freer-3 trials - ads-broadcast over vhf-stdma performance ... · freer-3 trial/vhf-stdma data...

Ref.: EEC/SUR6E1/WP/006Version: 1.6Issued: December 1999Owner: EEC/ASTPAuthors: C. Tamvaclis

G. RambaudL. Rabeyrin

Revision History

EUROCONTROL EXPERIMENTAL CENTRE

Brétigny-sur-Orge, FRANCE

ADS Studies and Trials Project

FREER-3 Trials

ADS-Broadcast over VHF-STDMA

Performance Analysis

EUROCONTROL

Abstract

The FREER-3 trials tested the feasibility of using ADS-Broadcast overVHF-STDMA (an early form of VHF Datalink Mode 4) to implement anautonomous airborne long-range deconfliction application developed atthe EEC. This paper presents a performance analysis of data collected inthese trials with regard to air-to-air operation of ADS-B and VHF-STDMA.

FREER-3 Trial/VHF-STDMA Data Analysis 2

EEC/ASTP Dec. 99

Version Issue Date Status Reason for Change

1.0 19-Jan-99 draft Creation of the document

1.1 March 99 draft Trial flight 21-11-98

1.2 April 99 draft Trial flight 05-02-99

1.3 June 99 draft Trial flight 15-02-99

1.4 August 99 draft Trial flight 18-05-99

1.5 December 99 draft Final Draft


EEC/ASTP Dec. 99

CONTENTS

ACRONYMS AND ABBREVIATIONS ..............................................................................................................4

LIST OF REFERENCES.......................................................................................................................................5

1. INTRODUCTION..........................................................................................................................................6

1.1 PURPOSE ...................................................................................................................................................61.2 FREER-3 TRIALS......................................................................................................................................6

1.2.1 FREER-3 Aircraft and Equipment ...................................................................................................71.2.2 FREER-3 Software and use of ADS-B..............................................................................................71.2.3 Data Logs .........................................................................................................................................8

1.3 ADS-B AND VHF-STDMA CONFIGURATION ...........................................................................................91.4 MEASURED PERFORMANCE PARAMETERS...............................................................................................101.5 SIMULATOR : E-SPS................................................................................................................................101.6 SCOPE .....................................................................................................................................................10

2. FLIGHT SCENARIOS ................................................................................................................................12

2.1 SESSION OF 21 NOVEMBER 1998.............................................................................................................122.2 SESSION OF 6 FEBRUARY 1999................................................................................................................142.3 SESSION OF 15 FEBRUARY 1999..............................................................................................................172.4 SESSION OF 18 MAY 1999 .......................................................................................................................202.5 SIMULATION SCENARIOS .........................................................................................................................22

3. DATA LOG ANALYSIS..............................................................................................................................24

3.1 POSITION REPORTS ..................................................................................................................................243.1.1 Success Rate variation with time....................................................................................................243.1.2 Range and Refresh Rate .................................................................................................................343.1.3 Comparison with simulation results ...............................................................................................39

3.2 FREE TEXT MESSAGES.............................................................................................................................433.2.1 Success Rate ...................................................................................................................................433.2.2 Simulation Results ..........................................................................................................................43

4. CONCLUSIONS AND FURTHER WORK...............................................................................................48


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Acronyms and abbreviationsACI Adjacent Channel Interference

ADS-B Automatic Dependent Surveillance - Broadcast mode

ASTP ADS Studies and Trials Project

CCI Co-Channel Interference

CDTI Cockpit Display of Traffic Information

EEC Eurocontrol Experimental Centre

FREER Free Route Experimental Encounter Resolution

FREER-3 Application for assisting long range path deconfliction by autonomousaircraft

FREER-3 FREER Air-to-Air VHF Data Link Trial

LFV LuftFartsVerket (Swedish Civil Aviation Administration)

LOS Line Of Sight

MASPS Minimum Aviation System Performance Standards

MER Message Error Rate

MSR Message Success Rate

NEAN North European ADS-B Network

nmi Nautical miles

RA Random Access

SARPs Standards And Recommended Practices

SPS STDMA/VDL Mode 4 Performance Simulator

STDMA Self-organising Time Division Multiple Access

T3 VHF-STDMA Transponder model developed by SAAB Dynamics AB

UTC Universal Co-ordinated Time

VDL Mode 4 VHF Data Link Mode 4

VHF Very High Frequency

VSS VDL Mode 4 Specific Service


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List of references1 ADS Studies and Trials Project Management Plan, EEC/SUR6E1/PRJ/001, Dec. 1998

2 Eurocontrol ADS Programme Management Plan, DIS/SUR, http://www.eurocontrol.be/projects/eatmp/ads/programme/index.htm

3 ADS Technology Assessment Task, Eurocontrol, Ref. DED.3/SUR/ADS.TSK.99.001, Sept.1999

4 Draft VDL Mode 4 SARPs, Version 5.4.2, AMCP/WG4/VSG, Sept. 1998

5 MASPS for ADS-B, Ref. DO-242, RTCA SC-186, Jan. 1998

6 FREER-3 Software Requirements Document, Ref. EEC/ATM/APO/R.J.Irvine, 15 January 1998

7 Equipment specification for the LINCS T3L/M Transponder, SAAB Dynamics AB, Nov. 1997

8 STDMA/VDL Mode 4 Performance Simulator - User Manual & Supplements Version 3.9,Swedish Civil Aviation Administration, 7 February 1998

9 STDMA/VDL Mode 4 Performance Simulator - Enhanced SPS User Manual, SYSECA, Ref.178200-SUM-982188-RAB, November 1998

10 SPS Compliance with VDL Mode 4 SARPs, EEC/ASTP, Ref. EEC/SUR6E1/WP/005, 03November 1998

11 SPS Validation, EEC/ASTP, Ref. EEC/SUR6E1/WP/007, April 1999

12 VDL Mode 4 Capacity Assessment, EEC/ASTP, Ref. EEC/SUR6E1/WP/008, August 1999

13 Study of alternative beacon based surveillance and datalink systems, MITRE, Ref. MTR-6517,April 1974

14 Simulation of ADS-B using VDL-4 in the LA Basin and Core Europe Airspace, Version 3.0, G.Frisk, LFV, 1999


EEC/ASTP Dec. 99

1. Introduction

1.1 PurposeThis document has been produced by the ADS Studies and Trials Project [1] (ASTP), underthe task - ‘VDL Mode 4 performance assessment’. ASTP is part of the Eurocontrol ADSProgramme [2]. VDL Mode 4 performance assessment is done in the context of the ADSTechnology Assessment Task [3] of the Eurocontrol ADS Programme.

The Eurocontrol ADS Programme aims toward the harmonised implementation of ADS in theECAC airspace. It started in 1998 and one of its early tasks is the assessment of candidateADS technologies. This assessment has been assigned to the ADS Studies and TrialsProject at the EEC.

One of the main candidate ADS-Broadcast (ADS-B) technologies is VHF Datalink Mode 41

(VDL-4). The FREER-3 trials (see Sec. 1.2) provided the opportunity to test air to air ADS-Boperation over an early form of VDL-4 which is known as VHF-STDMA. For this reasonASTP and the Eurocontrol ADS programme supported the FREER project in carrying outthese trials, and ASTP undertook an analysis of the collected data to measure theperformance of ADS-B over VHF-STDMA in air to air operation. The present report is thedeliverable of that analysis.

It should be noted that for the ASTP, this document also constitutes the third part of a seriesof four papers2 which assess and validate a VDL-4 simulation tool and then estimate VDL-4datalink capacity for FREER type applications.

