october trial report - asas tn · 2020. 6. 15. · eec/sur6e1/wp/006, astp, dec. 1999 4. phase one...

Eurocontrol Experimental CentreBrétigny-sur-Orge, FRANCE

Eurocontrol ADS ProgrammeTechnology Assessment Task

1999 ADS-B TrialsPart I

October Trial Report

EEC Ref: EEC/SUR6E1/AC/015Version: 0.6Issue date: 24-Dec-99Authors: C. Tamvaclis

G. RambaudL. Rabeyrin

Abstract

This report presents the first round of ADS-B trials organised in 1999 by the EurocontrolADS Programme in the context of the ADS Technology Assessment Task. The objectives ofthese trials were to measure performance and assess the maturity of three candidate ADS-Btechnologies, namely Mode S Extended Squitter, VHF Datalink Mode 4, and Universal Ac-cess Transceiver. The first trial round focused on air to ground performance. Two trial air-craft were equipped with all three technologies. Base stations were installed at the Experi-mental Centre. The 1st round trial flights took place in the period 6- 12/10/1999. This docu-ment describes the equipment configuration used, the flight scenarios, and presents analysisresults of the data logs collected in the trial flights.

Eurocontrol ADS-B Trials 1999 - 1st Round Version: 0.6

Ref. EEC/SUR6E1/AC/015 2

DOCUMENT CONTROL LOG

SECTION DATE Version REASON FOR CHANGE OR REFERENCE TO CHANGE

25-Oct-99 0.1 1st Draft

12-Nov-99 0.5 2nd Draft

24-Dec-99 0.6 3rd Draft



Table of ContentsREFERENCES............................................................................................................................................. 5

ACRONYMS AND ABBREVIATIONS ..................................................................................................... 6

1 INTRODUCTION ................................................................................................................................ 8

1.1 DOCUMENT OBJECTIVES ................................................................................................................. 81.2 BACKGROUND ................................................................................................................................. 81.3 1999 TRIAL OBJECTIVES ................................................................................................................. 91.4 ADS-B PERFORMANCE MEASURES................................................................................................. 91.5 SCOPE.............................................................................................................................................. 91.6 DOCUMENT OVERVIEW ................................................................................................................. 101.7 ACKNOWLEDGEMENTS.................................................................................................................. 11

2 TRIAL CONFIGURATION.............................................................................................................. 12

2.1 AIRCRAFT...................................................................................................................................... 122.2 GROUND VEHICLES ....................................................................................................................... 132.3 BASE STATIONS............................................................................................................................. 13

3 TRIAL SCENARIOS ......................................................................................................................... 15

3.1 FLIGHT REQUIREMENTS AND CONSTRAINTS................................................................................... 153.2 FLIGHT PROFILES .......................................................................................................................... 153.3 TEST PROCEDURES ........................................................................................................................ 16

3.3.1 Pre-flight tests....................................................................................................................... 163.3.2 Session I................................................................................................................................ 163.3.3 Session II .............................................................................................................................. 163.3.4 Session III ............................................................................................................................. 16

4 EXECUTION OF THE TRIAL ........................................................................................................ 17

4.1 PREFLIGHT TESTS .......................................................................................................................... 174.1.1 Cessna Installation ............................................................................................................... 174.1.2 Ilyushin Installation.............................................................................................................. 18

4.2 SESSION I....................................................................................................................................... 184.3 SESSION II ..................................................................................................................................... 204.4 SESSION III .................................................................................................................................... 21

5 ANALYSIS OF LOGGED DATA..................................................................................................... 23

5.1 ANALYSIS METHOD....................................................................................................................... 235.2 SESSION I : 6/10/99 – 1090 EXT. SQUITTER AND VDL-4 .............................................................. 24

5.2.1 Aircraft Trajectories ............................................................................................................. 245.2.2 Message Success Rates ......................................................................................................... 29

5.3 SESSION II: 8/10/99 – VDL-4 AND 1090 EXT. SQUITTER.............................................................. 315.3.1 Aircraft Trajectory................................................................................................................ 315.3.2 Message Success Rates ......................................................................................................... 33

5.4 SESSION III: 12/10/99 – UAT AND VDL-4 ................................................................................... 365.4.1 Aircraft Trajectories ............................................................................................................. 365.4.2 Message Success Rates ......................................................................................................... 39

6 CONCLUSIONS................................................................................................................................. 42

7 ANNEX A............................................................................................................................................ 44

7.1 TECHNOLOGY CHARACTERISTICS.................................................................................................. 447.2 INSTALLATIONS............................................................................................................................. 44

8 ANNEX B: 1090 EXT. SQUITTER TRIAL IN TOULOUSE ........................................................ 49



8.1 TRIAL SCENARIO ........................................................................................................................... 498.2 ANALYSIS OF LOGGED DATA ......................................................................................................... 49

8.2.1 Ext. Squitter Position Trajectories........................................................................................ 498.2.2 Ext. Squitter Success Rate..................................................................................................... 508.2.3 TCAS Acquisition Squitters................................................................................................... 52



REFERENCES1. ADS Technology Assessment Task Specification, Version 1.0, Ref.

DED.3/SUR/ADS.TSK.99.001, Eurocontrol ADS Programme, Sept. 1999

2. Eurocontrol ADS-B Trials 1999 Part II, Ref. EEC/SUR6E1/AC/021, ASTP, Jan. 2000

3. FREER-3 Trials ADS-B over VHF-STDMA Performance Analysis, Version 1.0, Ref.EEC/SUR6E1/WP/006, ASTP, Dec. 1999

4. Phase One Link Evaluation Report, SF21 Technical/Certification Subgroup ADS-B LinkEvaluation Team, Nov. 1999

5. ADS-B MASPS, DO-242, RTCA, Jan. 1998

6. ARTAS 2 Specifications, EATCHIP Document, Nov. 1996



ACRONYMS and ABBREVIATIONSADS Automatic Dependent Surveillance

ADS-B Automatic Dependent Surveillance - Broadcast

ARTAS ATC Radar Tracker And Server

ASTP ADS Studies and Trials Project (EEC)

ATC Air Traffic Control

CAA Civil Aviation Authority

CCI Co-Channel Interference

CDTI Cockpit Display of Traffic Information

CEV Centre d�Essais en Vol (Brétigny-France)

CPR Compact Position Reporting

CRC Cyclic Redundancy Code

DERA Defense and Evaluation Research Agency (UK)

DGAC Direction Générale de l�Aviation Civile (French CAA)

EATMP Eurocontrol Air Traffic Management Programme

ECAC European Civil Aviation Conference

EEC Eurocontrol Experimental Centre

EUROCAE European Organisation for Civil Aviation Electronics

FAA Federal Aviation Administration (USA)

FEC Forward Error Correction

GFSK Gaussian Frequency Shift Keying

GosNIIAS State Research Institute of Aviation Systems (Russia)

GPS Global Positioning System

LDPU Link Data Processing Unit (UPS/AT product)

MASPS Minimum Aviation System Performance Standards

MSR Message Success Rate

MTL Minimum Trigger Level

NLR National Aerospace Laboratory (Netherlands)

nmi nautical miles

PPM Pulse Position Modulation

PSU Power Supply Unit

RF Radio Frequency

RTCA Radio Technical Commission for Aeronautics (USA)

SCAA Swedish Civil Aviation Authority (also known as LFV)

SF21 Safe Flight 21

SNR Signal to Noise Ratio

SSR Secondary Surveillance Radar



STDMA Self Organising Time Division Multiple Access

STNA Service Technique de la Navigation Aérienne (part of DGAC)

SV State Vector

TCAS Traffic alert and Collision Avoidance System

TMA Terminal Manoeuvre Area

UAT Universal Access Transceiver

UPS United Parcel Service

UPS/AT UPS Aviation Technologies (formerly known as II Morrow)

VDL-4 VHF Datalink Mode 4

VHF Very High Frequency

VSWR Voltage Standing Wave Ratio



1 Introduction1.1 Document ObjectivesThis document presents results from the first round of ADS-B flight trials organised in 1999 by theEurocontrol Experimental Centre (EEC) ADS Studies and Trials Project (ASTP). These trials in-volved three ADS-Broadcast (ADS-B) technologies, namely

• VHF DataIink Mode 4 (VDL-4) operating at 136.975 MHz• Mode S Extended Squitter (also known as 1090 Extended [or Long] Squitter) operating at

1090 MHz• Universal Access Transceiver (UAT) operating at 966 MHz

The trials were organised as part of the activities of the ADS technology Assessment Task of theEurocontrol ADS Programme [1].

1.2 BackgroundThe Eurocontrol Automatic Dependent Surveillance (ADS) Programme1 is part of the EurocontrolAir Traffic Management Programme (EATMP) and aims towards the harmonised implementationof an ADS infrastructure in ECAC (European Civil Aviation Conference) capable of supporting thefull �Gate-to-Gate� concept.

The ADS Technology Assessment Task [1] of the Eurocontrol ADS Programme will evaluate thevarious ADS datalink technologies and determine their characteristics. The information collectedby this task will serve as input to the Business Case for ADS in ECAC in order to select the mostappropriate ADS-B datalink technology (or combination of technologies).

