Proceedings of ESAV'08 - September 3 - 5 - Capri, Italy

Engineering a US national AutomaticDependent Surveillance - Broadcast (ADS-B)

radio frequency solutionDr. Ronald Bruno Member, IEEE, and Glen Dyer Member, IEEE,

ITT Advance Engineering and Sciences12975 Worldgate Drive, 20170-6008 Herndon, Virginia USA

phone: 703-668-6000, fax: 703-668-6005 email: ron.bruno@itt.com

Abstract- This paper presents the methodology and results ofthe engineering effort that culminated in the ITT Corporationsolution for a System that provides a comprehensive set of ADS-Bservices in the entire airspace of the United States. It includes anoverview of the requirements to which the System had to bedeveloped, the constraints within which the System had tooperate, the methods through which requirements/constraintswere mapped to a System solution, and the final solutiondesigned by ITT. The paper will focus on the engineering TrafficInformation Service - Broadcast (TIS-B) and ADS-B surveillanceservices representative examples of the solution for all the ADS-Bservices.

I. INTRODUCTION

On August 31, 2007, the Federal Aviation Administration(FAA) awarded ITT a contract to develop and roll out aSystem to provide a comprehensive set of ADS-B services inthe United States. Prior to and since that milestone, ITTconducted numerous analyses and trade studies that providedthe basis for the overall System design. ADS-B services in theUnited States are based upon two non-interoperable data linktechnologies as follows:

• 1090 MHz Extended Squitter (1090ES): this data linkis applicable primarily to commercial aviation aircraft

• Universal Access Transceiver (UAT): this data linkoperates at 978 MHz and is applicable primarily togeneral aviation aircraft

Services provided by the system include surveillance to FAAair traffic control and a variety of 'in cockpit' services topilots. Figure 1 illustrates the ADS-B services required:

• ADS-B Surveillance of 1090ES and UAT aircraft toFAA Air Traffic Control

• ADS-B Rebroadcast (ADS-R): an 'in cockpit' servicethat supports translation between 1090ES and UATmessages to provide pilots awareness of aircraft with adifferent data link equipage.

• Traffic Information Services - Broadcast (TIS-B): an'in cockpit' service that provides pilots awareness ofaircraft that are not ADS-B equipped

• Flight Information Service - Broadcast (FIS-B): an 'incockpit' service that provides pilots Meteorological andAeronautical Information

Deployment of this system has commenced with theimplementation ofTIS-B and FIS-B services in the enrouteairspace over South Florida.

II. SYSTEM REQUIREMENTS AND CONSTRAINTS

The overall requirements of the system are to provide CriticalServices (ADS-B and ADS-R) and Essential Services (TIS-Band FIS-B) for the entire United States National AirspaceSystem (NAS). There are a number ofperformancerequirements associated with each of these services as well asa number of constraints. The key System constraints are thedata link parameters of A1 class avionics, specified aircrafttraffic densities, the 1090 MHz interference environment, andprotection limits to existing secondary surveillance radars(SSR).

A. System Service Requirements

The key System requirements are embodied in a number ofTechnical Performance Measures (TPM) includingavailability, latency, and information update interval, andapply to aircraft equipped with either 1090ES or UAT datalinks. Critical ADS-B service has an availability requirementof 0.99999, an aircraft position latency of700 ms, and arequirement to provide position at a defined update intervalthat depends upon the airspace domain: 1 second for Surface,3 seconds for Terminal and 6 seconds for Enroute. EssentialTIS-B service has an availability requirement of 0.999, aposition latency of 1500 ms, and a requirement to provideequipped aircraft with nearby non-equipped aircraft positionsat an update interval that depends upon the airspace domain: 2seconds for Surface, 6 seconds for Terminal and 12.1 secondsfor Enroute. For both ADS-B and TIS-B service the updateintervals must be met with a 95% probability.

B. System Constraints: 1090 MHz Interference

Because the 1090 MHz frequency is used for many othersurveillance applications (Mode S and ATCRBS), the Systemneeds to account for and work in high interferenceenvironments. Figure 2 provides example of the specifiedATCRBS Mode AlC interference at the aircraft that theSystem needs to operate under. Note that it contains a familyof curves corresponding to interference environments fromlow to very high. Each environment is specified as acumulative distribution of the number of False Replies

l090ES

Proceedings of ESAV'08 - September 3 - 5 - Capri, Italy

Cross-linking of ADS-B data for AIrcraft SituatIonal Awareness I

Figure 1: ADS-B Services to be Provided Over the US NAS

Unsynchronized in Time (FRUIT) per second vs. at a definedpower. Also, each interference scenario incorporatesinterference from Mode S as well as 1090ES from a highdensity of aircraft traffic (e.g., up to 400 aircraft in a largeterminal airspace).