1.2 FREER-3 TrialsThe Free Route Experimental Encounter Resolution (FREER-3) Trials were designed todemonstrate the technical feasibility of autonomous airborne separation assuranceoperations (including a long range deconfliction service developed at the EEC, see Sec.1.2.2) in low air traffic density airspace, based on ADS-Broadcast (ADS-B). Trial flights beganin September 1998 and continued until the 18 May 1999.

Several companies and organisations participated in the project. The FREER-3 applicationsoftware was specified by the Experimental Centre and developed by Carmenta AB ofSweden. There were trial flights firstly in Germany [organised by Deutsche Flugsicherung andinvolving aircraft from Lufthansa and Ostfriesische Lufttransport] and then in Sweden[organised by Luftfartsverket (LFV), the Swedish Civil Aviation Authority, and involving aircraftfrom Scandinavian Airlines]. Application software development and the trial flights werefunded by the Eurocontrol ADS Programme.

The performance analysis presented in this paper is based on data collected from theFREER-3 flights in the period February 1999 to May 1999. During that period the FREER-3system had reached a stable configuration and the most significant hardware/softwarereliability problems had been resolved.

1 The other candidates are 1090 MHz Ext. Squitter and Universal Access Transceiver).

2 The series consists of : (a) ref. [10] which specifies the compliance of the simulator to the VDL-4SARPs; (b) ref. [11], which presents the results of simulator validation tests using custom scenarios;(c) the present document in which the simulator is tested against live trial measurements; and (d) ref.[12], which presents the results of simulations designed to measure the capacity of VDL Mode 4 forrandom access and incremental broadcast.


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1.2.1 FREER-3 Aircraft and Equipment

For the trial flights in Germany 6 Lufthansa 747-200s were equipped with the FREER-3equipment along with a Fairchild Metroliner operated by Ostfriesische Lufttransport. In theSwedish trial flights two Fokker 28s operated by SAS were equipped, together with LFV ’sflight inspection aircraft (Beech 200).

Most of the trial sessions involved a commercial carrier (747 or Fokker-28) and a “obstacle”aircraft, e.g. the Metro or the Beech 200.

All the FREER-3 aircraft were equipped with a VHF-STDMA transceiver (SAAB Dynamicstransceiver Level T3 [7]) and a CDTI (MMI5000 airborne PC, running the FREER-3 softwaredeveloped by Carmenta according to EEC specifications). The CDTI and the T3 transceiverwere connected via a serial RS232 cable.

The “obstacle” aircraft trajectories were selected so as the create virtual trajectory “conflicts”3which the pilots were able to resolve by using the information provided on the CDTI displayby the FREER-3 application.

1.2.2 FREER-3 Software and use of ADS-B

The FREER-3 software runs on the onboard CDTI host and provides a Traffic SituationDisplay (TSD) to the pilot. It uses the separate VHF-STDMA transceiver to broadcastperiodically own position reports and also the planned trajectory (which is defined by thesequence next, next+1, … next+x waypoint). The desirable look ahead times covered by thetrajectory would be in the order of 10-20 minutes. The FREER-3 software takes into accountthe advertised trajectories of all aircraft in the vicinity, and if it detects any potential futureconflicts it displays them to the pilot on the TSD. All aircraft must have coherent pictures ofthe air situation and trajectories (through the received trajectory periodic broadcasts) and sothey should arrive at the same conclusions concerning any potential conflicts. In order tospeed up detection of any incoherence, every time an aircraft detects one or more conflicts itmust broadcast a conflict message (which is re-broadcasted periodically to ensure thateveryone receives it). A set of rules has been defined (“extended flight rules”) by which aparticular aircraft is assigned the responsibility to change trajectory in order to resolve theconflict. The pilot of that aircraft receives from the FREER-3 software an appropriate advisoryand can select any trajectory changes via the TSD.

The FREER-3 application uses the following three types of ADS-B reports.

a. State Vector (broadcasted periodically)

• a/c identification

• 4D position and speed

b. Trajectory (intent) report (broadcasted periodically and on any modification)

• sequence of future trajectory change points4 (next, next+1, next+2, …). Each TCP isdefined in terms of lat./long./alt. and time-to-go.

• Trajectory version identifier

c. Conflict reports5 (Broadcasted on conflict detection and repeated periodically)

3 The conflicts occurred in the horizontal plane, but the aircraft were in fact separated vertically.

4 TCP is a 3D point where the current operational trajectory is planned to change.


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• Applicable (own) trajectory version

• Per conflicting aircraft:

(a) identifier of aircraft with conflicting trajectory

(b) trajectory version taken into account

(c) calculated conflict priority

(d) conflict start time

• indication of manoeuvre responsibility

Trajectory and conflict reports are variable in length, because the former depend on thenumber of TCPs reported and the latter on the number of conflicting aircraft. Trajectoryreports should ideally be broadcasted periodically at a low rate, which should be increasedtemporarily should a trajectory change occur6. Conflict reports are also broadcastedperiodically. No other signaling is required on the datalink between the aircraft.

Currently there are no widely accepted ADS-B standards except for the RTCA ADS-BMASPS [5], which have not been adopted by either ICAO or EuroCAE. The latter do notprovide for either trajectory or conflict reports. The MASPS define Mode/Status (MS) and OnCondition (OC) reports which carry the next and next+1 TCP, respectively. These would beinsufficient for implementing the trajectory report, if more than two TCPs are required in thetrajectory, and this is usually the case if look-ahead times are to be of the order of 8-10minutes or more. In the FREER-3 trials it was assumed that up to four TCPs would beincluded in a trajectory report, but this was due to VHF-STDMA limitations.

1.2.3 Data Logs

In all the FREER-3 flights, data logs were collected by the MMI5000 software. Each log filecontains all the messages transmitted to and received from the VHF-STDMA transceiver,with their respective UTC time-stamp. An extract of log file is given below, as an example:

...981121 10:51:07.59 p ZZZ9999 N58ø35'25.50" E016ø11'59.40" 0 KN 36.0ø 80 feet, NO NAV,level981121 10:51:07.61 T GOT907 52 1 4 [ 58.0950 15.2733 83 10:55:33 ] [ 57.9850 15.5683 8310:59:28 ]981121 10:51:07.74 P GOT907 N58ø12'57.66" E014ø55'57.59" 175 KN 12.3ø 8363 feet, 3DD, level981121 10:51:08.13 C OWN 52 52 0981121 10:51:08.62 RADIO TRAFFIC STATUS $B000006ID:19892*48981121 10:51:08.64 C OWN 52 52 0981121 10:51:08.74 P GOT907 N58ø12'56.10" E014ø56'02.27" 175 KN 12.3ø 8363 feet, 3DD, level981121 10:51:09.00 G 10:51:09, 8 satellites of 11 possible, 11,6,6 (VDOP,EDOP,NDOP)981121 10:51:09.74 P GOT907 N58ø12'54.54" E014ø56'06.89" 175 KN 12.3ø 8363 feet, 3DD, level981121 10:51:10.22 p SAS9821 N56ø56'06.90" E014ø33'22.19" 374 KN 4.1ø 9440 feet, 3DD, level981121 10:51:10.64 p ZZZ9999 N58ø35'25.50" E016ø11'59.40" 0 KN 36.0ø 80 feet, NO NAV,level981121 10:51:10.74 P GOT907 N58ø12'52.98" E014ø56'11.58" 175 KN 12.3ø 8364 feet, 3DD, level981121 10:51:11.20 p ZZZ9999 N58ø35'25.50" E016ø11'59.40" 0 KN 36.0ø 80 feet, NO NAV,level981121 10:51:11.27 DATA LINK MANAGEMENT $9@ESSPB01010500001C24012C0000000000*2F....

The log file contains not only own and third party VHF-STDMA sync burst messages but alsostatus reports issued by the onboard VHF-STDMA station (GPS position, GPS status, etc.).

