effective UA flight operation. The functions of CNPC can be related to different types of information such as: telecommand messages, non-payload telemetry data, support for navigation aids, air traffic control voice relay, air traffic services data relay, target track data, airborne weather radar downlink data, non-payload video downlink data.

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Sense and avoid (S&A): S&A corresponds to the piloting principle “see and avoid” used in all air space volumes where the pilot is responsible for ensuring separation from nearby aircraft, terrain and obstacles.

Unmanned aircraft systems (UAS): Consists of the following subsystems:– UA subsystem (i.e. the aircraft itself).– UACS subsystem.– CNPC subsystem.– ATC communications subsystem (not necessarily relayed through the UA).– S&A subsystem.– Payload subsystem (e.g. Video camera …).

Intruder: An aircraft (manned or unmanned) that enters the S&A surveillance volume and tracked by the S&A system.

2.2 Airspace

To date, operations have been limited to segregated airspaces designated as “R” (Restricted), “D” (Dangerous) or “P” (Prohibited). For the purposes of this report, the airspace may be grouped into three categories, namely:– ATC Separation Assurance – Air traffic control is responsible for safe separation of all

aircraft. This comprises Classes A, B, and, if the UAS is operated in accordance with Instrument Flight Rules (IFR), Class C airspace.

– Limited or no ATC Separation Assurance– Air traffic control is not responsible for safe separation of all airspace users. This comprises Classes D, E, F and G airspace.

– Segregated – A defined volume of airspace is reserved for exclusive use of a particular UAS. In such airspace there would be no air traffic control service and therefore ATC is not responsible for separation but there are one or more aircraft, under the control of the same operator, in this airspace at a given time.

2.3 Applicability of sense and avoid to overall collision avoidance approach

An important point to consider in the design of a sense and avoid system is how it fits into the total systems approach to collision avoidance. ICAO Document 9854 describes conflict management as consisting of three layers: strategic conflict management (SCM), remain well clear (RWC), and collision avoidance (CA). A S&A system provides the RWC and CA layers.

Figure 1: Three layers of conflict management according to ICAO Doc 9854.

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2.3.1 Strategic Conflict Management

SCM includes preflight actions performed to minimize potential flight path conflicts with, and maximize separation from, intruders. SCM also includes risk mitigation that is achieved through airspace organization and management, demand and capacity balancing, and traffic synchronization.

2.3.2 Remain Well Clear/Separation Provision

At the RWC level of S&A, the system identifies the pilot to a potential violation of the S&A Well Clear volume. Based on the information provided by the S&A system, the pilot identifies whether, and if so, what type of a maneuver is necessary to avoid the intruder, and then executes that maneuver. If operating under an Air Traffic Control (ATC) clearance, the UA Pilot coordinates with ATC to obtain an amended clearance before executing the maneuver.

2.3.3 Collision Avoidance

CA is the last layer to of conflict management and aims to prevent an intruder from penetrating the Near Mid-Air Collision (NMAC) volume.” The Aircraft and Collision Avoidance System (ACAS) is a system that is currently used to this effect on manned aircraft.

As shown in Figure. 2, the approach to collision avoidance uses a layered approach. Current technologies that may accommodate these layers include ATC procedures, ground and surface ATC surveillance systems, automatic dependant surveillance-broadcast (ADS-B), airborne collision avoidance system (ACAS) also called traffic collision avoidance system (TCAS), and S&A. ACAS (or TCAS) for use on an UA is still being developed and requires modification to the ACAS algorithms not only for the UA, but also on existing manned aircraft prior to implementation. ADS-B is a newer situational awareness system that is currently being deployed on manned aircraft. It is likely that there will be cases where ADS-B is used without ACAS. In any event, the S&A system is the final layer in case the preceding layers do not provide sufficient separation to avoid a potential collision.