As part of the ADS technology assessment work in 1999, Eurocontrol decided to organise com-parative flight trials of the three main candidate ADS-B technologies [listed in Sec. 1.1]. Thesetrials consisted of two rounds. The first round focused on measuring the basic performance char-acteristics of the candidate ADS-B technologies for air-to-ground operation. The second round(done in December 1999) focused on air-to-air performance characteristics.

The first round trial flights were originally scheduled for the 29 and 30/9/99 but finally took placeon the 6, 8, and 12/10/99 for reasons explained in Sec. 4. Two aircraft and a ground vehicle par-ticipated in these trials. The following organisations contributed by providing equip-ment/services/technical support :

• Centre d� Essais en Vol (Brétigny, France)• Eurocontrol Mode S Programme• FAA Tech. Centre (USA)• GosNIIAS (Russia)• Swedish CAA• Melun Aerodrome Authority (France)• NLR (Netherlands)• Saab Celsius Transpondertech AB (Sweden)• UPS Aviation Technologies (USA)

The two aircraft were equipped with all three ADS-B technologies. Ground stations were de-ployed at the Eurocontrol Experimental Centre in Brétigny. The aircraft were supposed to fly inparallel in the air space around Paris and transmit ADS-B reports, which were to be logged on theaircraft and on the ground stations. Concurrently radar data were to be collected from the SSRradars located at Orly and Palaiseau. 1 More information can be found in the ADS Programme web page:http://www.eurocontrol.be/projects/eatmp/ads/default.htm



In addition to the two ADS-B trial rounds, ASTP analysed in 1999 results from two other ADS-Brelated trials, namely the 1090 Ext. Squitter Trial in Toulouse and the FREER-3 Trial of VHF-STDMA for airborne separation assurance. Results of the Toulouse trial are described in AnnexB. FREER-3 trial data analysis is described in Ref. [3]. The results of those preliminary trialsserved as benchmarks for comparison with the ADS-B trial results in Paris.

ADS-B Trials have also been conducted in 1999 by FAA under the Safe Flight 21 (SF21) Initiative(SF21) in the Ohio Valley and Los Angeles, see Ref. [4]. The results of the Sf21 activities havebeen taken into account in the analysis of the Eurocontrol ADS-B trial data.

1.3 Trial ObjectivesThe aims of the Eurocontrol 1999 ADS-B Trials were [1]:

• Measurement of the basic datalink performance characteristics for each technology in theairspace around Paris [this airspace is one of the most heavily loaded traffic environments inEurope];

• Collection of data for the calibration of the models used in ADS-B simulations;

• Assessment and comparison of the ADS-B performances of the three technologies under thesame environment and flight conditions;

• Collection of data for evaluation of the tracking performance of the ARTAS-2 prototype sur-veillance data processing system [6] (which is capable of using ADS, Mode S, and multi-radar data)

1.4 ADS-B Performance MeasuresIn line with the methodology described in ref. [1], datalink performance was measured in terms ofthe probability of successful message delivery (also known as success rate) and its variance withdistance of the receiver from the transmitter. Success rate measurements were used to derivemathematically estimates of the effective ADS-B position update periods versus distance. Thecalculation method is explained in Sec. 5.1. The resulting update period versus distance curveswere compared with application requirements stated in the ADS-B MASPS [5], to obtain an indi-cation of ADS-B system range. ADS-B MASPS were selected as comparison baseline for rea-sons explained in Sec. 1.5.

Message Success Rates were calculated as follows: Messages2 are transmitted periodically,therefore the number of messages transmitted per time unit is known. Receiver logs can be usedto determine the number of messages received in a time unit. This number divided by the numberof messages transmitted provides an estimate of the average success rate within the time unit.The resulting success rate can be associated with the horizontal distance between transmitterand receiver, i.e. the great circle distance between transmitter and receiver at the middle of thetime unit.

1.5 ScopeThe following issues must be taken into account in interpreting the results presented in thisdocument

a) ADS-B standards have yet to be agreed in ICAO. There is an ADS-B standard published byRTCA [5] but it has not yet been adopted by either ICAO or EUROCAE. Consequently both

2 The concept of a �message� is different in each ADS-B technology. The transmitted message units arecalled squitters in Mode S, bursts in VDL-4, and messages in UAT. Furthermore they carry different subsetsof ADS-B data. All three technologies define multiple types of squitters or bursts or messages. An ADS-Bposition report update may require more than one message unit, depending on the technology. Successrate calculations in this document always refer to a message unit, (i.e. a squitter in Mode S, a burst in VDL-4, and a message in UAT) of a specific type. Sec. 5.1 explains the assumptions made for the position up-date rate calculations



the content of ADS-B messages and ADS-B system performance requirements have yet tobe agreed in Europe or indeed internationally.

b) ADS-B information to be included on VDL-4 messages is not yet finalised in the VDL-4 stan-dards. The frequency of VDL-4 transmissions for ADS-B purposes is also not finalised.

c) There are no UAT standards.

d) The existing standards for Mode S ext. squitter define both the ADS-B information to be in-cluded in the various squitter types and the frequency of transmission of the latter.

e) Most equipment used had prototype status. Furthermore antennas and wiring installationsused in the trials may have not been optimal for the technologies under test. It is thereforepossible that the results obtained in the trials are not indicating the best performance achiev-able by each technology. In any case, one of the trial aims was to assess the maturity of ex-isting implementations and not to build new implementations. Every effort was made to verifythe quality of the trial installations to ensure conformance with manufacturer specifications3.Installations were performed by people and organisations with long experience in avionics in-stallations.

f) In the analysis presented in this report, RTCA ADS-B MASPS requirements [5] were used asbaseline for comparisons and mathematical modeling, together with the parameter settingsadopted in Safe Flight 21 [4]. This does not imply that Eurocontrol ADS Programme hasadopted either the RTCA ADS-B MASPS requirements [5] or the link specifications of SF21.These documents were used in order to facilitate comparisons with the results of the ongoinglink evaluation of the FAA .

g) In practice ADS-B performance will be dependent on the number of participating ADS-B sta-tions (e.g. aircraft, vehicles, and base stations) as well as the available frequency spectrumfor transmission. The Eurocontrol 1999 ADS-B trials involved a very small number of stationshence the impact of input traffic load on performance could not be evaluated. Complex sce-narios with multiple ADS-B terminals are best tested through simulation, and the results ofthe Eurocontrol ADS-B trial will serve to tune the simulation models to be used in the capacityanalysis work of the ADS Technology Assessment.

.

1.6 Document OverviewThe contents of this report are organised as follows:

Sec. 2 describes the trial configuration, e.g. the types of aircraft and ADS-B equipment used.

Sec. 3 presents the flight scenarios and the test procedures.

Sec. 4 describes what happened during the execution of the trial flights.

Sec. 5 presents the analysis of the data logged during the trials.

Sec. 6 presents the conclusions drawn from the execution of the trials and the data analysis.

Annex A provides details of equipment configurations and installation.

Annex B: 1090 Ext. Squitter Trial in Toulouse presents the results of that trial.

3 Saab/Celsius and SCAA staff participated in the installation tests. It is regrettable that other equipmentproviders did not follow their example



1.7 AcknowledgementsThe conduct of this trial was made possible thanks to the contribution of the organisations listedin Sec. 1.2. SAAB/Celsius and SCAA contributed not only equipment but also sent members oftheir staff to Brétigny to assist in the trial. The teams from the aircraft providers (NLR and Gos-NIIAS) made great efforts to ensure the success of the trials

Besides the Eurocontrol ADS Programme Team, many EEC and Eurocontrol HQ people, too nu-merous to list, helped in the conduct of the trial. The EEC Mode S Team successfully handled allMode S related installation and test issues.

The organisation of the trial flights was made possible thanks to the huge contributions of ReneCamus (CEV), and Bernard Brunner (EEC).

Finally, the authors of this report note that Michel Biot (EEC) performed most of the analysis ofMode S trial data. His help is gratefully acknowledged.



2 Trial Configuration

2.1 AircraftThe following two aircraft were used in the 1st trial round:

• Cessna 550 Citation II, provided by NLR, see Figure 2• Ilyushin 18D, provided by GosNIIAS, see Figure 1

Figure 1 GosNIIAS Ilyushin 18D at Melun

Figure 2 NLR Cessna 550 Citation at Brétigny



The flight characteristics of the two aircraft were as follows:

Aircraft Endurance,hours

Rangenmi

CruisingSpeed4

(TAS)

Ceilingft

Rate of Climbfpm

Cessna 550 Citation II 3 :20 600-1600

360 Knots 43000 2900

Ilyushin 18D 8 3000 290 Knots 32000 1125

Each aircraft was meant to be equipped with the following ADS-B systems:

Technology Equipment AntennasVDL Mode 4 • one SAAB Celsius T4 Transceiver

• one SAAB Celsius WINLINCS datalogger

• one VHF omnidirectionalantenna (located at aircraftbottom)

• one GPS antenna1090 Ext. Squit-ter

• one Honeywell Transponder5

• one GPS receiver• one Dassault 1090 ADS-B receiver5

• two L-Band antennas (lo-cated top and bottom of thea/c)6

• one GPS antennaUAT • one UPS/AT LDPU7

• one FAA Tech Centre CDTI Simu-lator8

• two L-Band antennas (lo-cated top and bottom of thea/c)9

• one GPS antenna

The above equipment was loaned to Eurocontrol by the organisations indicated. Antennas weresupplied by the aircraft providers, who also carried out all the wiring installations required.