5 Notification of detected conflicts with notification of responsibility for action to resolve conflict

6 In the FREER-3 trials the trajectory reporting rate was kept constant (30 sec period).


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1.3 ADS-B and VHF-STDMA ConfigurationThe T3 transceiver comprises a single VHF transceiver, and a single GPS receiver. It uses asingle VHF channel 25 KHz wide. The VHF frequency assigned to NEAN (136.95 MHz) wasused in most trial sessions. In at least one session the 136.975 MHz channel was used. Asingle VHF antenna was used on each aircraft for STDMA transmission and reception. Theantenna was located at the rear bottom of the aircraft frame. Transmission power was set to10 W.

VHF-STDMA is an early form of VDL-4 (e.g. the ICAO standard, still in draft form) and hassome significant differences from the latter:

• STDMA uses GMSK modulation at 9600 bits/sec. The VDL-4 SARPs specify GFSK at19200 bits/sec. In theory GMSK offers better discrimination capability than GFSK, soSTDMA should be more resistant than VDL-4 to interference.

• STDMA uses a single VHF channel while VDL-4 can use two.

• STDMA provides a periodic synchronisation burst (known as sync burst) service whichuses slot reservation and which is similar but less sophisticated than the periodic syncburst service specified in the VDL-4 SARPs.

Each STDMA sync burst was deemed to supply all the State Vector elements required by theFREER-3 application. The sync burst transmission period was set to one sec. It should benoted that in an operational VDL-4 system, the en route sync burst transmission period wouldbe 10 sec.

STDMA provides a random access service (known as freetext mode) which allows fortransmission of fixed length7 ASCII coded messages without prior reservation. VDL-4SARPs also specify a similar random access specific service but which uses a moresophisticated algorithm and allows for variable length binary coded messages. The STDMAfreetext mode was used to broadcast both trajectory and conflict FREER-3 reports. Thereporting period was set to 30 sec for both report types.

One freetext message was used per trajectory/conflict report. Up to four TCPs could be fittedwithin a single freetext message. Similarly up to four conflicts could be fitted within a singlefreetext message.

It should be noted that

• the pilots of two aircraft communicated via a specially assigned VHF voice channel. Insome flights the frequency used was 136.7 MHz. It will be seen in the analysis resultsthat this may have caused noticeable performance deterioration due to co-siteinterference from the VHF voice system to STDMA.

• the trial flights took place in areas where a number of other NEAN stations (on aircraft,cars, and ground stations) were also active. Consequently in most trial sessions (exceptthe very few ones which used a different VHF frequency) the channel had to be sharedwith NEAN stations. Analysis and simulations have taken account of the channel trafficload due to NEAN.

7 each message occupies three consecutive slots (one slot is 256 bits) allowing for about 160 usersupplied ASCII characters per message.


EEC/ASTP Dec. 99

1.4 Measured Performance ParametersPerformance analysis was done according to the criteria established in the ADS TechnologyAssessment Task [3], namely

- Reliability measured by the probability of correct message reception (also calledmessage success rate or MSR) versus time and versus range

- Refresh Rate measured by distribution of message refresh periods (= time intervalbetween successive updates from the same station) versus time and versus range

The environment was characterised in terms of :

a) Traffic load on the VHF channel,

b) number, trajectory, and type of non FREER VHF-STDMA stations, and message typesthey emit

c) GPS status in terms of the numbers of GPS satellites seen and used by the T3transponders on the FREER aircraft

MSR and Refresh Periods were calculated on the air to air communications between the trialaircraft.

1.5 Simulator : E-SPSIn order to interpret the behaviour of VHF-STDMA that was observed in the trial flights, eachtrial session was also simulated using identical flight and environment scenarios. Thesesimulations were done with a tool developed by the Swedish CAA [8] for validating the VDLMode 4 SARPs. That simulator is known as the SPS. ASTP enhanced the SPS to supportadditional VDL-4 features. These added features (namely the VDL-4 Specific Services onrandom access and incremental broadcast) were necessary for the simulation of the FREER-3 application. The enhanced SPS is referred to as the E-SPS.

The (E-)SPS uses input scenarios describing the trajectories of each station over ageographic area and can calculate performance measures such as those listed in Sec. 1.4either per aircraft or as averages over the total aircraft population.

The FREER-3 simulations used the trajectories recorded during the trial flights as stationsposition reports. All the stations that were active during the session were added in thesimulation scenarios including their movement (if any). The same reporting rates were usedas those recorded in the trial sessions.

The SPS incorporates a simplified VHF physical channel model which does not explicitlyaccount for factors such as detailed radio transmitter/receiver characteristics, VHF antennaradiation patterns, relative aircraft orientation, geographic terrain profile, and multipath. Inorder to simulate the trial sessions, the SPS scenarios implemented over a spherical earththe flight trajectories recorded in the trials (maximum distance was of the order of 190 nmi).The scenarios included the base stations which appeared in the FREER-3 logs. They alsoincluded other mobile stations (aircraft and vehicles) which appeared in the FREER-3 logs(through their recorded position reports). The E-SPS does not model ACI.

It should be noted that the E-SPS maintains logs of sent and received messages for eachaircraft of interest. Consequently, one can apply the same analysis as in the logs for theFREER-3 trials (see Sec. 1.4).

1.6 ScopeThe FREER-3 trials were designed to demonstrate the feasibility of autonomous airborneseparation assurance in low air traffic conditions using ADS-B/VHF-STDMA implemented as


EEC/ASTP Dec. 99

described in Sec. 1.2. The performance measurements presented in this paper provide anindication of performance to be expected for air to air ADS-B broadcast in an STDMAenvironment in non-congested VHF channel. Use of VDL-4 compliant equipment may deliverbetter air to air performance, and this issue will be tested in future Eurocontrol trials.

Interpretation of the results must take into account the following factors:

• Prototype hardware and software equipment (transponders and CDTI/TSD) was usedthroughout the trial. Equipment failures may have affected the results. Nevertheless aconsiderable number of flights were performed in order to test the equipment and theresults presented in this paper refer to flights done after that period of testing

• VHF-STDMA has differences with VDL Mode 4 as defined in the ICAO SARPs. Thesedifferences were outlined in Sec. 1.3. In the trials the datalink was lightly loaded (usuallybelow 25%). In these conditions the more sophisticated slot selection managementprocedures envisaged in the VDL-4 SARPs should not play a critical role. Furthermore,VHF-STDMA uses a lower transmission rate (9600 versus 19200) and has betterdiscrimination capability, hence its results should be positively influenced.

• Antenna placement was not necessarily optimal in the trial aircraft. VHF antenna isolationmay have been inadequate to prevent co-site interference from VHF voice and ACARS.Since a single VHF antenna was used for STDMA on each aircraft, blocking by theairframe may well have affected air to air communications. LFV tried to find goodantenna positions on the trial aircraft but it is recognised that VHF antenna placement isan open issue.

Performance analysis focused on reliability and surveillance refresh rate for the periodicposition reports and also trajectory messages. The latter although periodically broadcastedwere transmitted using a random access method which is not the most efficient way of usingSTDMA.. Unfortunately VHF-STDMA does not provide any other method for transmitting longmessages. VDL-4 provides more options. This issue is explored in a separate ASTP report,see ref. [12].

Finally, it should be noted that ASTP is continuing the evaluation of VDL Mode 4 performanceas part of the ADS technology Assessment Task of the Eurocontrol ADS Programme. Trialswith VDL Mode 4 compliant equipment are being conducted, see ref. [3RENV], and theresults will be compared with the VHF-STDMA results presented in this paper.