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FIGURE 2

Layered collision avoidance approach

Fig 1: Collision Protection Layers

Air TrafficServices

Procedural

SelfSeparation

Collision Avoidance

Air TrafficServices

Procedural

Air TrafficServices

Procedural

SelfSeparation

SelfSeparation

Collision Avoidance

2.4 Existing aeronautical radionavigation allocations

Currently, there are a number of frequency bands with worldwide ARNS allocations. These ARNS bands could potentially be used to accommodate the UAS S&A applications. These frequency bands are listed in subsequent sections.

2.5 Existing radionavigation allocations

Currently, there are a number of frequency bands with worldwide with radionavigation allocations. Since ARNS is a subset of radionavigation, ARNS systems can be operated in radionavigation allocations provided compatibility can be achieved with other radionavigation systems operating in that allocation. These radionavigation bands could potentially be used to accommodate the UAS S&A applications. These frequency bands are listed in subsequent sections.

3 Technical considerations for S&Asense and avoid

3.1 Aircraft-based S&Asense and avoid

There are a number of factors that drive the performance requirements needed from an RF-based AB Airborne S&A sensor as shown in Figure. 3. Indeed, tThe number of factors that drive the performance requirements for an AB Airborne S&A sensor is large resulting in a very difficult multidimensional trade space containing both dependent variables and independent variables. These factors include characteristics of the encounter including near miss aircraft collision (NMAC) volume, the latencies in the actual AB Airborne S&A system implementation, and the performance parameters of the radar used as the AB Airborne S&A sensor.

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FIGURE 3

S&ASense and avoid sensor performance requirement factors

3.1.1 Encounter characteristics

The first major factor driving the sensor performance requirements shown in Fig. 3 is Encounter characteristics, which include collision course geometries, closing speeds between the UA and another aircraft known as “the intruder”, the selected NMAC or well clear volume, collision avoidance zone size and overall aircraft traffic density. The second major class of factor driving the needed sensor performance includes the latencies in the system implementation. Specific latency drivers that need to be taken into account are minimum allowable detection/detection times, pilot response latencies, UA communications system latencies, AB Airborne S&A sensor data processing delays and expected UA aero-performance characteristics. The last major factor driving AB Airborne S&A sensor performance includes the characteristics of the radar such as angular accuracy, available power-aperture (i.e. detection range), radar cross-section of the intruder aircraft and track rate.

Closing speeds between the UA and an intruder and the NMAC or well clear volume, on the other hand, do have bearing on the necessary detection range needed to detect, track and perform a collision avoidance maneuver. Obviously, the faster the closing speed between the two aircraft, the longer the detection range from the radar that is needed. Additionally, larger NMAC or Well Clear volumes also increase What might not be obvious is the impact of NMAC size on needed detection range. Each plot in Figure. 4 shows the distance between the UA and the intruder aircraft as a function of time before a maneuver is needed, and the time at which each curve is at a minimum is the point of closest approach assuming that the UA can perform a turn at a 15° bank angle. Using the minimum as a proxy for horizontal NMAC distance, one can see that a larger NMAC volume drives the system designer to needing a longer detection range.

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FIGURE 4

NMACNear mid-air miss aircraft collision size vs. time required to start avoidance manoeuver

3.1.2 System latencies

The S&A encounter timeline shown in Figure. 5 defines the 8 major elements that need to be accounted for in an AB Airborne S&A scenario. Again, as with the considerations associated with encounter geometries, system latencies ultimately drive the radar sensor detection range.

UA performance has a large impact on the system latencies. Obviously, a slower moving aircraft has more time to devote to detecting an intruder, but is often less maneuverable so it will probably have less ability to affect a collision avoidance maneuver in order to avoid a collision. On the other hand, UA that fly faster will have less time to devote to detecting an intruder, but these UA are often more maneuverable so they will probably have a greater ability to turn away from the collision.