2.2 Ground VehiclesA single10 ground vehicle (Citroen Xantia, owned by the EEC) participated in the trial. The EECcar was fitted with a VDL-4 transceiver (same as the one used on the aircraft, see previous Sec-tion), a roof mounted VHF whip antenna, and a GPS antenna.

.

2.3 Base StationsA single base station for each technology was installed at the Eurocontrol Experimental Centre,Brétigny, using the following equipment:

4 at around FL 3005 supplied by the Eurocontrol Mode S programme6 Two TX and two RX L-Band antennas were required for Mode S, but neither aircraft was able to providefour L-band antennas for Mode S as well as two L-Band antennas for UAT.7 The UPS/AT LPDU contains a UAT transceiver and two Mode S Ext. squitter receivers. The latter were notused in this trial flights described in this report. They were used however in the 2nd trial round (see [2])8 Software running on portable PC equipped with ARINC 429 interface for connection to the LPDU.9 UAT Antenna installation at the bottom of the Cessna airframe was not feasible (see Sec. 4.1.1).10 Effort was made to get a second vehicle from STNA, Toulouse, for use with Mode S. Unfortunately STNAcould not send the car to Paris.



Technology Equipment AntennasVDL Mode 4 • one SAAB Celsius T4 Base Station

• one SAAB Celsius WINLINCS datalogger

• one VHF omni antenna11

• one GPS antenna

1090 Ext. Squit-ter

• one ERA Mode S Ground Station5

• one GPS receiver• one helical L-Band omni

antenna12

• one GPS antennaUAT • one UPS/AT LDPU

• one FAA Tech. Centre CDTI simu-lator

• one avionics L-Band bladeantenna13

• one GPS antenna

The EEC contributed a car which was equipped with a T4 transceiver (identical to the ones usedin the aircraft) with one VHF omni antenna and one GPS antenna..

Annex A lists the characteristics of each technology as deployed in the trial and includes installa-tion photographs.

11 KATHREIN model K512631, frequency range 116-152 MHz, 0 dB Gain. It was located at EEC building top� height 21 m.12 ERA L-Band antenna, 5 dB gain, with a 5 deg. cone of silence on the vertical plane and no cone of silenceon the horizontal plane. All the ground station antennas were placed at the roof of the EEC building at aheight of ~21 m.13 UPS/AT Model AT-130 antenna tuned at 966 MHz



3 Trial Scenarios3.1 Flight requirements and constraintsOriginally it was planned that both aircraft use the Brétigny aerodrome (administered by theCEV). Due to CEV constraints, only the Cessna was able to use the Brétigny aerodrome. TheIlyushin had to use the Melun aerodrome, which is located 15 nmi east of Brétigny.

Three flight sessions were planned. Each flight session were to last 2 hours and both aircraftwould participate. The aircraft should follow a racetrack profile (described in Sec. 3.2) that wouldprovide air to ground (i.e. to EEC) distances up to 200 nmi and air to air horizontal distances up to100 nmi. In practice only the first session fulfilled this requirement (for reasons explained in Sec.4).

The aircraft should maintain a constant flight level near FL 300, except for ATC constraints. Inpractice, the aircraft had to fly at FL 270 due to ATC constraints. Vertical separation was keptwithin 3000 ft.

3.2 Flight ProfilesFigure 3 shows the planned flight trajectories of the two aircraft. The first flight leg (~ 1 hour)would be in the direction of Brest (west of Paris) reaching a distance of 200 nmi from Brétigny(points B, B�). The two aircraft were to fly in parallel maintaining a lateral separation of about 50nmi and a longitudinal. separation of <= 10 nmi.

Then both aircraft would do a 180 deg. turn and the Cessna would gradually increase its lateraldistance from the Ilyushin to about 100 nmi (points C, C�). The two aircraft would then graduallyapproach to about 10 nmi laterally (points D, D� and stay within this lateral separation until ap-proaching the airports. The longitudinal separation should stay within 10 nmi for most of the time.

A1

D

C’A’

C

D’'10 nmi

Brétigny

B

B’

Melun50 nmi

50 nmi

N

200 nmi

100 nmi 80 nmi

A

20 nmi

Figure 3 Planned profiles of 1st Round ADS-B Trial flights



3.3 Test procedures

The flight profiles described in Sec. 3.2 were to be used in all three sessions.

3.3.1 Pre-flight tests1. Aircraft installations were to be tested prior to the trial flights by Eurocontrol ADS and Mode S

project personnel. These tests were meant to determine the quality of the antennas and wir-ing installed by the aircraft providers.

2. The VDL-4 installations (including the base station and the VDL-4 car) were also to be testedby SCAA and SAAB/Celsius personnel.

3.3.2 Session I1. The NLR aircraft had to activate the 1090 Ext. Squitter transponder and also the VDL-4

transceiver.2. The Ilyushin had to activate the 1090 Ext. Squitter transponder.3. Both the 1090 Ext. Squitter and VDL-4 base stations were to be activated at the EEC. The

VDL-4 equipped car (located at Melun) was also to be activated.4. The received ADS-B messages were to be logged on both base stations and also in the VDL-

4 car for the duration of the session.5. Radar plots were to be recorded from multiple SSR in the Paris area and also at Toulouse14.6. The VDL-4 equipped car was to be driven from Melun to Brétigny and back to Melun where it

would wait for the arrival of the Ilyushin.

3.3.3 Session II1. The NLR aircraft had to activate both the 1090 Ext. Squitter transponder and the VDL-4

transceiver plus a VDL-4 message logger (WINLINCS).2. The Ilyushin had to activate the 1090 Ext. Squitter receiver15, and also the VDL-4 transceiver

and WINLINCS logger.3. Both the 1090 Ext. Squitter and VDL-4 base stations were to be activated at the EEC. The

VDL-4 equipped car (located at Melun) was also to be activated.4. ADS-B message logs were to be logged on both base stations, the two aircraft and also the

VDL-4 car for the duration of the session.5. SSR plots were to be recorded from multiple radars in the Paris area and also at Toulouse.6. The VDL-4 equipped car was to be driven from Melun to Brétigny and back to Melun where it

would wait for the arrival of the Ilyushin.

3.3.4 Session III1. The NLR aircraft had to activate the UAT transceiver (with LDPU recording enabled), and

also the VDL-4 transceiver and WINLINCS logger.2. The Ilyushin had to activate the UAT transceiver (with LDPU recording enabled), and also

the VDL-4 transceiver and WINLINCS logger.3. The VDL-4 base station was to be activated at the EEC. The VDL-4 station on the EEC car

was also to be activated.4. The received ADS-B messages were to be logged on the VDL-4 base station and the two

aircraft as well as the VDL-4 car for the duration of the session.5. SSR plots were to be recorded from multiple radar stations in the Paris area and also at

Toulouse.6. The VDL-4 equipped car was to be driven from Melun to Brétigny and back to Melun where it

would wait for the arrival of the Ilyushin. 14 The Toulouse radar was necessary for achieving 100% SSR coverage of the flight paths.15 It was not possible to activate both the 1090 Ext. Squitter transponder and receiver due to lack of suffi-cient Mode S antennas on the aircraft, see Sec. 4.



4 Execution of the TrialOriginally the three trial sessions were scheduled for the 29 and 30/9/99. Due to delays experi-enced by the aircraft providers in acquiring connectors and racks for the UAT LDPU, the sessionshad to be rescheduled for the 6 and 7/10/99.

4.1 Preflight tests

4.1.1 Cessna InstallationThe NLR aircraft arrived at Brétigny on the 1/10/99 for pre-flight testing. The installation checksshowed the following: (see Annex A for pictures of the installed equipment and installation dia-grams)

♦ VDL-4:

• A single VHF antenna was used for VDL-4 which was located under the fuselage nearthe left wing of the aircraft. It would have been preferable to install it under the middle offuselage but this was not feasible.

• VHF Antenna VSWR was measured by SAAB/Celsius at 1.33 (the T4 manufacturerspecifies a VSWR < 1.50).

• A GPS antenna was installed in a antenna box on top of the aircraft fuselage and con-nected to the T4 transceiver. GPS SNR was measured by SAAB and found to be ade-quate for T4 operation.

• Operation was tested with the ground station (distance ~ 1 nmi) and the car (distance ~50 m) and it was found to be correct.

♦ 1090 Ext. Squitter:

• Two L-Band antennas had been installed in an antenna box on top of the aircraft fuse-lage. It was not feasible to install additional antennas under the fuselage of this aircraft.

• Antenna and wiring installation was found to be according to spec, and passed the testsprescribed by the transponder manufacturer.