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2. Flight ScenariosFour sessions were selected for presentation in this paper as they are representative of theFREER-3 and VHF-STDMA behaviour observed during the totality of trial sessions. Thesesessions are:

2.1 Session of 21 November 1998This session was selected because it comes from the period of system testing. It is meant todemonstrate the level of performance achieved prior to the completion of system debuggingand optimisation.

The following two aircraft participated and flew the trajectories shown in Figure 1:

• The LFV Beech 200 took off from point A and flew, with callsign GOT905, to point Bwhere there was an datalink interruption (due to equipment reconfiguration) for about fiveminutes. The callsign was changed to GOT907 and the aircraft continued to point C andthen on to D where it landed.

• the SAS Fokker 28, whose trajectory is only partially shown, first flew a parallel path toAB and on its return flight it followed the path crossing CD. It used the callsign SAS9821.

Figure 1 Aircraft trajectories, Trial Session 25/11/98The longest time period during which the FREER application was turned on for both aircraftwas from 9:37:00 am to 11:04:00 am, giving a total trial time of 87 minutes.

Two conflicting situations for testing the FREER-3 application were triggered during this flight:


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1. overtake by SAS9821 (F28) of GOT90X (B200) along path AB

2. SAS9821 (F28) approaching GOT907 (B200) near point D.

Figure 2 plots the great circle distance between the two aircraft during the session. Theirdistance decreased steadily as they approached point B, where the B200 station wasswitched off for five minutes, and then it increased to the maximum of 200 nmi at point C.

Figure 3 shows the altitudes of the two aircraft as recorded on the B200. The two aircraft keptconstant flight levels, except during the F28 landing, with the B200 being at a lower altitudethan the F28.

Var iation of distance between B200 and F28 (as seen on B200)

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Figure 2 Aircraft separation distance, trial session 25/11/98

Var iation of flight altitudes, 21/11 /99

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Figure 3 Aircraft Altitude Variation, Session 25/11/98


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The FREER-3 aircraft logs showed that a number of other VHF-STDMA were active andwithin range during the session. These were NEAN base stations, aircraft, and vehicles. Thepeak numbers of NEAN participants were:

• Up to 5 base stations: These stations were broadcasting their position every 8 or 20 sec,as well as other types of NEAN messages including DGPS corrections.

• Up to 4 mobile stations (two aircraft and two vehicles). These stations were broad-casting their position once per sec.

The following table calculates the peak traffic load per minute:

Type ofstation

Type ofreport

Number oftransmitters

Transmissionrate (msg/min)

Length(slots)

Channel load

Base station Positionreport

5 3, 7.5 1 1.3%

DGPS 2 15 2 2.7%Datalink

Management3 5 1 0.7%

(see Note 1)Networkstatus

3 1 3 0.4%8

PING 1 2.5 2 0.2%8

ACK 1 2.5 1 0.1%8

Aircraft Positionreport

4 60 1 10.7%

Trajectory 2 2 3 0.5%Conflict 2 2 3 0.5%PING 1 2.5 3 0.3%

PETAL 1 1 3 0.1%8

Vehicle Positionreport

2 60 1 5.3%

= 22.8%

Note 1 : Datalink Management reports correspond to STDMA messages type 130, which isused by base stations to reserve periodic blocks of slots for their uplinks, such as positionreports, DGPS and Datalink Management. It was found that 30 blocks of 9 slots werereserved within each superframe. Consequently 12% of channel capacity was reserved bythe ground stations, while only 4.7% were actually used (ground station position reports plusDGPS plus datalink management).

2.2 Session of 6 February 1999This was a typical FREER-3 trial session after the completion of system debugging andoptimisation. In this session the T3 transponders and MMI5000 equipment onboard eachaircraft had been upgraded to fix the problems detected in previous trials. A particularcharacteristic of this session was the important presence of STDMA stations on vehicles(from the NEAN project).

Figure 4 plots the trajectories of the two FREER-3 aircraft on the lat/long plane during thesession. The two trajectories were constructed by plotting the position reports logged on eachaircraft. The SAS Fokker 28 took off from point αααα, flew through point ββββ to point γγγγ, where itlanded, and then flew back to a via point δδδδ. The Beech 200 departed from point A and flew topoint C via point B. Around point B the Beech 200 flew a holding pattern waiting for theFokker 28 to pass by. It then followed the F28 in a station keeping formation until the F28landed at C.

8 Network status, PING, ACK, and PETAL messages are transmitted using the freetext mode.


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Figure 5 plots the altitudes of the two aircraft (SAS Fokker 28 and LFV Beech 200) for theduration of the trial session, as derived from the position reports logged on each aircraft.There was a loss of contact in the period from 60 to 70 minutes (trial time). During that timethe Fokker 28 was landing (at point γγγγ).

Figure 6 plots the great circle distance between the two aircraft for the duration of thesession. The aircraft came together at point β β β β and moved apart up to a maximum distance of160 nmi. Then they came back together again at around point B, where the B200 was waitingin its holding pattern. The B200 then followed closely the F28 up to landing presumably in astation keeping exercise.

Figure 5 shows that the B200 kept a constant flight level for most part of the session and upto the point B where the station keeping manoeuvre was started. The F28 was at a higherflight level for most of this period. During the station keeping manoeuvre the two aircraft wereat roughly the same flight level and closely spaced up to the final approach of the F28 wherethe B200 took off again.

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Figure 4 Flight T

Figure 2-2a Flight trajectories, 5-02-99

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Dec. 99

rajectories, Session 25/11/98


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e, n

mi

Dis tance, nmi

Figure 6 Aircraft Horizontal Separation, 6/2/99The peak traffic load was estimated following the same method as for the previous trials. Theresult is shown in the following table. The estimated total peak traffic load is 52.1%, which isconsiderably higher that the traffic load observed on the flight trial of the 25/11/98 (23%). Thisis due to a much more significant presence of STDMA stations on vehicles.


EEC/ASTP Dec. 99

Type ofstation

Type ofreport

Number oftransmitters

Transmissionrate (msg/min)

Length(slots)

Channel load

Base stations Sync Burst 5 7.5 1 1.7%DGPS 2 15 2 2.7%

DatalinkManagement

3 5 1 0.7%

Networkstatus

3 1 3 0.4%

PING 1 2.5 2 0.2%

ACK 1 2.5 1 0.1%

Aircraft Sync Burst 4 60 1 10.7%Trajectory 2 2 3 0.5%Conflict 2 2 3 0.5%PING 0 2.5 3 0.0%

PETAL 0 1 3 0.0%Vehicles Sync Burst 13 60 1 34.6%

= 52.1%

2.3 Session of 15 February 1999This is another typical FREER-3 session done after the completion of system debugging andoptimisation, but with a lower traffic load and a smaller separation range between the twoaircraft. It is meant to demonstrate that the levels of performance measured in the session of6/2/99 were representative.

Figure 7 plots the lat/long trajectories of the two aircraft during the session. These trajectorieswere constructed from the position reports received on each aircraft. The Beech 200departed from near point A, flew a waiting pattern through point B and point C waiting for theFokker 28 to take off and then flew through point D to E near which it landed. It then took offand flew through points F and G to return at the airport of departure. The Fokker 28 took offnear point A’ and flew to point B’ in the proximity of which it landed. There was a virtualconflict with the B200 between points D and E (aircraft flying in opposite directions). Thenthe F28 took off again (becoming visible to the B200 at point C’) for the return flight leg losingcontact with the B200 at point D’ where it proceeded to land. There was another virtualconflict with B200 around point F.

The observed gaps between points A’ and D’ and also between B’ and C’ indicate that thetwo aircraft had no contact during F28 take off and landing.

Figure 8 plots the altitude variations of the two aircraft during the session. The B200 cruisedat a lower flight level than the F28. Contact was lost while the two aircraft were descendingand the B200 had reached a low altitude. They re-established contact when they took offagain, and finally lost contact at the end of the session when the B200 was at a low altitude.