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FIGURE 5

Collision encounter timeline

3.1.3 Radar performance considerations

Lastly, specific requirements on the performance of the radar component of the S&A system will affect the radar design and performance and be a main driver in frequency band selection. In particular, SWAP and the required accuracy of intruder position (as measured by angular and range resolution) will affect:

1. The detection range of an intruder

The detection range of an intruder is highly dependent on the transmit power-gain product of the radar. Transmit power is usually the highest power consuming element in the radar, which will affect power consumption directly and other SWAP elements indirectly. In order to constrain the radar transmit power and power consumption, lower frequencies are preferred for two factors

Lower frequencies have lower rain attenuation and atmospheric absorption factors as shown in figure 7 below

Higher efficiency amplifiers are more readily available at lower frequencies and hence same output power can be obtained at lower power consumption

For antenna gain evaluation, higher frequencies will have higher antenna gain for same antenna size.

2. The accuracy of the intruder position

In order to estimate the intruder position, the radar measures the range, azimuth (bearing) and elevation angle of the intruder relative to the ownship UAS.

The range resolution and accuracy is mainly dependent on the frequency bandwidth of the waveform and is usually sufficient to achieve required accuracy.

The radar usually relies on monopulse processing to improve angle accuracy. Typical accuracies are in the 1:10-1:20 of the 3-dB beamwidth of the antenna. For same antenna size, the beamwidth is

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reduced as the frequency is increased, and hence the accuracy is improved as the frequency is increased. Note that reducing the beamwidth may have the adverse effect of needing a longer frame time to cover all the required Field of Regard (FOR), hence these parameters are usually traded in a system optimization

Figure 6

Qualitative evaluation of parameters that affect choice of frequency Band

Low

er S

WAP

Frequency

C Band X Band Ku Band

It has been shown that an NMAC rate decreases with an increasing angular resolution (see Figure. 6), and several competing parameters come into play, all driven by size, weight and power (SWaP). In addition, available SWaP plays a significant role in designing an airborne radar, all being severely constrained on many UA.

In the volume search form of the radar range equation, the detection range depends on the power aperture product, the solid angle to be searched, the target cross section, total search frame time, system losses and noise figure. Obviously, the power aperture product is a function of the available UA power as well as the available volume for the antenna array.

A related consideration is the physical size of the antenna, which is inversely proportional to the 3 dB beamwidth of the antenna. Studies have shown that in order to have a reasonable NMAC rate, the angular accuracy of the radar must be of the order of a degree. In order to achieve a 1° angular accuracy assuming a 10-12:1 beam sharpening factor, the 3 dB beamwidth of the antenna must be of the order of 10°. Assuming one available antenna technology with a horizontal dimension antenna array that can be mounted on many UA is of the order of 1 foot, for this example it implies that 5 GHz would be the lowest frequency that can be used for a ABS&A radar. Another important consideration pertaining to frequency selection of an ABS&A radar sensor is the uncertainty inherent in determining and establishing the track of an intruder. If the track determination is incorrect, the collision avoidance software could be lead to an incorrect conclusion regarding the probability of a collision and an incorrect computation of an optimal collision avoidance maneuver.

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As illustrated in Figure. 7, the range uncertainty for a given frequency selection as a function of range is relatively constant, but the azimuth uncertainty is larger with lower frequency radars. Thus, for example, for a 5 GHz radar with a conventional Kalman filter based tracker, it would take longer to establish the true track of the intruder versus a 15 GHz radar using the same filter resulting in additional detection range needed for the 5 GHz solution.

FIGURE 6

Probability of missed collision as a function of available angular resolution

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FIGURE 7

Range and azimuth uncertainty as a function of range

Collision course geometries and overall aircraft traffic density impact the scan volume needed by an ABS&A radar, and have only indirect impacts on the specific radar frequency used. However, the size and placement of the antenna array are dependent on these. As shown in Figure. 8, the atmospheric attenuation due to rain increases as frequency increases so the lower frequencies would require less power aperture than higher frequencies for a given detection range.

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FIGURE 78

Plot of atmospheric absorption at microwave frequencies

According to Figure. 8, lower frequencies are favored over higher frequencies for a given detection range because of the lower attenuation due to rain. Further, in terms of antenna size, higher frequencies are favored over lower frequencies because, in general, smaller antennas can be used at higher frequencies (lower wavelengths).