• A GPS receiver had been connected via ARINC 429 to the Honeywell transponder tosupply position information. The GPS antenna was installed in an antenna box at the topof the aircraft fuselage.

• An ADC system was connected via ARINC 429 to the Honeywell transponder to supplybarometric altitude data.

• Operation was tested with the ground station (distance ~ 1 nmi) and it was found to becorrect.

♦ UAT :

• Two L-Band antennas had been installed in an antenna box on top of the aircraft andthey were connected to the LDPU. Normally one antenna should be installed under thefuselage but this was not feasible. Neither antenna was tuned to 966 MHz. Their DC re-sistance exceeded 300 Ohm while the LDPU manual requires a DC resistance of lessthan 10 Ohm. Eurocontrol loaned to NLR two tested 966 MHz avionics antennas16 whichwere then installed on the same positions on top of the aircraft17. The antenna wiring in-stallation was tested again on the 6/10/99 and was found to be correct.

• A GPS antenna had been installed in an antenna box on top of the aircraft fuselage andconnected to the LDPU.

16 UPS/AT model AT-130 L-Band blade antenna, tuned to 966 MHz.17 UPS/AT recommended against the use of both antennas, since they were located on top of the aircraft.



• The LDPU had not been connected to a barometric altimeter , and no air/ground switchhad been installed.

4.1.2 Ilyushin Installation

The GosNIIAS aircraft arrived at Melun on the 4/10/99. Installation checks showed that:

♦ VDL-4:• A single VHF antenna18 was used for VDL-4 placed on the rear bottom of the aircraft.• SAAB Celsius measured the VHF antenna VSWR and found it to be 2.05, which exceeds

the specified maximum (=1.5).• A GPS antenna19 was installed on the top of the aircraft fuselage and connected to the

T4 with a HF-cable. SAAB measured the GPS SNR and found it to be adequate for T4operation.

• The T4 transceiver was powered by a +27 V DC source with an autonomous connector.The T4 manufacturer specifies +28V DC.

• VDL-4 system operation on the aircraft was tested with the car (distance ~ 50 m) andwas found to be correct.

♦ 1090 Ext. Squitter:• Two L-band antennas20 had been placed at the rear top and bottom of the aircraft fuse-

lage and were connected to the Honeywell transponder via HF L-Band cables;• A GPS receiver (TOPSTAR 100 receiver by SEXTANT, France) was connected to the

Honeywell transponder via ARINC 429;• The Honeywell transponder had been connected via ARINC 429 to �baroaltimeter simu-

lator� driven by the GPS altitude output;• The EEC supplied a 115V 400Hz power supply unit (psu) for use with the Honeywell

transponder; the psu was unearthed (isolated from the body bus);• Operation was tested with a transportable receiver (distance ~ 50m) and it was found to

be correct.

♦ UAT:• The same two L-band antennas20 and HF L-Band cables used for Mode S were also to

be used for UAT;• The measured antenna DC resistance impedance was < 1 Ohm, and hence conformant

to LDPU manufacturer specs;• The same type of GPS antenna and cable was used as for VDL-4;• The LDPU was not connected to a barometric altimeter, and no air/ground switch had

been installed.• The LDPU was driven by a +27V DC supply (the LDPU manufacturer specifies +28V DC)

4.2 Session IThis session took place as planned on the 6/10/99 with both aircraft participating.

On the Cessna both Mode S Ext. Squitter and VDL-4 transceiver were activated. On the Ilyushinonly the Mode S ext. squitter transponder was activated21.

18 Russian-made АШС model.19 AeroAntenna Technology Inc., USA Model AT575-9 S/N: 1042920 Sensor Systems, Model SD65-5366-7L, bandwidth 960-1220 MHz21 It was later discovered that the Ilyushin crew had also activated a VHF-STDMA station operating at136.95 MHz, which was not foreseen in the trial scenario. In any case this did not appear to affect in anysignificant way VDL-4 operation on 136.975 MHz (25 KHz separation).



The Brétigny VDL-4 and Mode S Ext. Squitter stations were active during the session. AWINLINCS terminal was used to monitor VDL-4 operation on the base station, while the ERA sta-tion provides its own logging and display facilities. Onboard the two aircraft Dassault 1090 receiv-ers were used to monitor Mode S transponder transmissions.

The following problems occurred concerning the ADS-B equipment

♦ 1090 Ext. Squitter:• The ERA Base Station froze repeatedly during the session. The extended squitter re-

cordings of the ERA station were found to be corrupted. ERA suggests that the failurewas due to overloading of the receiver by SSR/TCAS traffic. However the station hadbeen working correctly in preflight tests. An earlier version of the base station softwarewas finally used and it worked but in the meantime the larger part of the session hadbeen lost;

• On the Ilyushin, the Honeywell transponder was switched off for 30 minutes. This wasdone on pilot request at takeoff, because of supposed interference problems with VHFradio voice. It transpired that the Honeywell transponder had nothing to do with this in-terference;

• The Honeywell transponder on the Ilyushin transmitted �pseudobarometric� (GPS) alti-tude on Mode C replies. This caused problems with ATC and the transponder had to bereset a number of times during the flight.

.♦ VDL Mode 4

• The aircraft position as received on the ground station became incorrect when the NLRaircraft crossed the Greenwich meridian. SAAB/Celsius reported that this was due tosome error in the transceiver software concerning the encoding of longitude sign.

• Saab/Celsius restarted the ground station (by manual switching) when the aircraft wasapproaching maximum distance from Brétigny. This was due to the above problem withthe passage of the Greenwich meridian. About 35 min of flight time were lost.

Radar recordings were collected during the sessions and the observed radar tracks are shown inFigure 4. The two aircraft flew closely to the prescribed profile (compare with Figure 2). In thebeginning of the flight the Cessna had to keep a holding pattern waiting for the Ilyushin to climb tothe allocated flight level (270). On the return leg, the Ilyushin had to keep a holding pattern wait-ing for the Cessna to approach at the 10 nmi distance. The maximum distance from Brétigny ex-ceeded 200 nmi, going beyond radar cover as Figure 4 indicates.



Figure 4 Trial Session I: Aircraft radar tracks

4.3 Session IIThis session was planned for the morning of the 7/10/99. On the afternoon of 6/10/99 FrenchATC decided to block the trial flights because they were busy with the operational introduction of8.33 KHz VHF channels. After negotiation, a new time slot was allocated for the afternoon of8/10/99. Then on the 8/10/99, the Ilyushin engines failed to start at the allocated slot start time.Consequently, only the NLR aircraft was able to participate in this session, and its flight profilewas simplified to follow the trajectory originally intended for the Ilyushin.

Both VDL-4 and Mode S Ext. Squitter were activated on the Cessna. Both the Mode S Ext.Squitter and VDL-4 Base Stations were active at Brétigny. WINLINCS terminals were used tomonitor VDL-4 operation on the aircraft and on the base station. For Mode S Ext. Squitter, theERA position display was used.

The EEC car equipped with VDL-4 participated in the session. A WINLINCS terminal onboard thecar was used to monitor VDL-4 reception.

Radar recordings were collected during the sessions and the observed radar track of the Cessnais shown in Figure 5. The maximum distance from Brétigny was about 160 nmi.

Ilyushin

Cessna

MelunBrétigny



Figure 5 Trial Session II : Cessna radar track

There were no ADS-B or equipment related incidents during this flight, except for the knownproblem of incorrect longitude sign on VDL-4 (see previous Sec.). The absence of the Ilyushinprevented the logging of air to air messages on 1090 Ext. Squitter.

4.4 Session IIIThis session was originally planned for the afternoon of the 7/10/99 but it had to be rescheduledfor the 11/10/99 due to the French ATC problems mentioned in the previous section. However, bythat date the NLR aircraft was no longer available, so only the Ilyushin could participate. The lat-ter became ready to fly only at the evening of the 11/10 and so the third session was eventuallycarried out on the 12/10/99.

In this flight, both UAT and VDL-4 were activated on the aircraft. VDL-4 used the Russian GPSantenna (certificate IEMA.464656.001 PC)) of the VHF-STDMA system A UAT pseudo-aircraftstation was installed at Brétigny using material recovered from the Cessna. The FAA Tech CentreCDTI was used to monitor LDPU performance on the aircraft and on the ground. Similarly aWINLINCS terminal was used to monitor VDL-4 operation on the aircraft and on the ground.

The EEC car equipped with VDL-4 participated in the session. A WINLINCS terminal onboard thecar was used to monitor VDL-4 operation.

At the beginning of the flight the aircraft crew by mistake switched off the power supply of theUAT and VDL-4 systems, consequently there was no recording of the first few minutes of theflight.

The radar track of the Ilyushin is shown in Figure 6. Due to ATC constraints the flight was di-rected to the south and the return path was shifted to the east. The aircraft arrived at a maximumdistance of about 160 nmi from Brétigny.

Brétigny



Figure 6 Trial Session III: Ilyushin radar track

Brétigny



5 Analysis of logged data

5.1 Analysis MethodThe collected data consist of the messages received on each mobile and base station timestam-ped with GPS UTC time.