Figure 9 shows the variation of great circle distance between the two aircraft during thesession. The two aircraft move towards each other and then move apart in both flight legs.The maximum separation distance is about 100 nmi. The gap in the mddle is the period oflost contact due to loss of LOS.


EEC/ASTP Dec. 99

56.5

57

57.5

58

58.5

59

59.5

60

14 15 16 17 18 19

Longitude (deg.)

Latit

ude

(deg

.)

SAS Fokker 28

Beech 200

AB C

DE

FG

A'

B'

C'

D'

Figure 7 Flight Trajectories, 15/2/99

0

5000

10000

15000

20000

25000

30000

0 10 20 30 40 50 60 70 80 90 100

Trial Time, min

Alti

tude

(ft)

Fokker 28

Beech 200

Figure 8 Flight altitudes, 15/2/99


EEC/ASTP Dec. 99

0

20

40

60

80

100

120

0 10 20 30 40 50 60 70 80 90 100 110

Trial Time, min

Dis

tanc

e, n

mi

Dis tance,nm i

Figure 9 Aircraft horizontal separation, 15/2/99Figure 10 plots the average traffic load per minute (=number of slots used over the maximum2250) as derived from the reception logs of each FREER-3 aircraft. For the F28, themaximum traffic load was around 24%, while for the B200 the traffic load was up to 16%.

0%

5%

10%

15%

20%

25%

-30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110Trial Time, min

Cha

nnel

Loa

d, %

Fokker 28

Beech200

Figure 10 VHF-STDMA Channel Load variation, 15/2/99


EEC/ASTP Dec. 99

2.4 Session of 18 May 1999In this session the 136.975 MHz VHF channel instead of the usual 136.95 MHz. Theobjective was to measure performance without co-channel interference from other NEANsystems. Only one ground station was active in the same channel during this trial session.During this session, the pilots recorded the times of their VHF voice communications. Thiswas done in order to check any co-site interference impact between VHF voice (at 136.7MHz) and VHF-STDMA.

The flight trajectories are shown in Figure 11. The F28 flew the usual two leg trajectory fromA to B and back as in the previous sessions. The B200 flew two flight legs both taking offfrom point C. In both legs the B200 flew a holding pattern designed to meet with the F28while flying in opposite directions.

∀

∀

∀

SAS1153 (F28)GOT910 (B200)GOT903 (B200)

A

B

C

Figure 11 Flight Trajectories, 18/5/99Figure 12 shows the evolution of the altitudes of the two aircraft during the session. Thealtitude values were extracted from the broadcasted position reports. Altitude evolution issimilar to the session of 15-02-99 (see Sec. 2.3). The F28 kept a much higher cruisingaltitude than the B200.

Figure 13 shows the variation of the great circle distance between the two aircraft during thesession. During each flight leg the aircraft moved first toward each other and then movedapart. they lost contact while both were descending to land (66th min – 125 nmi apart, at FL60). The B200 took off first for the second leg and was at a cruising FL of 95 when itrecovered contact with the F28 which was climbing after takeoff (120th min, FL=40). At that


EEC/ASTP Dec. 99

point their separation distance was at its maximum (160 nmi). The gap in the middle is theperiod of lost LOS.

Variations of altitude

0

5000

10000

15000

20000

25000

30000

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180

Trial Time (min)

Alti

tude

(ft

)

F28B200

Figure 12 Flight altitudes, 18/5/99


EEC/ASTP Dec. 99

Variations of distance between aircraft

0102030405060708090

100110120130140150160

20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

Trial Time (min)

Dis

tanc

e (n

mi)

Figure 13 Aircraft horizontal separation, 18/5/99

2.5 Simulation ScenariosThe four trial sessions were simulated with the E-SPS. In each simulation the messageSuccess Rate (MSR) was calculated similarly to the trial measurements. The simulationscenario included the two trial aircraft and all the stations that appeared in the FREER-3 datalogs collected during the trial sessions.

The kinematics of aircraft and vehicles, as well as the locations of base stations wereextracted from the log files of the Fokker 28 and Beech 200.The trajectory of each mobilestation was determined as a list of 3D positions, derived from the recorded position reports.Speed was derived from the report timestamps. Between successive scenario definedpositions the E-SPS applies linear interpolation to determine intermediate positions.

The two trial aircraft were set to transmit position reports every one second, trajectoryreports (3 slot long) every thirty seconds, and conflict reports (3 slot long) on a random basis(uniform probability distribution) and then retransmitted periodically every 30 sec for 6minutes.

In the simulations, trajectory and conflict reports were transmitted using the VDL-4 randomaccess mode (p-persistent algorithm with parameter settings TM1=80, TM2=1500, p=13/256,and VS3=135), while in the trial they were transmitted as freetext messages. The VHF-STDMA freetext mode uses a 1-persistent type algorithm.

All mobile stations were set to follow the trajectories indicated by their position reports aslogged in the corresponding trial session. The NEAN station position report period was setper station to the period recorded in the corresponding trial session. The NEAN station non-position reports were included as random access mode message transmissions of equallength to those recorded in the trials. In the trials NEAN stations used the VHF-STDMAfreetext mode.


EEC/ASTP Dec. 99

Base stations were set to transmit position reports within reserved blocks of slots predefinedin an E-SPS input file. Consequently no transmission conflict could arise among them. VHF-STDMA base stations operate in this mode.

The simulations used a channel width of 2250 slots per minute (identical to VHF-STDMA).

The terrain profile, other than earth curvature, was not accounted for.

In the absence of colliding STDMA transmissions the (E-)SPS assumes that the messageerror rate in LoS conditions is determined by a piecewise linear relationship with distance.The default setting for the error rate is for no errors at distances within 200 nmi. This defaultsetting was used for the simulations presented here.

The E-SPS was set to Q2 parameter default values recommended in the SARPS v.54.2 forPeriodic Broadcast (see §3.3.11.5.2 of SARPs v5.4.2). There is no equivalent feature in VHF-STDMA.

E-SPS was set to use slot selection levels 0-4 both for Periodic Broadcast transmissions andRandom Access transmissions. There is no equivalent feature in VHF-STDMA.

E-SPS was set to apply the procedure for reservation conflict resolution that is specified inthe SARPs v5.4.2 (see §3.3.6.5). Consequently the E-SPS re-applied the slot selectionprocedure to assess the availability of conflicting slots prior to cancelling a reservation. Thereis no equivalent feature in VHF-STDMA.

In the simulation, performance was measured in terms of F28/B200 position and trajectoryreport success rate. MSR was calculated as average per minute for the duration of thesimulation scenario. The duration of the scenario was of equal duration to the correspondingflight trial plus a startup time to let NEAN base stations come to steady state


EEC/ASTP Dec. 99

3. Data Log AnalysisThis section presents the results of performance analysis done on the data logs collectedduring the four trial sessions described in Sec. 2.

3.1 Position reports

3.1.1 Success Rate variation with time

Position reports were broadcasted every second as single slot sync bursts. Consequently thereceiving aircraft should log 60 reports per minute. The logs of the two aircraft were analysedto count the number of position reports received per minute. The percentage of reportsreceived over the theoretical maximum of 60 was considered to be a measure of the positionreport success rate (air-to-air probability of successful delivery). Figure 14-Figure 179 showthe success rates measured for the sync bursts (position reports) of each aircraft in each ofthe four trial sessions described in Sec. 2. In each figure, success rate is plotted against trialtime. Note that the large gaps in the middle of the session are due to loss of LOS (see Sec.2)

Comparison of the four plots indicates the following points:

• Some significant MSR variations from one minute to the next occurred in all sessions.Most minute to minute MSR drops were within 20% but there were some peaks wherethe drop exceeded 40-50%.