C. System Constraints: A1 Avionics

The data link parameters of A1class avionics are a keydeterminant of the overall link budget for both downlink(ADS-B) and uplink (TIS-B) services. Table 1 contains thedownlink aircraft parameters in the context of the baseline linkbudgets for the terminal and enroute domains, where the

Figure 2: ATCRBS Interference (Mode AlC FRUIT)

200000

180000

160000

140000

u 120000II

~ 100000II

a:: 80000

60000

40000

20000

-90 -80 -70 -60Amp (dbm)

-50 -40 -30

nominal maximum ranges are assumed to be 60 and 120nautical miles, respectively. An Al avionics installation has aminimum transmit power of 125 watts (or 51 dBm), which isspecified by the 1090 MOPS to be at the antenna inputs.Assuming a minimum gain of 0 dBi for the antennainstallation, this results in an EIRP of 51 dBm, which is takenas the benchmark for the lowest transmit power that will beaccommodated reliably in the Service Volume (SV) design.The A1 class avionics parameters applicable to the uplink TIS­B budget are illustrated in Table 2. The driving avionicsparameter for the uplink is the required power at the aircraftreceiver to achieve the needed detection probability for asingle uplink message. This is determined by the Al classequipage receiver sensitivity and interference degarblingperformance specified in the MOPS [1].

A1 receivers are assumed to use only a minimum set of theenhanced reception capabilities described in the MOPSAppendix I. The MOPS also provides a detailed testprocedure to measure the performance of the receiver indecoding the 1090ES preamble and data block overlappedwith Mode Ale and Mode S FRUIT, and for the Al receiver,the success criteria are provided by Tables 2-157 and 2-160.Since the interference can occur at random at any time, with aknown rate, provided by the Figure 2 FRUIT curves, a Poissondistribution is the best method to model the probabilisticbehavior of this environment. Therefore the probability of acorrect 1090ES message reception is computed by using the

Proceedings of ESAV'08 - September 3 - 5 - Capri, Italy

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&':0co

..0ea..c:oa<Do~

-80 -75 -70 -65Received Signal Strength (dBm)

Reception Probability withCumulative FRUIT Rateat 5 dB below Rx SignalStrength

• 1 Mode NC ~ 89%• 2 ModeNC~ 64%• 3 Mode NC ~ 52%Rx with 1 Mode Sat 5 dB

-60 -55

Figure 3: Airborne Al Receiver Performance with Interference

Poisson probability of k occurrences of overlaps (with k = 0,1, 2, 3 for ATCRBS and k = 0, 1 for Mode S). The cumulativeFRUIT rates used in the Poisson equation were taken from theprovided curves at 5 dB below the amplitude level of thedesired signal, and each probability of overlapping occurrencehas been scaled with the factors provided by the MOPS.Using the specified MOPS performance, Figure 3 wasgenerated to illustrate the predicted probability of correctreception of an A1 receiver for each interference environmentspecified in Figure 2.

D. System Constraints: Victim Receiver

While ADS-B service provision requires RF coverage over theentire NAS, there are concurrent requirements to limit thenumber of 1090 MHz transmissions and to maintain the powerlevels at secondary surveillance radars (SSR) below thespecified protection limit of -65 dBW1m2 at all times. BecauseTIS-B service requires the transmission of messages at 1090MHz, Radio Stations must be located away from SSRs.

III. METHODOLOGY FOR SYSTEM DESIGN DEVELOPMENT

The development of the System design was driven by the needto provide robust RF coverage and meet the required ADS-Band TIS-B update intervals on the uplink and downlinkeverywhere in the NAS.

A. Robust RF Coverage

For the prediction of the received signal from aircraft(downlink) and signal in space (uplink), we turned to ITU-RRecommendation P.528-2 [2], "Propagation Curves/orAeronautical Mobile and Radionavigation Services using theVHF, UHF, and SHF Bands," as an authoritative source ofguidance. ITU-R P.528-2 notes the aeronautical serviceprovides a safety of life function and therefore requires ahigher standard of availability than many other services.Accordingly, ITU recommends the use of the IF-77 Johnson­Gierhart Model for RF coverage modeling to determine thebasic transmission loss and further recommends a time

Table 1: Downlink ADS-B Link Budget Table 2: Uplink TIS-B Link Budget

Slant Range (Nm) 60 i20 Slant Range (Nm) 60 120

Aircraft Transmitted EIRP (dBm) 51 51 Radio Transmitted Power (dBm) 59.5 59.5

Free Space Path Loss (dB) 134.1 140.1 Tx Cable Loss (dB) 3 3

Rx Antenna Gain (dBi) 12.7 12.7 Tx Antenna Gain (dBi) 12.7 12.7

Rx Cable Loss (dB) 3 3 Free Space Path Loss (dB) 134.1 140.1

Ground Received Power (dBm) -73.4 -79.4 Aircraft Received Power (dBm) -64.9 -70.9