The logged messages were decoded using, for VDL-4 the WINLINCS software, for Mode S ERAand Dassault software, and for UAT software supplied by UPS/AT. The resulting lat./long. infowas used to

• plot the 2xD aircraft trajectories (e.g. positions along lat. and long. axis).• determine great circle distance from the ground station

For each mobile station two lat./long. trajectories have been derived: one from its own ADS-Bstation (which corresponds to own GPS log and shows any equipment operation interruptions)and one from the ground station. The former trajectory represents the information input to theADS-B system while the latter represents the information delivered by the ADS-B system.

The timestamp information is used to calculate

• the success rate (probability of successful delivery of a datalink message) per minute =number of received messages per minute divided by the number of theoretically transmittedmessages in one minute

• the update period = time elapsed between successive messages

The calculated success rates were correlated with distances (minute average) to produce plots ofmessage success rate variation with distance. These plots were then be used to estimate thevariation of ADS-B report update period with distance, which were compared with the require-ments for state vector update periods stated in the RTCA ADS-B MASPS.

The ADS-B state vector update period was not measured directly in the trials because none ofthe three systems operated in its �standard� configuration22. For example the trial VDL-4 equip-ment transmitted one burst (=single slot message) per second, but the future VDL-4 system isexpected to transmit at a variable rate ranging from 1 burst/sec to 1 burst/10 sec depending onthe operational conditions. Therefore it would not be fair to compare measured update periodsunder the trial configurations. For this reason update periods were estimated from the measuredsuccess rates assuming that the message transmission rates were those that would be applied inan �operational� ADS-B system.

The update periods depends also on the relationship of datalink messages to ADS-B state vec-tors. In the case of Mode S Ext. Squitter it was assumed that the critical part of the state vectorrequires reception of both a position long squitter and a velocity long squitter. Both squitter typesare supposed to be transmitted twice per second23.

In the case of VDL-4, it was assumed that

! a single 1-slot VDL-4 message (=burst) provides sufficient information for the critical part ofthe ADS-B state vector.

22 e.g. the configuration projected for the future Operational ADS-B system. In fact, there are no formallyagreed system descriptions for any of the three technologies. The SF21 Link Descriptions [4] have beenused as reference for the present analysis.23 Note that the trial Mode S transponders transmitted only position long squitters. ADS-B state vector up-dates require also velocity long squitters



! These bursts are transmitted at a variable frequency24, namely 1 burst/sec at the airport, 1burst /5 sec at TMA, and 1 burst/10 sec en route. To facilitate update period calculations itwas assumed that the airport rate would apply at distances up to 3 nmi from the base station,the TMA rate at distances up to 50 nmi from the base station, and the en route rate for dis-tances beyond 50 nmi. All these numbers correspond to assumptions made in VDL-4 simula-tions up to now.

In the case of UAT, the following assumptions were made based on the UAT system descriptionin Safe Flight/21:

! A single UAT message conveys the information needed for a state vector report

! The UAT message is transmitted once per second

Based on the above assumptions, the state vector update rates were calculated using the follow-ing equations:

VDL-4 and UAT: P= 1-(1-p)Tc/T => Tc = T * ln(1-P)/ln(1-p)

Mode S: P= ((1-(1-p)Tc/T)2 => Tc = T * ln(1-√P)/ln(1-p)Where P = required percentile of update period, Tc = update period at percentile P, T= datalinkmessage period, p = (measured in the trial) success rate.

5.2 Session I : 6/10/99 – 1090 Ext. Squitter and VDL-4Both the Cessna and the Ilyushin participated in this session (see Sec. 4.2). The Cessna hadboth Mode S Ext. Squitter and VDL-4 stations active. The Ilyushin was supposed to have only theMode Ext. Squitter station active (see Sec. 4.2). A Mode S Ext. Squitter (Dassault) receiver wasalso activated on each aircraft to capture broadcasted squitters (e.g. aircraft own positions)25.

5.2.1 Aircraft TrajectoriesFigure 7 shows a two dimensional plot of the Cessna lat./long. positions transmitted by the Hon-eywell transponder and recorded by the on-board Dassault receiver. These positions correspondto the GPS input to the ADS-B system. It can be seen that the Honeywell transponder and theGPS receiver were operating for almost the complete duration of the flight. Unfortunately there isno equivalent recording from the Ilyushin.

24 Note that in the trial the VDL-4 transceiver transmitted one burst per second.25 The Dassault receiver was not connected to an external antenna hence it could not capture incomingsquitters from the other aircraft. Two additional Mode S antennas would have been required, but they werenot available on either aircraft.



Ce ssna Mode S Ex t. Squitte r Tra nsm itte d Positions

47.4

47.6

47.8

48

48.2

48.4

48.6

48.8

49

49.2

-4 -3 -2 -1 0 1 2 3

longitude, deg

latii

tude

, de

g

B retigny

Figure 7 Mode S Ext. Squitter positions broadcasted by the Cessna � 6/10/99

Figure 8 plots the received position squitters on the Mode S 1090 Ground Station (ERA) at Bré-tigny. Due to the base station problems mentioned in Sec. 4.2, only the last 20 minutes of theflight were recorded successfully. Consequently, the trajectories depicted in Figure 8 contain onlythe return approach of the two aircraft to the Brétigny and Melun airports.

The trajectory of the Ilyushin was lost well before landing, but this is due to the low altitude of theaircraft, which made the Ilyushin invisible (loss of line of sight) from Brétigny. The trajectory of theIlyushin contains many more gaps than the Cessna trajectory. The better quality reception fromthe Cessna was surprising because the Cessna did not have a bottom Mode S antenna (see Sec.4.2) and hence it was expected to perform worse than the Ilyushin. This poor Ilyushin Mode Sperformance can be partly attributed to multiple power resets that occurred during the flight whiletrying to fix the �pseudo baro altitude� problem described in Sec 4.2.



M ode S Ex t. Squitte r Tra je ctory

47.8

47.9

48.0

48.1

48.2

48.3

48.4

48.5

48.6

48.7

1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9

longitude, deg

latii

tude

, de

g

Cessna

Ilyushin

Bretigny

M elun

Figure 8 Mode S Ext. Squitter recording on the ERA ground station � 6/10/99

Figure 9 plots the position reports as received on the VDL-4 Ground Station at Brétigny. Sinceonly the Cessna VDL-4 station had been activated, only the Cessna trajectory appears. It can beseen that:

• VDL-4 covered 3/4 of the flight in air-ground mode. The missing part of the flight correspondsto the period where the a/c was furthest from Brétigny. During that period the ground stationwas reset (see Sec. 4.2) and hence no positions were received.

• There are discontinuities in the reported positions around longitude 0. In fact the recordedpositions presented negative longitude signs as positive. We have corrected this error inFigure 9, but even so there remain some discontinuities around longitude 0 as well as somemisplaced positions between longitude �1 and �0.5. SAAB reported that the longitude signproblems are due to some transceiver software error in burst encoding/decoding.

• There are also a few intermediate gaps within the reported trajectory. The reasons are notclear.



Cessna Trajectory - 6/10/99

47.4

47.6

47.8

48

48.2

48.4

48.6

48.8

49

49.2

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3

Longitude, deg

Latit

ude,

deg

CessnaEEC ground station

Figure 9 VDL-4 Recording on Ground Station � 6/10/99

Figure 10 and Figure 11 zoom on the take-off and landing trajectories for the Cessna as receivedon the VDL-4 Base Station. For comparison Figure 12 zooms on the Cessna landing trajectory asreceived on the Mode S Ext. Squitter Base Station. Both appear to be of good quality but mathe-matical analysis in the following section will show whether they would meet the MASPS require-ments for ADS-B state vector update period.

V DL-4: Ce ssna Ta ke Off

48.597

48.602

48.607

48.612

2.32 2.325 2.33 2.335 2.34 2.345 2.35 2.355 2.36 2.365

Longitude, deg

Latit

ude,

deg

EEC

Figure 10 Cessna take off as recorded by VDL-4 base station � 6/10/99



Ce ssna La nding : V DL-4

48.565

48.575

48.585

48.595

48.605

2.29 2.3 2.31 2.32 2.33 2.34 2.35

Longitude, deg

Latit

ude,

deg

EEC

Figure 11 Cessna landing as recorded on the VDL-4 Base Station � 6/10/99

Ce ssna La nding: M ode S Ex t. S quitte r

48.565

48.575

48.585

48.595

48.605

2.29 2.30 2.31 2.32 2.33 2.34 2.35

Longitude

Latit

ude

EEC

Figure 12 Cessna landing as recorded on the Mode S Ext. Squitter Base Station � 6/10/99



5.2.2 Message Success Rates

Figure 13 shows the variation of the one-minute average 1090 Ext. Squitter success rate versusdistance from the EEC base station. Success rate has been plotted separately for each aircraft.As expected from the trajectory graphs of the previous section, the Ilyushin had a much lowersuccess rate than the Cessna. The distance covered goes only up to 40 nmi due to the base sta-tion logging problems encountered during the session

Figure 14 shows the corresponding plot of Cessna VDL-4 message success rate versus distancefrom the EEC VDL-4 base station. The success rate achieved is in the order of 90-100% up toaround 130 nmi and then drops off. There are a few intermediate drops below 90% correspondingto the holes observed in the VDL-4 trajectory plotted in Figure 9.