Thus the proper selection of frequency is critical to the success of the AB Airborne S&A radar. There is no optimum solution that fits all UAS classes, the solution will depend on the UAS class, and operational environment. As shown in figure 6, the X band gives a compromise for trading all parameters. A lower band would have lower power consumption for same range requirement, but may need a larger size antenna to achieve the required antenna gain and intruder position accuracy. Frequencies below C band are not expected to achieve required angle accuracy.

On the high frequency side, ku band and higher can achieve required gain and position accuracy with smaller size, but may need higher power to achieve required detection range If the selected frequency is too low, the antenna will become too large for the UA and if the frequency is too high, the atmospheric attenuation will require more power than is available onboard the UA and increase the size and weight of components required for the ABS&A radar.

3.1.4 Other technical considerations

Another factor that must be taken into account in the determination of a suitable frequency for an airborne radar sensor is electromagnetic interference (EMI) compatibility, both local compatibility on the UA, as well as compatibility with co-primary users of the spectrum. For example, if a UA is carrying another radar as part of its mission payload, one would prefer that

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the S&A sensor frequency be out-of-band from the mission payload radar in order to minimize interference between the two radars. In addition, the prevalence of other radars (e.g. weather radars) in a certain band may impact the use of that band for ABS&A.

3.2 Other technical issuesConsiderations for Ground Based S&A

A Ground Based S&A system may be operated as one unit, or a network of units may be used to cover a large area and improve accuracy. Covering large areas with a Ground Based S&A system is expected to be cost prohibitive, but there are situations where a Ground Based S&A system is the preferred S&A sensor for non-cooperative traffic. Such scenarios include

Limited area coverage: there are cases where the required airspace is limited in area, e.g when the UAS transitions from one controlled zone to another. In such cases, it maybe more cost effective to cover the area with a Ground Based S&A system than needing a sensor on the UAS

Complimenting Airborne S&A systems for scenarios where the an airborne radar may not operate well, including low altitude operations and terminal area, where it is expected that clutter return will dominate the airborne radar return

Cases where the UAS is small and cannot provide the SWAP necessary to carry an airborne radar and hence uses a network of Ground Based S&A system to cover its area of operation

Figure 9 shows a sample representation of how a Ground Based S&A system surveillance volume may be set up.

There are scenarios that occur when traditional existing ground-based surveillance radars are unable to detect the UA when they are flying at low altitude. These scenarios are caused by terrain, man-made structures, flight below normal radar coverage and lack of a transponder. A ground-based radar that can fill this gap would be an ideal GBS&A system.

One approach has been developed utilizing a GBS&A system. This approach involves self separation that requires the UAS operate in airspace inside a fixed threat detection airspace with no other air vehicles. If an aircraft enters the threat detection airspace, a ground-based surveillance system warns the operator. The operator will then execute the second phase and fly the UAS to a safe state. A safe state exists when the UAS lands, exits from its operating area into a safe area, or controlled or restricted airspace.

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FIGURE 9

Ground-based sense and avoid

It is anticipated that GBS&A may be a critical component to the overall S&A solution, because many operations are conducted with relatively much small classes of UAS that do not have the power, cooling and physical space to accommodate the currently projected size and weight of current onboard ABS&A systems. In addition, an ABS&A system may add significant cost and may therefore not be affordable for all UAS. Therefore, GBS&A may be the predominant solution to support all classes of UAS to operate in non-segregated airspace.

There are currently no standards for GBS&A, so standards will need to be developed.

4 Spectrum considerations for UASunmanned aircraft sense and avoid system

To ensure safe flight operations with other aircraft in non-segregated airspace, UAS S&A may require operation in multiple frequency bands allocated to the ARNS bands. Allocations for ARNS can be grouped in three general categories; airborne radars, ground-based radars, and other ARNS allocations. Thus, ABS&A may not use frequency bands reserved for ground-based ARNS systems and GBS&A may not use frequency bands reserved for airborne ARNS systems.