• The table below shows the average MSR per aircraft in each session (ignoring theperiods of lost LOS). It indicates also the peak channel load and the maximumseparation for each session. There is no correlation between mean MSR and peak trafficload values, but there seems to be some correlation between mean MSR and max.separation. The session of 15-02-99 achieved the best average performance and hadthe least separation while the session of 25/11/98 had the worst performance and themaximum separation. MSR differences in sessions of 06/02/99, 15/02/99, and 18/05/99are minor.

Average Success Rate per min

Broadcasting Aircraft 25/11/98 06/02/99 15/02/99 18/05/99

F28 64% 80% 86% 80%

B200 49% 80% 84% 85%

Max. Separation, nmi 200 160 100 160

Channel Load 22.8% 52.1% 24% 6.8 %

• The session of 25-11-99 had the worst performance particularly during the return flight ofthe F28. However, system testing was not yet complete and results may have beeninfluenced by T3 software bugs which were fixed later.

9 The F28 line corresponds to the success rate of F28 position reports (as received on B200) and theB200 line corresponds to the success rate of B200 position reports (as received on the F28).


EEC/ASTP Dec. 99

• The observed success rate variations do not correlate with the variations of the trafficload during the session. This is illustrated by Figure 18 which compares the number of(mobile and base) stations visible by the B200 during the session 06/02/99 with the F28MSR as received on the B200. That session presented the highest peak traffic load(51%). It can be seen that changes in the number of received stations do not correlatesignificantly with MSR variations.

• In session 18-05-99 there was no NEAN traffic because of the different frequency used.Yet there was no noticeable impact on performance as comparison of Figure 17,Figure16,Figure 15 indicates. This demonstrates the capability of STDMA to share the channelamong multiple stations without performance degradation. The channel congestion limitis clearly well above the traffic load range 5-50% that occurred in the FREER-3 trials.

Analysis of the reports received by the ground stations showed that ground-to-airperformance was more stable than air-to-air. This is illustrated by Figure 19 which plots thenumber of uplink messages received per minute on the F28 per source ground station.

Reception of NEAN mobile station (aircraft and vehicles) messages presented the samevariations as those seen between the FREER-3 aircraft, as shown in Figure 20.

The observed success rate variations could be attributed to the following factors:

(a) Line of Sight Obstruction: Line of Sight may be lost temporarily due to blocking by theaircraft airframe and/or the terrain profile. The available data were not sufficient to permitreliable detection of this condition in the analysis.

(b) Colliding VHF-STDMA transmissions: Collisions may occur in STDMA when the stationshave to select an unreserved slot randomly, or when they are unaware of reservationsmade by other stations. The latter case occurs for hidden terminals10, and also for newlyvisible stations. As the aircraft moves, new STDMA stations come into range and untiltheir reservations are detected there is a chance of colliding reservations for a shorttransition period. Some evidence of the hidden terminal situation occurring have beenfound11. Some of these conflicts should also be detectable in the simulations which arediscussed in Section 3.1.3.

(c) Co-Site Interference from VHF voice: STDMA reception could be affected by VHFtransmissions in adjacent channels on the same aircraft. Such transmissions could bemade for pilot voice communications and also for ACARS. The strength of any suchinterference would depend on VHF channel separation and the isolation between theVHF antennas on the aircraft. There was no ACARS system active on the two FREERaircraft, and the nearest voice communications channel used was at 136.7 MHz. Thischannel served for pilot to pilot communications between the FREER-3 aircraft. In thesession of 18-05-99, the times of pilot voice communications were recorded. Figure 22and Figure 23 show the success rate of F28 position reports as received on the B200 (foreach flight leg). The times where the B200 pilot recorded a voice transmission aremarked in red. B200. It can be seen that pilot voice transmissions correspond to deepdrops in the success rate. Figure 24 and Figure 25 show the corresponding graphs for

10 For the purpose of this report, hidden terminal is defined as a user that lies within LOS distancebut has one or more transmissions that are not visible due to garble, signal attenuation, or terrainobstruction.

11 In the 25-11-98 session, the F28 and B200 reception logs indicated that SAS9821 (F28) receivedsync bursts from two mobile stations [namely, an aircraft, SAS590, and a ground vehicle, #DCAAV01]which were not visible on the B200. Figure 26 compares the received report rates on the F28 fromthese two stations and the B200. It can be seen that report success rate lows tend to occur at thesame minute for the three stations. This alignment would suggest the occurrence of conflictingtransmissions.


EEC/ASTP Dec. 99

reception on the F28. Again, F28 pilot voice transmissions (marked in red) coincide withdeep lows in the success rate. Clearly co-site interference from VHF pilot voice could bea cause of at least some of the observed peak success rate variations.

(d) Antenna Gain variations: In FREER-3 both aircraft used a single VHF antenna located atthe bottom of the aircraft. It is well known that there can be significant 3D variations in theantenna gain patterns, and a substantial shielding effect from the airframe. It is thereforelikely that as aircraft move, RF signal strength will vary depending on their relativeorientation. This could lead to lost transmissions, especially when an aircraft passesthrough a cone of silence of the other.

(e) STDMA Implementation or aircraft installation problems: During the 1998 trial sessions,the T3 software had a bug concerning slot selection when the selection procedurehappened to start from a slot contained within a block of slots already reserved by a basestation (these slots are used for DGPS uplinks). Then, the algorithm implemented in theT3 had difficulty moving out of the reserved zone hence leading to report loss. Fig. 3-1.1hfrom Session 21/11/98 shows the report rates received on the F28 log for the variousmessage types. It can be seen that during the second flight leg the F28 traversed anarea of high DGPS uplink rates. This could be a reason for the drop of performanceobserved in the second part of the 25/11/98 session. In any case, the slot selectionproblem was subsequently fixed and the equipment used in all the trials after Jan. 1999did not suffer from this problem.

Some evidence of colliding reservations was found which might be attributed to hiddenterminals, but it is suspected that it might be due to STDMA implementation errors. It wasnoted that some position report losses tended to occur at the same second in severalsuccessive minutes. This is illustrated by Figure 27 which plots the instances of B200 reportsreceived on the F28 (session: 15/02/99). In this Figure, the vertical axis shows sixty positions(0 to 59) corresponding to the seconds contained in a minute. The horizontal axis shows oneposition per trial minute. For each report received a dark square is plotted at the point (sec,min) of its arrival time. Consequently, blank horizontal lines mean that in successive minutesa report was lost at the same second. Since periodic slot reservations are made with a periodof a minute for the next 3-8 minutes, it can be expected that colliding periodic reservationswould lead to report losses that correlate up to the next 8 minutes. Figure 27 shows a lot ofhorizontal blank lines especially in the second part of the session. Such reservation collisionsmight be at least partly due to hidden terminals, but it is surprising that some blank linesexceed 20 minutes in length.

Figure 28 shows the corresponding plot for the 18-05-99 session, where there was no NEANtraffic and hence no hidden terminals. Figure 28 shows again the occurrence of horizontalblank lines mainly during two specific periods, i.e. from the 30th to 40th minute and from the125th to the 135th minute. B200 Pilot voice transmissions occurred at the same periods (34th,44th, and 134th-135th minute). The horizontal blank lines however seem to start before theoccurrence of these voice transmissions.

If the above correlation was resulting from some inherent feature of STDMA, it should also beobservable in the simulated sessions. This subject is discussed further in the presentation ofthe simulation results, see Sec. 3.1.3.


EEC/ASTP Dec. 99

Position Report Success Rate per minute

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

1 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86

Trial Time, min

Suc

cess

Rat

e %

B200

F28

Figure 14 Position MSR, 25-11-9812

Position Report Success Rate versus trial time

0.0%

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0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160Trial Time, min

Suc

cess

Rat

e %

F28B200


12 F28 reports disappeared in the period 41 to 45 min due to a reconfiguration of the MMI5000 on theB200. During this time, the B200 did not record received position reports, although it continued tobroadcast its position.