The success rates achieved by VDL_4 are much higher than those achieved by Mode S ext.squitter. However, one should remember that

• VDL-4 operated in a channel free of co-channel interference (CCI), since only two VDL-4transmitters were active.

• Mode S had to cope with CCI from SSR, TCAS and military systems, but then, it will alwayshave to face CCI from such systems

The observed lower success rates for Mode S are not necessarily an indication of poorer per-formance, because Mode S transmits at a higher rate (twice per second) than VDL-4. Figure 15provides a more meaningful performance quality comparison for the two technologies. It plots the95th percentile of state vector update period against distance for the two technologies. It alsoshows the MASPS requirement for the state vector update period. The update period values havebeen calculated from the measured success rates according to the method described in Sec. 5.1.

As Figure 15 shows, the observed trial VDL-4 performance would meet MASPS requirements forthe SV update period up to a range of 130 nmi except possibly for distances below 3 nmi andalso a gap at 50-70 nmi. The observed trial Mode S Ext. Squitter performance would meetMASPS requirements up to the range of 30 nmi except for the gap around 15-20 nmi. For bothtechnologies, the intermediate performance gaps might be due to equipment problems ratherthan some inherent technology failure.



Mode S Ext. Squitter Success Rate

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 5 10 15 20 25 30 35 40 45distance, nmi

Suc

cess

Rat

e, %

CessnaIlyushin

Figure 13 1090 Ext. Squitter Success Rate: Reception on ground station - 6/10/99

VDL-4 Message Success R ate

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

110%

0 20 40 60 80 100 120 140 160 180

Distance, nmi

Suc

cess

Rat

e, %

Figure 14 VDL-4 Message Success Rate: Reception on ground station � 6/10/99



Ce ssna S V Upda te P e riod - 95% confide nce

0

5

10

15

20

25

30

35

40

45

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Distance, nmi

Upd

ate

Per

iod,

sec

VDL-4

Mode S

MASPS

Figure 15 Comparison of ADS-B state vector update periods � 6/10/99

5.3 Session II: 8/10/99 – VDL-4 and 1090 Ext. SquitterOnly the Cessna participated in the second session (see Sec. 4.3), and it had both Mode S Ext.Squitter and VDL-4 stations active. A VDL-4 equipped car was also active during that session.

5.3.1 Aircraft Trajectory

Figure 16 shows a two dimensional plot of the lat./long. positions recorded on the Cessna VDL-4station. Own positions reflect the input from the GPS receiver contained within the station, andhence describe the GPS input to the ADS-B system. It can be seen that the T4 transponder andthe attached GPS receiver were operating for most of the flight, with one notable gap on the re-turn leg at longitudes 1.8-2.0 degrees. The reason for this gap is an automatic re-boot of thetransponder (power reset). Note also that the VDL-4 car was detected on the aircraft.

Figure 17 shows a two dimensional plot of the lat./long. positions recorded on the VDL-4 groundstation that was installed at the EEC. We have corrected the wrong longitude signs that the T4actually transmitted when the aircraft was in negative latitudes. There remain Cessna positiondeviations around the Greenwich Meridian similar to those observed in the first session (seeFigure 9). In any case the ground station managed to capture the greatest part of the transmittedCessna trajectory.

Figure 18 shows a two dimensional plot of the lat./long. positions recorded on the Mode S Ext.Squitter Base Station(ERA) that was also installed at the EEC. The range of the captured Cessnatrajectory is considerably better than what Mode S delivered in the first trial session (see Figure8), where Mode S operation was plagued by equipment problems. It is clear however that

a) the Mode S trajectory becomes progressively sparser as the distance from the base stationbecomes larger. This behaviour is similar to what was seen in the Toulouse trial of June 1999(see Annex B8.2.1, Figure 36).

b) The Mode S Base station captured a smaller part of the aircraft trajectory than VDL-4.



Ce ssna V DL-4 Re cording

48

48.1

48.2

48.3

48.4

48.5

48.6

48.7

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3Longitude, deg.

Latit

ude,

deg

NLR aircraft

EEC car

EEC ground s ta tion

Figure 16 Cessna VDL-4 Station Log - 8/10/99

VDL-4 Base Station recording

48

48.1

48.2

48.3

48.4

48.5

48.6

48.7

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3

Longitude, deg

Latit

ude,

deg

N LR a ircra ft

EEC ca r

EEC ground s ta tion

Figure 17 EEC VDL-4 Base Station log - 8/10/99



M ode S Ex t. S quitte r Ba se S ta tion Re cording

48.0

48.1

48.2

48.3

48.4

48.5

48.6

48.7

-1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5

longitude, deg.

latit

ude,

deg

.EEC

Figure 18 Mode S Ext. Squitter Base Station Log - 8/10/99


Figure 19 shows the variation of the one-minute average success rate of Cessna Mode S longsquitters versus distance from the Mode S ground station. This session provided data up to 180nmi, unlike the previous session (see Figure 13), but still the quality of the trajectory deterioratesrapidly as the aircraft is moving away from the base station. In comparison with the Toulouse trialresults (see Annex B.8.2.2, Figure 39), the success rates of Figure 19 are significantly lower. Forexample at 40 nmi the Toulouse success rate ranged from 60 to 70% while in Figure 19 it rangedfrom 5 to 35%. The Toulouse environment has a lower Mode S fruit rate than Paris, and this fac-tor might contribute to the observed performance difference. However, the Toulouse trial permit-ted also success rate measurements in Paris while the aircraft was en route from and to the UK(see Annex B.8.2.2, Figure 38). In that case the measured success rate at 70 nmi ranged from 5to 35% while in Figure 19 it is below 10%. The ground station and antenna were the same in bothcases, but the Toulouse trial aircraft (BAC 1-11) had two L-Band antennas located top and bot-tom of the fuselage (unlike the Cessna).

Figure 20 shows the corresponding plot for the Cessna VDL-4 message success rate versusdistance from the EEC Base station. The plot is similar to what was obtained in the previous ses-sion (see Figure 14). Success rate is in the order of 90-100% up to around 130 nmi then drop-ping off, and there are a few drops below 90% in intermediate distances.

It should be noted that VDL-4 success rate calculations were somewhat perturbed by a problemwith UTC reception timestamps in the WINLINCS recordings. The UTC reception timestamp issometimes frozen for a period of a few minutes or it can jump backwards by up to 10-20 sec. Thereason for this problem is not known. It is mitigated by the fact that the log includes the UTCtransmission timestamp. The latter may also suffer from similar problems but usually at differenttimes. This problem caused the loss of a few data and may have introduced some additionalvariation in the success rate estimates26.

Figure 21 compares the performance of the two technologies in terms of the 95th percentile of thestate vector update period. The ADS-B MASPS requirement for the 95th percentile of the statevector update period is also shown. The update period values have been calculated from themeasured success rates according to the method described in Sec. 5.1.

Figure 21 indicates that the observed trial VDL-4 performance would meet MASPS requirementsfor the SV update period up to a range of 130 nmi. In the case of Mode S Ext. Squitter, the up-

26 The same problem happened also on the other two sessions but it was less noticeable



date period would meet MASPS requirements up to the range of 40 nmi, although there are twoupdate period peaks exceeding the MASPS requirement around 15 and 25 nmi.

M ode S Ex t. Squitte r Succe ss Ra te

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 20 40 60 80 100 120 140distance, nmi

Suc

cess

Rat

e, %

Figure 19 Variation of Mode S Extended Squitter Success Rate versus distance � 8/10/99



V DL-4 M e ssa ge S ucce ss Ra te

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

110%

0 20 40 60 80 100 120 140 160 180

Dis tance, nmi

Suc

cess

Rat

e, %

Figure 20 Variation of VDL-4 Message Success Rate versus distance � 8/10/99

SV Upda te Pe riod a t 95% confide nce

0

5

10

15

20

25

30

35

40

45

50

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150distance, nmi

upda

te p

erio

d, s

ec

VDL-4

Mode S Ext. Squitter

"MASPS"

Figure 21 Cessna State Vector Update Period at 95% confidence � 8/10/99



5.4 Session III: 12/10/99 – UAT and VDL-4Only the Ilyushin participated in the third session (see Sec. 4.4), and it had both UAT and VDL-4stations active. The second UAT station was installed at the EEC. A VDL-4 equipped car wasalso active during that session.

5.4.1 Aircraft Trajectories

Figure 22 shows a two dimensional plot of the Ilyushin lat./long. positions as recorded on its on-board UAT station (LDPU). Own positions reflect the input from the GPS receiver containedwithin the LDPU, and hence represent the input to the ADS-B system. It can be seen that theLDPU and the attached GPS receiver were operating for most of the flight, with two notable gapson the first flight leg at longitudes 2.3-2.4 degrees. In fact, the LDPU log indicated that five resetsoccurred during the flight.