Ground Receiver Required Power (dBm) -82 -88 Aircraft Receiver Required Power (dBm) -74 -74

Margin (dB) 8.6 8.6 Margin (dB) 9.1 3.1

Required Confidence Level (%) 95 95 Required Confidence Level (%) 95 95

Required Update Interval (seconds) 3 6 Required Update Interval (seconds) 6 12

Reception Chances per Update Interval 6 12 Reception Chances per Update Interval 6 6

Required Message Detection Probability 0.39 0.22 Required Message Detection Probability 0.39 0.39

Proceedings of ESAV'08 - September 3 - 5 - Capri, Italy

170160150140

Freq =1090 MHzTx Antenna = 5100A-DTx Height = 110 ftRx = 18,000 ft

13012080 90 100 110Distance From Tx (nm)

rJ)

~ 14

:615co

a.

16

17010 20 30 40 50 60 70

Figure 4: Coverage Model Predictions and Comparisons

availability of 0.95 to provide a reliable service. However,since the IF-77 model does not consider terrain, we conducteda trade study of alternative propagation models that consideredterrain, but agreed with IF-77 where terrain was not a factor.That study led to the selection of the eRe Predict® model forRF coverage prediction.

eRe Predict® is a deterministic RF propagation predictiontool that uses a quasi-exact two-dimensional Fresnel-Kirchoffwave-front method employing a chained calculation ofsuperimposed specular reflection and diffraction based onHuygens principle. A more complete description is providedby Whitteker [3]. The eRe Predict model was validatedagainst the IF-77 Johnson-Gierhart Model bothcomputationally, and through flight checks. Figure 4compares the modeling performance of the IF-77 model withthat of the eRe Predict model and against free space lossesfor two cases of time variability: 95% (T95) and 50% (T50).The figure shows close agreement between the two models atboth 50% and 95%. As shown, a time variability of 50%,representing median path losses, closely follows free spacepath loss. Most significant is the fact that the T95 curves havea significant excess path loss relative to the T50 and free spacepredictions. This fact was a major driver in the design of thelink performance parameters for the Radio Stations for theSystem.

B. Meeting the ADS-B and TIS-B Update Intervals

ADS-B and TIS-B update interval requirements are met byproviding a target signal power while taking advantage ofmultiple reception opportunities over each update interval.Table 1 summarizes this approach for the ADS-B downlink.In this table, the power received at the Radio Station is derivedassuming free space path loss and 3 dB cable loss between theantenna and the Radio. The required power at the receiver ineach domain is driven by the requirement to achieve an ADS­B position in each update interval with a 95% confidencelevel. The 95% confidence is met by a much lower singlemessage detection probability on multiple message receptionopportunities each update interval. Table 1 applies to aircraftat the farthest range envisioned in the terminal and enroutedomains, where the squitter from both top and bottomantennas may be received at a Radio Station. Thus, in theterminal domain, the Radio Station has 6 chances to receive an

ADS-B position message in the 3 second terminal updateinterval, and 12 chances in the 6 second enroute updateinterval. This is supported by the TLAT study [4] andmeasurements of aircraft gain patterns that show both top andbottom antennas have favorable gain patterns at low elevationangles. The 39% single hit probability for the terminaldomain ensures that at least one of the 6 messages is received(at 95% confidence). Moreover, the Radio Station designrequires a power of -82 dBm for a 39% detection probabilityso that the link margin is 8.6 dB. A similar deduction of an8.6 dB margin for the enroute domain is based on a -88 dBmreceived power and 12 message reception opportunities in the6 second update interval.

For meeting the required TIS-B update interval, a similarapproach is taken. For TIS-B service, a radar position updateof a non-equipped aircraft triggers the System to broadcast aset of TIS-B messages to equipped aircraft in the proximity.In terminal and enroute domains, radar position updates occurat periodic intervals (e.g., 6 and 12.1 seconds, respectively fora worst case where there is only coverage by a single radar).In response to each radar position update, the Systembroadcasts two message sets from a chosen Radio Station,each composed of three messages: even and odd positionmessages, plus a velocity message. Thus, 6 messages aretransmitted each update interval for both terminal and enroutedomains. To achieve a 95% confidence of meeting the updateinterval requirement, the detection probability of a singlemessage needs to be 0.39, and the receiver performance curvesof A 1 class aircraft in Figure 3 indicates that a signal in spaceof -74 dBm (at the Al class aircraft omni antenna) must besupplied to meet this probability. This results in an uplinkmargin of9.1 dBm and 3.1 dBm in the terminal and enrouteenvironments, respectively.