The sections below summarize the status of the frequency bands allocated to the ARNS bands from the Radio Regulations without prejudice to a specific frequency band. It should be noted that there may be exceptions to this categorization that could be considered when selecting an appropriate band. For example, the frequency bands from the other ARNS allocations category may also be appropriate for airborne S&A radars. Consideration should also be given to the need for ABS&A radars to use worldwide ARNS allocations rather than allocations for a single Region. Conversely, GBS&A radars only require allocations within regions and areas of the world where they will be used and allocations for GBS&A radars may be best considered within individual administrations.

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4.1 Aeronautical radionavigation spectrum currently allocated for airborne radars

There are currently five frequency bands allocated to the ARNS that are used to support airborne aeronautical radionavigation radar systems worldwide. These frequency bands are listed in Table 3. There are currently existing standards for these airborne radars in all of these frequency bands.

TABLE 3

ARNS allocations for airborne radars

Band StatusApplicable RR

(Edition of 200816)

footnotes

Exampleof Aviation Standards

ITU-RRecommendations

4 200-4 400 MHz primary 5.536, 5.438, 5.439, 5.440

C687a(1), C92c(2) M.2059-0M.2085-0

5 350-5 470 MHz primary 5.449, 5.448B, 5.448C, 5.448D

C63ce(3) C212(5)

M.1638-1

8 750-8 850 MHz primary 5.470, 5.471 C65a(4) C212(5)

M. 1796-2

9 300-9 500 MHz primary 5.474, 5.475, 5.475A, 5.475B, 5.476A

C65c3e(3) C212(5)

M. 1796-2

13.250-13.400 GHz primary 5.497, 5.498A, 5.499 C65a(4) C212(5)

M.2008-1

(1) Department of Transportation, Federal Aviation Administration, Aircraft Certification Service, Washington DC, Technical Standard Order TSO-C687a, Airborne Low Range Radar Altimeter Equipment (For Air Carrier Aircraft), 15 November 1960. 31 May 2012 (2) Department of Transportation, Federal Aviation Administration, Aircraft Certification Service, Washington DC, Technical Standard Order TSO-C92c67, Airborne Ground Proximity Warning Equipment, 19 March 1996. (3) Department of Transportation, Federal Aviation Administration, Aircraft Certification Service, Washington DC, Technical Standard Order TSO-C63ce, Airborne Weather and Ground Mapping Pulsed Radars, 18 August 1983 Equipment, 1 October 2016.(4) Department of Transportation, Federal Aviation Administration, Aircraft Certification Service, Washington DC, Technical Standard Order TSO-C65a, Airborne Doppler Radar Ground Speed and/or Drift Angle Measuring Equipment (for Air Carrier Aircraft), 18 August 1983. Note: This TSO has been canceled. Equipment that has been previously approved under this TSO may continue to be produced and installed on aircraft. 5) Department of Transportation, Federal Aviation Administration, Aircraft Certification Service, Washington DC, Technical Standard Order TSO-C212, Air-to-Air Radar (ATAR) for Traffic Surveillance, 22, September 2017.

4.2 Aeronautical radionavigation spectrum currently allocated for ground radars

There are currently six frequency bands allocated to the ARNS that are used to support ground-based aeronautical radionavigation radar systems worldwide. These frequency bands are listed in Table 4.