Figure 23 shows the corresponding two dimensional plot of lat./long. positions recorded on theIlyushin VDL-4 station. This log indicates the information transmitted by the onboard T4 and alsoshows that the EEC car and the EEC VDL-4 base station were detected. The recorded aircrafttrajectory matches the trajectory recorded on the LDPU since they both result from (different) on-board GPS receivers. There were two major T4 operation interruptions and they occurred in thesame period as in the UAT case but they lasted longer.

Figure 24 shows a two dimensional plot of lat./long. positions recorded on the UAT LDPU thatwas installed at the EEC. Clearly, only a small part of the trajectory transmitted was captured, andthere were many more disruptions than in the transmitted trajectory.

Figure 25 shows the corresponding plot of lat./long. positions recorded on the VDL-4 ground sta-tion installed at the EEC. Both the aircraft and the car were detected. The range of the capturedIlyushin trajectory is wider than that of UAT but the quality seems equally poor, especially if com-pared to the VDL-4 trajectories produced in the previous sessions.



Ilyushin own UAT Track

46

47

48

49

2 2.5 3 3.5 4 4.5 5

longitude, deg

latit

ude,

deg

.

Melun

Figure 22 Ilyushin UAT LDPU own position log� 12/10/99

Ilyushin VDL-4 recording

46

46.5

47

47.5

48

48.5

49

2 2.5 3 3.5 4 4.5 5

Longitude, deg.

Latit

ude,

deg

.

Ilyushin

EEC car

EEC VDL-4 base station

Figure 23 Ilyushin VDL-4 station log - 12/10/99



UAT reception log

47.4

47.5

47.6

47.7

47.8

47.9

48

48.1

48.2

48.3

48.4

48.5

48.6

48.7

2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9

longitude, deg.

latit

ude,

deg

.

IlyushinBretignyMelun

Figure 24 UAT ground station reception log, 12-10-99



VDL-4 Ground Station Log

46

46.5

47

47.5

48

48.5

49

2 2.5 3 3.5 4 4.5 5

Longitude, deg.

Latit

ude,

deg

.

IlyushinEEC carEEC ground station

Figure 25 EEC VDL-4 Ground Station Log � 12/10/99


Figure 26 shows the variation of the one-minute average success rate of Ilyushin VDL-4 mes-sages versus distance from the EEC ground station. VDL-4 messages were received up to 150-160 nmi away from the base station just like in the previous sessions, but the measured successrates are well below what was seen then.

Figure 27 shows the corresponding plot for the Ilyushin UAT message success rate versus dis-tance from the EEC. UAT messages have been received from up to 70 nmi away but the successrate varied widely. This was somewhat surprising since UAT operated in an environment free ofco-channel interference, unlike Mode S.

Figure 28 compares the quality of ADS-B performance of the two technologies in terms of the 95th

percentile of the state vector update period that would result from the measured success rates.The ADS-B MASPS requirement for the 95th percentile of the state vector update period is alsoshown. The update period values have been calculated from the measured success rates ac-cording to the method described in Sec. 5.1. The observed VDL-4 performance is widely off theMASPS requirements. This contradicts the results of the previous two sessions, suggesting thatthere must have been a serious problem in the Ilyushin VDL-4 installation. The only problemidentified in the pre-flight tests was a poor VHF antenna VSWR .

For UAT, Figure 28 suggests that MASPS requirements could be met up to 60-70 nmi, althoughthere are ten peaks exceeding the MASPS upper limit for the SV update period. These peaks are



linked due to the resets noted in the LDPU log. UAT performance may also have suffered by thenon optimised L-Band antennas used on the aircraft.

Ilyushin VDL-4 Message Success Rate versus distance

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

0 20 40 60 80 100 120 140 160

Distance, nmi

Suc

cess

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e, %

Figure 26 VDL-4 message success rate at the EEC ground station � 12/10/99

UAT Sucess Rate versus Distance

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30%

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70%

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0 10 20 30 40 50 60 70 80

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Suc

cess

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e, %

Figure 27 UAT Message Success Rate air to ground � 12/10/99



SV Update Period at 95% confidence

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20

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0 20 40 60 80 100 120Distance, nmi

Upd

ate

perio

d, s

ec

UATVDL-4MASPS

Figure 28 ADS-B State Vector Update period (at 95% confidence) versus distance � 12/10/99



6 Conclusions6.1 Performance/Range• The VDL-4 implementation (T4 transceiver) produced the better performance among the

three ADS-B implementations tested. The T4 implementation achieved air to ground ranges27

in the order of 120- 130 nmi. This performance is similar to results reported from previous T4trials in Sweden. Longer air to ground ranges have been reported in VHF-STDMA trials,where higher transmission powers have been used (10 W versus 5 W in the T4). The SF21LET specification for VDL-4 indicates that VDL-4 transmitters might use 10 - 50 W TX power.Therefore such transmitters might well achieve ranges beyond 150 and even 200 nmi but thishas yet to be tested. There is also the untested option of using two VHF antennas for VDL-4on the aircraft which might have a positive impact on air to ground performance28.

• The Mode S Ext. Squitter implementation (Honeywell Transponder, ERA receiver) achievedthe lowest air to ground range (~ 40 nmi) of the three ADS-B trial systems. This is somewhatlower than what was measured in the previous Eurocontrol trials in Toulouse and Paris (seeAnnex B) with the same equipment (base station, transponder, antenna) but a different air-craft (DERA BAC-1-11). In fact TCAS acquisition squitter measurements in Toulouse sug-gested that the air to ground range might reach 100 nmi (see Annex B8.2.3), but the Tou-louse airspace has a much lower Mode S fruit rate than Paris. However SAA/SF21 trial re-sults [4] from Los Angeles Basin and the Ohio Valley have also Mode S ext. Squitter rangesin excess of 100 nmi [using the same type of transponder from Honeywell]. These perform-ance differences might be attributed to the following factors:

! The Cessna did not have a Mode S antenna at the bottom of the fuselage (but the BAC1-11 and the Ilyushin did have such antennas)

! The ERA station had a lower sensitivity (higher MTL) than the equipment used in theSF21 trials. However, on paper the difference does not seem significant (-86 versus �87dBm).

! The ERA station did not use the advanced decoding techniques that were implementedin the Mode S receivers of the SF21 trials. Indeed the SF21 Link Evaluation report [4]states that �improved 1090 MHz receivers (relative to existing TCAS receivers) will beneeded to meet all ADS-B MASPS range and integrity requirements�.

! Neither the ground nor the aircraft Mode S antennas used pre-amplification (used in theSF21 trials)

! The ground Mode S station omni antenna was less efficient than the (DME and 6-sector)antennas used in the SF/21 trials particularly regarding its sensitivity to multipath.

• The UAT trial implementation (UPS/AT equipment) achieved an air to ground range of about70 nmi. This is somewhat less than the ranges reported in the SF/21 Ohio Valley trials [4].The transceiver was identical to what was used in the SF/21 trials but performance musthave been penalised by the generic L-Band antennas used in the aircraft and also the avion-ics L-Band antennas used on the ground station.

6.2 Equipment maturity• Only the Mode S Honeywell transponder was a commercial and certified product. All the

other equipment used were uncertified prototypes and not available commercially. Eurocon-trol could not find any alternative sources of equipment for any of the three technologies.

27 In this discussion range is measured as the maximal distance at which MASPS requirements [5] for the95th percentile of the state vector update period are met.28 The Eurocontrol trials as well as all previous T4 and VHF-STDMA trials have used a single VHF antennaon the aircraft.



• Equipment related problems were encountered with all three technologies, and they involvedboth hardware and software. These problems were relatively minor but they did cause someloss of data and in some cases may have impacted negatively on performance. In particularall airborne receivers suffered from random albeit infrequent resets in both trial aircraft. Theseresets caused some data loss. Their cause is not known (the aircraft providers assert thattheir power supplies are very stable). The VDL-4 implementation [T4] suffered from positionencoding and UTC timestamp errors (software problems?). The latest versions of the Mode SExt. Squitter receiver {ERA} did not work.

• There were striking variances between the results obtained on the two aircraft despite thefact that identical equipment was used. Antenna and wiring implementation differences ap-pear to have played a critical role for all three technologies. Yet, no manufacturer had definedinstallation test procedures that would permit reliable measurement of installation quality priorto the flights. The variances observed suggest also that the equipment was not robustenough to deal with variations in wiring, connections, and antennas that are usually encoun-tered in aircraft installations.

6.3 Next stepsThe results obtained in the 1st round did not permit to define with confidence the capabilities ofthe three candidate technologies. The reasons were explained in the previous two subsections. Anumber of potential improvements were listed, and these will be tried in the 2nd round of the Euro-control ADS-B trials for 1999. This 2nd round will focus on air to air performance, since unfortu-nate circumstances prevented the collection of air to air measurements in the 1st Round.

ADS Technology Assessment will be continued in the year 2000 and will address any issuesarising from these trials by conducting studies, simulations, and further trials. The case of ADS-Bfor surface movement in airports will be one of the items to investigate in 2000. Progress in thedefinition of ADS-B operational requirements should allow also further refinement of performanceanalysis to enable more reliable range measurements and technology comparisons. If new ADS-B products appear in the market, they will also be tested.