EEC/ASTP Dec. 99


0%

10%

20%

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-10 -5 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

Trial Time, min

Suc

cess

Rat

e %

F28

B200



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e %

B200

F28



EEC/ASTP Dec. 99

0123456789

101112131415161718

-20

-11 -2 7 16 25 34 43 52 61 70 79 88 97

106115

124133

142

Trial Time. min

No

of s

tatio

ns

0%

10%

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30%

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50%

60%

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90%

100%

Suc

cess

Rat

e %

A irc raft

Vehicules

Base S tations

F28 SB M SR

Figure 18 Number of Stations visible from the B200 – 06/02/99

Position reports from NEAN ground stations

0

1

2

3

4

5

6

7

8

9

10

1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77 81 85

Trial Time, min

Rep

orts

per

min

@ ESM SB01

@ ESGJM 01

@ ESDBB01

@ EKCHB01

@ ESGGB01

@ ESSPB01

@ ESSAB01

Figure 19 Base station uplinks in F28 log – 25/11/98


EEC/ASTP Dec. 99

Position reports from NEAN mobile stations

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10

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30

40

50

60

1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77 81 85

Trial Tim e, m in

Rep

orts

per

min

#DCAAV01

#SA338

#SA318

#SA308

#B200

DLH430

SE-DGL

SAS590

ZZZ9999

Figure 20 NEAN mobile station messages in F28 log - 25/11/98

STDMA and application specific messages

0

5

10

15

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25

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35

1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77 81 85

Trial Tim e, m in

repo

rts/

min

PETAL

P ING+ACK (Up/Downlink )

Network S tatus

Datalink M anagem ent

DGPS

Figure 21 Non FREER messages appearing in F28 Log – 25/11/99


EEC/ASTP Dec. 99

B200 Reception

12:33

12:23

0%

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12:1012:12

12:1412:16

12:1812:20

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12:3012:32

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12:4612:48

12:5012:52

12:5412:56

12:5813:00

Trial Time

Suc

cess

Rat

e %

F28 report success rate

Figure 22 VHF Voice Transmissions versus position MSR – 18/5/99

B 2 0 0 re c e p t io n

1 4 :0 4

1 4 :0 3

0 %

1 0 %

2 0 %

3 0 %

4 0 %

5 0 %

6 0 %

7 0 %

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1 0 0 %

13 :48

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14 :20

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14 :24

14 :26

14 :28

Tria l T im e

Suc

cess

Ra

te %

F 28 repor t suc c ess ra te

Figure 23 VHF Voice transmissions versus position MSR – 18/05/99


EEC/ASTP Dec. 99

F28 Re ce ption

12:3712:3312:29

0.0%

10.0%

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12:10 12:15 12:20 12:25 12:30 12:35 12:40 12:45 12:50 12:55

Trial Time

Suc

cess

Rat

e %

B200 report success rate

Figure 24 VHF voice transmissions versus position MSR – 18/5/99

F28 reception

14:17

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14:1214:14

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14:2014:22

14:2414:26

14:28

Trial Time

Suc

cess

Rat

e, %

B200 report success rate

Figure 25 VHF Voice transmissions versus position MSR – 18/5/99


EEC/ASTP Dec. 99

Mobile Station Position Reports received on the F28

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40

50

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70

33 38 43 48 53 58 63 68 73 78 83

Trial Tim e, m in

repo

rts/

min

#DCAAV01

SAS590

B200

Figure 26 Success Rate drop alignment – 25/11/98

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T rial T ime, min

seco

nds

F28 Pos ition Report

Figure 27 Time correlation of F28 report losses on B200 – 15/02/99


EEC/ASTP Dec. 99

F28 Position Re ports, re ce iv e d on the B200

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60

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Trial Time, min

Sec

ond

(0-5

9)

F28 Report

Figure 28 Occurrences of F28 position reports received on the B200, 18/5/99

3.1.2 Range and Refresh Rate

Figures 29 to 31 plot the mean success rate (minute average) of position reports versus thegreat circle distance (minute average) between the trial aircraft for trial sessions 06/02/99,15/02/99, and 18/02/99, respectively. Position report success rates are indicated pertransmitting aircraft.

All three figures show a peak MSR of 100% but with wide variations even in close range.Potential factors of the observed variations have been discussed in the previous section.Figure 29 and Figure 31 suggest that MSR declines in the 140-160 nmi range, but the rate ofdecline is markedly lesser in the 18/5/99 session. In that session the VHF channel was freeof NEAN traffic.

MSR versus range curves can be used to calculate ADS-B position refresh update periodpercentiles (i.e. update periods not exceeded with x% probability) versus range. calculation ismade on the basis of the following equation13 [5]

P= 1-(1-p)Tc/T => Tc = T * ln(1-P)/ln(1-p)

Where P = required percentile of update period, Tc = update period at percentile P, T=datalink message period, p = (measured in the trial) success rate

This calculation is useful because


EEC/ASTP Dec. 99

" ADS-B RENVperformance requirements are usually expressed in terms of state vectorrefresh update period percentiles for different ranges

" in FREER-3 the VHF-STDMA system operated at a rate of one position report persecond. However the future operational VDL-4 system is expected to operate with 1, 5,and 10 sec report periods depending on the operational context (on airport surface, TMA,and en route, respectively).

Position refresh periods were calculated from MSR measurements for reporting periods of 5and 10 sec. If the requirements of long range deconfliction were defined, one could thendetermine the range of the ADS-B system (e.g. the distances for which the applicationrequirements are met) for this application. Such requirements have yet to be defined, soinstead, comparison was made with ADS-B MASPS requirements for separation assuranceand flight path deconfliction14. The calculated 95% refresh periods are shown in Figure 32,Figure 33, and Figure 34.

The above Figures suggest that the observed MSR performance could meet ADS-B MASPSrequirements depending on the reporting rate used. However, the results are different foreach session. In particular,

" Session 18/5/99: ADS-B MASPS requirements would be mostly met for distances up to120 nmi and especially with a 5 sec reporting period. The 10 sec period would givemarginal performance.

" Session 15/2/99: ADS-B MASPS requirements would be mostly met up to 100 nmi for a5 sec reporting period. The 10 sec period would not be sufficient.

" Session 6/2/99: ADS-B MASPS requirements would not be met with a 5 sec reportingperiod.

It should be noted that LFV has produced SPS simulation results [14] showing that VDL-4would meet MASPS requirements under various heavy traffic scenarios using a 10 secreporting rate.

13 It was assumed that one VHF-STDMA report equals one ADS-B state vector report.

14 95% percentiles of refresh rate: 7 sec up to 20 nmi and 12 sec up to 120 nmi. It should be notedthat so far the MASPS requirements have not been adopted by either EUROCAE or ICAO.


EEC/ASTP Dec. 99

0%

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0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160Distance, nmi

MS

R %

F28

B200

Figure 29 Position report MSR versus range, 5/2/99

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R %

Fokker 28

Beech 200

Figure 30 Position Report MSR versus range, 15/2/99


EEC/ASTP Dec. 99

Position MSR vs Distance for Sync Bursts

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Figure 31 Position Report MSR variation with range, 18/5/99

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3.1.3 Comparison with simulation results

Figures 35 to 38 plot the F28 position report success rate (as received on the B200),averaged per minute, versus simulation time for four corresponding to the trial sessionspresented previously. Each figure also plots the MSR measured in the corresponding trialsession. The following observations can be made

• The simulations produced mostly stable MSR at its peak value (100%) with fewtemporary variations occurring when the aircraft first entered in LoS of each other.