TABLE 4

ARNS Aeronautical radionavigation service allocations for ground radars

Band StatusApplicable RR

(Edition of 200816)

footnotes

Aviation standards

ITU-RRecommendations

1 215-1 240 MHz primary 5.329, 5.330, 5.331, N/A M.1463-3, M.1479

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5.332 M.1904-01 240-1 300 MHz primary 5.282, 5.329, 5.330,

5.331, 5.332, 5.335, 5.335A

N/A M.1463-3, M.1479M.1904-0

1 300-1 350 MHz primary 5.337, 5.337A N/A M.1463-3, F.1584-0

1 350-1 370 MHz primary 5.334, 5.338 N/A M.1463-3, F.1242-0

2 700-2 900 MHz primary 5.337, 5.423, 5.424 N/A M.1464-219 000-9 200 MHz primary 5.337, 5.471, 5.4735A N/A M. 1796-2

4.3 Other aeronautical radionavigation spectrum

There are currently twelve other frequency bands allocated to the ARNS that are used to support a variety of ARNS applications. These bands are listed in Table 5. There are existing standards for the systems operating in the ARNS systems in many of these frequency bands. It should be noted that some of these frequency bands may be appropriate for ABS&A or GBS&A radars.

TABLE 5

Other ARNSaeronautical radionavigation service allocations

Band StatusApplicable RR

(Edition of 200816) footnotes

Exampleof Aviation Standards

ITU-RRecommendat

ions190-285 kHz primary 5.68, 5.69, 5.70, 5.71 C41d(1)

325-405 kHz primary 5.72 C41d(1)

415-435 kHz primary 5.77, 5.805.72 C41d(1)

510-535 kHz primary 5.845.72 C41d(1)

74.8-75.2 MHz primary 5.180, 5.181 C35d(2)

108-117.975 MHz primary 5.197, 5.197A C36e(3), C40c(4)

328.6-335.4 MHz primary 5.258, 5.259 C34e(5)

960-1 215 MHz primary 5.328, 5.328A, 5.328AA C66c(6)

5 000-5 030 MHz primary 5.3675.443AA

TABLE 5 (end)

Band StatusApplicable RR

(Edition of 201608)

footnotes

Exampleof Aviation Standards

ITU-RRecommendations

5 030-5 150091 MHz primary 5.3675.443C, 5.443D, 5.444, 5.444A, 5.446, 5.447, 5.447B, 5.447C

C104(7)

5091-5150 MHz primary 5.443AA, 5.444, 5.444A, 5.444B

C104(7) M.1827-1

5 150-5 250 MHz primary 5.367, 5.444, 5.444A, 5.446, 5.447, 5.447B,

M.1454-0,M.2007-0,

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5.447C S.1426-0, S.1427-1

15.400-15.700 GHz primary 5.511A, 5.511C, 5.511D

C63c(8)

C212(9) M.1730-1,S.1340-0, S.1341-0

(1) Department of Transportation, Federal Aviation Administration, Aircraft Certification Service, Washington DC, Technical Standard Order TSO-C41dTSO-C41d, Airborne Automatic Direction Finding (ADF) Equipment, 6 May 1985.(2) Department of Transportation, Federal Aviation Administration, Aircraft Certification Service, Washington DC, Technical Standard Order TSO-C35dTSO-C35d, Airborne Radio Marker Receiving Equipment, 5 May 1971.(3) Department of Transportation, Federal Aviation Administration, Aircraft Certification Service, Washington DC, Technical Standard Order TSO-C36eTSO-C36e, Airborne ILS Localizer Receiving Equipment Operating within the Radio Frequency Range of 108-112 Megahertz (MHz), 25 January 1988.(4) Department of Transportation, Federal Aviation Administration, Aircraft Certification Service, Washington DC, Technical Standard Order TSO-C40cTSO-C40c, VOR Receiving Equipment Operating within the Radio Frequency Range of 108-117.95 Megahertz (MHz), 25 January 1988.(5) Department of Transportation, Federal Aviation Administration, Aircraft Certification Service, Washington DC, Technical Standard Order TSO-C34eTSO-C34e, Airborne ILS Glide Slope Receiving Equipment Operating within the Radio Frequency Range of 328.6-335.4 Megahertz (MHz), 15 January 1988.(6) Department of Transportation, Federal Aviation Administration, Aircraft Certification Service, Washington DC, Technical Standard Order TSO-C66cTSO-C66c, Distance Measuring Equipment (DME) Operating within the Radio Frequency Range of 960-1215 Megahertz (MHz), 18 January 1991.(7) Department of Transportation, Federal Aviation Administration, Aircraft Certification Service, Washington DC, Technical Standard Order TSO-C104TSO-C93, Microwave Landing System (MLS) Airborne Receiving Equipment, 22 June 1982.(8) Department of Transportation, Federal Aviation Administration, Aircraft Certification Service, Washington DC, Technical Standard Order TSO-C63eTSO-C63c, Airborne Weather and Ground Mapping Pulsed Radars, 18 August 1983 Equipment, 1 October. 2016.(9) Department of Transportation, Federal Aviation Administration, Aircraft Certification Service, Washington DC, Technical Standard Order TSO-C212, Air-to-Air Radar (ATAR) for Traffic Surveillance, 22, September 2017.