Eurocontrol seeks partners for the above technology assessment activities. Interested parties areinvited to contact Dr. C. Tamvaclis (EEC) or Mr. P. Van Der Kraan (Eurocontrol HQ) to discussthe possibilities for collaboration.



7 Annex A7.1 Implementation Characteristics

The following table lists the characteristics of the ADS-B equipment used in the trial.

Characteristic 1090 ext. squitter VDL-4 UATFrequency, MHz 1090 136.975 966

TX Rate 1 Mbps 19.2 Kbps 1 MbpsModulation PPM GFSK � 2400 KHz GFSK � 312 KHz

Synchronisation 4 pulse preamble first 24 bits first 36 bitsMessage length, bits 112 192 (after sync) 246-372

Parity, bits 24 16 48 FEC and 24 CRCAddress, bits 24 27 25Lat/Long, bits 17 unspecified 24TX power29, W 500 5 50

Polarisation vertical vertical verticalTX Rate, msg/sec 2 1 1

RX MTL, dBm -7430, -8631 unspecified -92

Notes! The Mode S equipment implemented CPR encoding for position coordinates as specified in

the Mode S standards.! The VDL-4 transceiver did not implement CPR encoding, although it is required in the latest

versions of the VDL-4 standards.

29 At the output of the transmitter30 Dassault Ext. Squitter Receiver31 ERA Mode S Ground Station



7.2 Installations

Figure 29 Honeywell Transponder and Control Panel installed on Cessna

Figure 29 shows the installation of the 1090 Ext. Squitter transponder and its control panel on theCessna. The portable PC in the background is the Dassault Ext. Squitter receiver.

Figure 30 shows the installation of the UPS/AT LDPU on the Cessna. The LDPU was used onlyfor UAT. The LDPU was connected to a control panel and a portable PC running CDTI software(from the FAA Tech. Centre) as shown in that Figure.

Similarly Figure 31 shows the Saab/Celsius T4 transceiver installed on the Cessna. The trans-ceiver was connected to a WINLNCS terminal (portable PC) not shown in the Figure.

Figure 32 and Figure 33 show the corresponding Honeywell, LDPU, and T4 installations on theIlyushin.

Figure 34 shows that T4 transceiver and WINLINCS terminal installed on the car. Figure 35shows the antenna installation on the car. The aircraft in the background of this Figure is the Ilyu-shin at Melun.



Figure 30 LDPU with Control Panel and CDTI installed on Cessna

Figure 31 VDL-4 T4 Transceiver installed on Cessna



Figure 32 Honeywell Transponder with Control Panel and LDPU installed on Ilyushin

Figure 33 VDL-4 T4 Transceiver and WINLINCS terminal installed on Ilyushin



Figure 34 VDL-4 Transceiver and WINLINCS terminal installed on car

Figure 35 VHF and GPS antennas installed on car



8 Annex B: 1090 Ext. Squitter Trial in Toulouse8.1 Trial ScenarioThe Toulouse trial was held in June 1999 with the participation of a single aircraft. The latter wasa BAC 1-11 (200) provided by DERA. There was also a single 1090 Ext. Squitter Base Stationsupplied by the EEC. The trial involved a single flight of the BAC 1-11 in the Toulouse airspaceand it was organised by STNA. The objective of the Toulouse trial was to measure air to groundextended squitter performance.

The BAC 1-11 was equipped with the same Honeywell transponder (supplied by the EEC) thatwas also used in the October trials. Two L-Band antennas (supplied by DERA) were installed onthe aircraft, located at the top and bottom of the fuselage. There was no 1090 Ext. Squitter re-ceiver in the aircraft. The BAC 1-11 was also equipped with a GPS receiver and antenna. TheHoneywell transponder was connected to the aircraft FMS, which supplied GPS data and baro-metric altitude to the transponder. All installation activities were carried out by DERA personnel.The installation was tested by EEC staff while the aircraft was in the UK and then again in Tou-louse.

The 1090 Ext. Squitter Base Station was the same ERA system and antenna used in the Octobertrials (see Sec. 2.3). Initially this Base Station was located at Brétigny to capture the ext. squittersemitted by the aircraft on its way to Toulouse from the UK. Then the station was installed at theToulouse airport, where it logged BAC 1-11 ext. squitters while the aircraft was flying a racetrackpattern around Toulouse. Finally the station was brought back to Brétigny, where it capturedBAC 1-11 ext. squitters while the aircraft was en route to the UK.

During the trial flight in Toulouse, TCAS Acquisition Squitter (DF11) messages and SSR plotswere also logged on a Mode S SSR station located near the Toulouse airport. Analysis of thoserecordings is presented in Sec. 8.2.3.

8.2 Analysis of logged data8.2.1 Ext. Squitter Position TrajectoriesFigure 36 plots BAC 1-11 positions received on the 1090 Ext. Squitter Base Station at the EEC,near Paris, while the aircraft was en route to Toulouse and also on its return trip. The position ofthe EEC station (coordinates 2.347, 48.599) does not appear on the plot because it is outside itsrange (to the right). Figure 36 indicates that the return trajectory was much more sparsely re-ceived than the incoming one. This must be due to the greater distance from the EEC Base Sta-tion. Indeed both trajectories become sparser as the distance from the EEC station becomesgreater.

Figure 37 plots BAC 1-11 positions received on the 1090 Ext. Squitter Base Station in Toulouse,during the trial flight there. The Toulouse Base Station was the same ERA system and antennaused at the EEC. The Toulouse flight trajectory is much more complete than the ones recorded inParis, but air to ground distances were smaller than in the Paris case. It is noticeable that theFigure 37 trajectory has some gaps while the aircraft was manoeuvering at the longer distancesfrom the Base Station. Aircraft turns may provoke signal strength variations, due to shielding oraircraft antenna sidelobes. If the signal is already marginal due to distance, this factor wouldcause the observed gaps.



46

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-1.4 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4

Longitude, deg.

Latitude, d

towards Toulouse

back from "

Figure 36 BAC 1-11 Trajectories recorded on the Base Station at the EEC

42.9

43

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Longitude, deg.

Latitide,d

Toulous

Figure 37 Bac-111 Trajectory recorded on the Base Station at Toulouse

8.2.2 Ext. Squitter Success RateFigure 38 plots the success rate (averaged per 30 sec sliding window) of the BAC1-11 positionsquitters received on the EEC Ground Station during the flight to Toulouse and the return flight tothe UK. Success rate is plotted versus distance from the EEC Base Station. Similarly Figure 39plots the success rate of the BAC 1-11 position squitters received on the Toulouse Base Stationduring the trial flight.

Both figures confirm that air to ground long squitter success rate tends to drop rapidly with dis-tance. The Paris success rate plot covers ranges above 60 nmi. The Toulouse plot shows thatsuccess rate became very variable above 45 nmi ranging from less than 10% to 40-60%. Thisrange variation is in agreement with the Paris measurements



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Success Rate, southward

return

Figure 38 BAC 1-11 Position Squitter Success Rate on the incoming and return flights - Paris

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Figure 39 BAC 1-11 Position Squitter Success Rate� Toulouse Flight



8.2.3 TCAS Acquisition SquittersThe Toulouse Mode S Station was used to log TCAS Acquisition Squitters (DF11) broadcastedevery second by all TCAS equipped aircraft during the trial flight. At the same time SSR positionplots were logged. TCAS Acquisition Squitters contain an identification code, which was used toidentify concurrent SSR position plots of the same aircraft. The results of this correlation areplotted in Figure 40. The latter shows on an orthogonal horizontal plane the logged SSR posi-tions in two colours:

♦ Red colour indicates positions for which a corresponding TCAS acquisition squitter(DF11) has been received,

♦ Blue colour indicates that no corresponding TCAS acquisition squitter was received.

The position of the Toulouse Mode S station is at the centre of the diagram, and the x- and y- axisare scaled in terms of distance from the base station. There are few blue lines within 100 nmi ofthe base station. This suggests that DF11 squitters were received with a high success rate fordistances within 100 nmi of the base station.

DF11 squitters are only 56 bits long, but extended squitters (112 bits) ought to achieve fairlysimilar performance under the same traffic environment. Consequently, Figure 40 suggests thatextended squitters should also be able to achieve high success rates up to 100 nmi from thebase station [Toulouse airspace].

DF11 measurements could be considered as more reliable than the extended squitter ones [inthe flight trial], because the former took into account a much larger population of aircraft. The ex-tended squitter trial measurements were based on only one aircraft. On the other hand the SSRand Mode S logs from which Figure 40 was derived covered less than three minutes (due to thehuge amount of data that had to be logged and analysed).

-200

-100

0

100

200

-300 -200 -100 0 100 200 300 400

radar only

radar and DF11

Figure 40 DF11 squitter correlation with SSR plots - Toulouse

october trial report - asas tn · 2020. 6. 15. · eec/sur6e1/wp/006, astp, dec. 1999 4. phase one...

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