• The simulations tended to overestimate the LOS condition at low aircraft altitudes, whichcould be attributed to the fact that the (E-)SPS does not take into account the terrainprofile (except earth curvature).

• In the case of the trial session of 25/11/98, significant MSR degradation had beenobserved during the second leg of the F28 flight (see Figure 14). The simulation of thatsession did not produce any significant MSR degradation in the same period. It is thoughtthat the observed MSR degradation in the trial was due to a bug within the VHF-STDMAimplementation of the T3 transceivers.

• For the trial session of 18/05/99, simulation did not produce any significant MSRdegradation, unlike the trial results. This supports the idea that external interference maybe a cause of the observed MSR variations. The E-SPS does not model externalinterference.

In the case of the trial session 15/02/99 it had been found that there was significantcorrelation between message losses from one minute to the next (see Figure 27). In order tocheck whether this correlation appears also in simulation, the same type of analysis wasapplied to the corresponding simulation session. The result is shown in Figure 39. The latterfigure shows some message loss correlation occurring while the aircraft is climbing. Thispoints out a hidden terminal effect. However correlation lasts only for a few minutes incontrast to the trial results.

The same type of analysis was attempted with the simulated session of 18/5/99. The result isshown in Figure 40 and should be compared with Figure 28 which shows the equivalentresults from the trial session of 18/5/99. Again the simulation produced much lessercorrelation of position report losses. The few that occurred were at roughly the same periodas in the trial session. In conclusion, it is suspected that the abnormal report loss correlationseen in the trials might be due to VHF-STDMA implementation problems within the T3transceiver and particularly the logic handling slot reservation conflicts.


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F28 Position Report Success Rate

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Figure 35 Simulation versus trial results: Position MSR Comparison, 25/11/98


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Figure 39 F28 position report reception on the B200, simulated session 15-02-99

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Figure 40 F28 position report reception on the B200, simulated session 18-5-99


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3.2 Free Text messages

3.2.1 Success Rate

The VHF-STDMA freetext mode was used in the FREER-3 Trials to transmit trajectory andconflict reports. Freetext message success rate was measured as the percentage of freetextmessages received over the number of messages transmitted within a sliding time window.Trajectory and conflict messages were analysed separately. Since the results were verysimilar, only trajectory message results are presented in this section. There were only twotrajectory messages per minute, therefore a 10-minute window was used for averaging.

Figure 41 to 44 plot15 the measured air-to-air success rates of F28 and B200 trajectorymessages over trial time for the four trial sessions. Due to the wider averaging window, thetrajectory MSR curves are smoother than the position MSR ones (see Figure 14 to 17) butthe evolution is similar.

The following table compares the MSR of trajectories and position reports when averagedover the session (excluding periods where the aircraft were out of contact):

Average Success Rate

BroadcastingAircraft

Message Type 21/11/98 06/02/99 15/02/99 18/05/99

Position 64% 80% 86% 80%F28

Trajectory 55% 64% 87% 63%

Position 49% 80% 84% 85%B200

Trajectory 52% 68% 80% 71%

Peak Channel Load 22.8% 52.1% 24% 6.8%

Average trajectory MSR was somewhat lower than the position MSR. This should beexpected, since random access is less efficient than slot reservation. The efficiency ofrandom access depends on the channel traffic load. It can be seen that performancedropped somewhat on the 6/2/99 where peak channel load was highest. It is surprisinghowever that mean trajectory performance was relatively low on the 18/5/99 where there wasno NEAN traffic (other than the ground station)

3.2.2 Simulation Results

Figures 45 to 47 compare the trajectory MSR obtained in the trials against the same MSRobtained in the corresponding simulation. Similarly to the trials, the worst case results in thesimulations were obtained for the session of 6/2/99 which had the maximum peak channelload. Clearly however the simulations produced a better trajectory MSR than what wasactually measured in the trials. The simulations produced some trajectory MSR fluctuationsbut not to the extent seen in the trials. The difference is most obvious in the second part ofthe 18/5/99 session. In that session there was virtually no NEAN traffic. As Figure 47 shows,the simulator produced MSR=100% for trajectory messages, while the trial MSRmeasurements peaked at 80%.

15 the F28 curve shows the MSR of F28 trajectories as received on the B200, while the B200 curveshows the MSR of B200 messages as received on the F28.


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Trajectory Message Success Rate

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Figure 41 Air-to-air trajectory messages success rate, 25-11-99


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Figure 42 Air-to-air trajectory messages success rate, 06/02/99


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Figure 43 Air-to-air trajectory messages success rate, 15/02/99

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Figure 44 Air to air trajectory messages success rate, 18/5/99


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4. Conclusions and further workThe FREER-3 system delivered

" Position reports with mean reliability of the order of 80-90%

" Freetext messages with mean reliability of the order of 60-70%

In both cases mean message success rate per minute tended to vary widely from one minuteto the next.

In all trial sessions the FREER-3 system operated well below its congestion threshold (i.e. theinput traffic load was well within datalink capacity). A study on the congestion threshold ofVDL-4 for FREER-3 type freetext messages can be found in ref. 12 based on simulations.Concerning VDL-4 capacity for position reports, e.g. single slot periodic broadcast traffic,simulation results can be found in ref. 14.

The trial results clearly showed that air to ground performance was significantly better thanair to air. Reception of the air to air information seems to be the key issue for VHF-STDMAand VDL-4 performance.

Regarding the ADS-B air to air performance requirements stated in the RTCA ADS-BMASPS16, FREER-3 performance would be sufficient for ranges up to 100-120 nmi providedthe reporting period was kept well below 10 sec. Another option would be to increase thetransmission power above 10 W but this needs to be tested.

Four potential factors were identified to explain the observed fluctuations, namely

- Co-site interference from VHF voice

- Handling of slot reservation conflicts in the T3 VHF-STDMA prototype

- VHF Antenna radiation pattern variations

- Hidden terminals (see footnote 10)

Voice co-site interference appeared to correlate with some peak fluctuations. Better antennaisolation, more efficient filtering, and reception muting are among the possible remedies thatshould be investigated. Aircraft VHF antenna placement and guard bands for the variousVHF links are open issues that need to be addressed by the avionics standardisationcommunity.

There was evidence of transmission collisions that repeat for too long periods and this wasnot reproduced by the simulator. It seems that the T3 implementation must have a problemwith detecting/handling reservation conflicts.

The trial session of 18/5/99 and the simulation results did not showed any significant impactfrom hidden terminals in comparison to the other factors.

There were not enough data collected to evaluate the impact of antenna gain variations. It islikely however that the use of a single VHF antenna penalised performance, see VHFantenna measurements reported by MITRE [13].

Comparison of trial and simulation results indicated that the simulation tool (E-SPS) tendedto show better air to air performance than what was measured in the corresponding trialsessions. The E-SPS does not model the factors mentioned previously except hiddenterminals17 and its physical layer model may need more elaboration. Trial performance may

16 The ADS-B MASPS do not include long range deconfliction applications. However, no performancerequirements could be found in any other standard concerning this type of applications.

17 The simulator does not take into account the terrain profile, e.g. obstructions due to mountains,buildings etc.


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have also been penalised by the prototype equipment used (see Sec. 1.6). FurthermoreSPS complies to VDL-4 rather than STDMA and it may be that VDL-4 achieves better air toair performance. It is planned to test VDL Mode 4 performance (air to air and air to ground) inthe forthcoming ADS-B trials in the context of the Eurocontrol ADS technology Assessmenttask [3]. The results of these trials will be compared with the FREER-3 results presented inthis paper.

freer-3 trials - ads-broadcast over vhf-stdma performance ... · freer-3 trial/vhf-stdma data...

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