4.4 Radionavigation spectrum

There are currently twelve frequency bands allocated to the radionavigation service that could be used to support a variety of ARNS applications. The frequency bands are listed in Table 6. It should be noted that some of these frequency bands may be appropriate for ABS&A or GBS&A radars.

TABLE 6

Radionavigation allocations for ARNS airborne radars

Band StatusApplicable RR

(Edition of 2016) footnotes

Exampleof

Aviation Standards

ITU-RRecommendatio

ns

24.25-24.65 GHz Primary (Region 2 & 3)

C212(1)

31.8-32.0 GHz primary 5.547, 5.547A, 5.548 M.1466-1

32.0-32.3 GHz primary 5.547, 5.547A, 5.548 M.1466-132.3-33.0 GHz primary 5.547, 5.547A, 5.548 C212(1) M.1466-1

33.0-33.4 GHz primary 5.547, 5.547A C212(1) M.1466-143.5-47.0 GHz primary

66.0-71.0 GHz primary95.0-100.0 GHz primary

123.0-130.0 GHz primary

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191.8-200.0 GHz primary235.0-238.0 GHz primary

252.0-265.0 GHz primary(1) Department of Transportation, Federal Aviation Administration, Aircraft Certification Service, Washington DC, Technical Standard Order TSO-C212, Air-to-Air Radar (ATAR) for Traffic Surveillance, 22, September 2017.

5 ConclusionsSummaryThis Report provides an explanation of the requirements for S&A sensors to support UAS operations as well as the current status of allocations that allow this usage. Tables 3 through 56 provide alternative bands that have allocations for airborne and ground-based applications of UAS S&A. As discussed in § 3, ABS&A systems are constrained by UA SWaP. Also, weather or atmospheric attenuation must be considered when choosing spectrum for ABS&A because of the impact in meeting the operational requirements of the S&A function described in § 2. GBS&A on the other hand is largely free from SWaP considerations and are less constrained by weather and atmospheric attenuation. Thus, selection of the most suitable band for any particular UAS S&A application must consider performance aspects, which may be constrained due to available technology. Many of the existing frequency bands allocated to the ARNS or radionavigation service are already in use and the existing users of these bands will need to be considered when selecting ARNSfrequency bands for S&A applications.

GLOSSARY

ABS&A Aircraft-based sense and avoid

ACAS Airborne collision avoidance system

ADS-B Automatic dependant surveillance broadcast

ARNS Aeronautical radionavigation service

ATC Air traffic control

BLOS Beyond line-of-sight

CNPC Control and non-payload communications

EESS Earth-exploration satellite service

EMI Electromagnetic interference

FSS Fixed-satellite service

GBS&A Ground-based sense and avoid

ICAO International Civil Aviation Organization

IFR Instrument flight rules

ITU-R International Telecommunications Union Radiocommunications Sector

MSS Mobile-satellite service

NMAC Near-miss aircraft collision

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RF Radio frequency

RNSS Radionavigation-satellite service

S&A Sense and avoid

SWaP Size, weight and power

TCAS Traffic collision avoidance system

TSO Technical standard order

UA Unmanned aircraft

UACS Unmanned aircraft control station

UAS Unmanned aircraft system

WRC World Radiocommunication Conference

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