C. Protection of Victim Receivers

If not for the -65 dBW/m2 protection limit at 1090 MHz forSSRs, the optimum placement of System Radio Stationswould be close to the center of a service area (e.g., one RadioStation in the center of a terminal domain). Such a RadioStation could provide complete and reliable coverage over the60 nautical mile radius of a terminal domain. However, basedon the protection limit and the EIRP of the Radio station, a17.4 nautical mile separation is needed between any SRR and

Proceedings of ESAV'08 - September 3 - 5 - Capri, Italy

Figure 5: Radio Station Placement forInterference Protection of SRRs

a Radio Station, and so two Radio Stations are needed toprovide robust RF coverage over the tenninal domain. Figure5 illustrates the approach establishing a search ring around atenninal domain with an SRR at the center. In this case, twoRadio Stations are sited no closer than 17.4 nautical miles andno further than 28 nautical miles from the SRR. In addition toproviding the required interference protection, this approach topositioning Radio Stations leads to very robust coverage of thetenninal domain in that it provides overlapping coverage fromtwo Radio Stations. Thus, an aircraft will always have at leastone Radio Station in view of the active antenna even duringbanking maneuvers.

IV. SYSTEM ARCHITECTURE

The System is comprised ofRadio Stations, Network andControl Segments. The Control Segment is centralized withfour Control Stations that will be located in existing ultra-highavailability data centers. The Control Stations process andmanage all communications with the Radio Stations andsupport all interfaces with FAA air traffic control, radar datapickup and System independent monitoring. The NetworkSegment is provided by an existing data network, which offersdedicated bandwidth, low latency, high availability, andsecurity. Its backbone core is a Multi-Protocol Label

1090Antennas

Figure 6: Radio Station Architecture

Switching (MPLS) network that provides a Virtual PrivateNetwork (VPN). Radio Stations connect with Control Stationsthrough MPLS backbone via dedicated, redundant, and diversecircuits.

The Radio Station Segment provides the required RF coverageand supports the air interface at both 1090 and V ATfrequencies for all designated ADS-B services. Driven by theneed to provide both robust RF coverage and SRR interferenceprotection throughout the NAS, the Radio Station Segmentembodies over 800 Radio Stations that will be distributed overthe NAS. Figure 6 illustrates the typical Radio Station design,with four (4) antennas at 1090 MHz that provide sectorizedcoverage in concert with primary and backup 4-channel 1090radios. The sectorized approach is driven by the need for highgain on both the ADS-B downlink and the TIS-B uplink aswell its effect on decreasing the amount of FRVIT seen ateach 1090 MHz radio channel. Figure 6 also illustratesredundant VAT antennas (primary and backup), which aresupported by primary and backup signal channel VAT radios.Figure 7 illustrates the location of all Radio Stations as well asthe coverage that the System will provide over the NAS. Thecoverage is indicated at altitudes of 1800, 5100, 18,000 and24,000 feet above mean sea level (MSL). Coverage at 1500feet MSL is also indicated over the Gulf of Mexico assupported by Radio Stations located on existing offshoreplatfonns for oil extraction.

The System implementation in South Florida is the first step ina nationwide rollout of Essential and Critical Services. TheSouth Florida implementation of TIS-B and FIS-B EssentialServices is being evaluated by ITT and the FAA in support ofan in-service decision (ISD) by the FAA in the 4th quarter of2008. After the Essential lSD, ITT will proceed with therollout of Essential Services over the entire NAS.

Proceedings of ESAV'08 - September 3 - 5 - Capri, Italy

Figure 7: Coverage Model Predictions of System Design Over the NAS

The Essential ISD will be followed by the implementation ofADS-B and ADS-R Critical Services in Louisville, KYandPhiladelphia, PA tenninal and surface airspaces, as well asenroute airspaces in the Gulf of Mexico and Juneau, AK.These four implementations of Critical Services will beevaluated by ITT and the FAA in support of a CriticalServices ISD in 2010. Following this lSD, ITT willcommence the rollout of Critical Services over the entireNAS.

REFERENCES

[1] RTCA DO-260A, Minimum Operational Performance Standards[MOPS] for 1090ES ADS-B and TIS-B, RTCA Inc.

[2] Rec. ITU-R P.528-2 1, Recommendation ITU-R P.528-2, PropagationCurves for Aeronautical Mobile and Radionavigation Services Using theVHF, UHF and SHF Bands

[3] Whitteker, James H. (Jim) Physical, Optics and Field-StrengthPredictions for Wireless Systems, IEEE Journal on Selected Areas InCommunications, VOL. 20, NO.3, APRIL 2002, pp. 515-522

[4] ADS-B Technical Link Assessment Team (TLAT), Technical LinkAssessment Report, March 2001

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