AMCP/7 IP/UK-FAAx 3/15/00 AERONAUTICAL MOBILE COMMUNICATIONS PANEL (AMCP) SEVENTH MEETING Montreal, 22 to 30 March 2000 Agenda Item 1: Development and Validation of Draft SARPs for the VDL Mode 3 System, including the Manual on Technical Details and the Implementation Manual
VDL Mode 3 Interoperability Testing (Presented by Dave Farncombe & Bill Blake) Summary This information paper discusses the results of some recent interoperability testing between US and UK VDL Mode 3 radios conducted after the recent Working Group D meeting in Rio de Janeiro, Brazil. The two radios were able to fully communicate voice and management information in a variety of set-ups.

Introduction

1.1 To fully validate a standard, it is usually desirable to be able to verify that separate design teams can build units that can interoperate. This indicates that the standards material is clear and concise enough that separate parties interpret the requirements and specifications similarly, and that the technology is such that more than one person can build it. Since there were two flight-capable prototypes developed for SARPs validation of the VDL Mode 3, it was desired to conduct such testing between the two units to verify the clarity of the specifications and to reinforce the maturity of the standards. The NATS radios were not available for inter-op testing until flight testing of the new vocoder was completed on the 16th of February. After that testing, the equipment was available for interoperability testing. 1.2 This paper documents the interoperability testing conducted between the US FAA/MITRE VDL Mode 3 prototype and the UK NATS VDL Mode 3 prototype from 2-6 March 2000 in MITRE's Bedford, Massachusetts location. These tests were performed using the voice-only system configuration 4V, as that was the only common system configuration between the equipment. Both radios are transceivers capable of being switched between Ground Station and Aircraft Station operation, and are illustrated in Figure 1. 1.3 The NATS equipment was received on March 2nd and voice communication was established. Voice recordings were made on the 3rd. Some tests were re-run on the 6th to gather log data on burst formatting. 2 Interoperability Testing
2.1 The following three radio set-ups were explored during the testing. These set-ups allow us to verify ground-air, air-ground and air-air operation of the equipment.

Set-up 3

Ground Station NATS FAA/MITRE FAA/MITRE
Aircraft Station FAA/MITRE NATS NATS and FAA/MITRE

FAA/MITRE NATS

Figure 1 - NATS and FAA/MITRE Validation Prototypes 2.2 The systems were capable of successfully exchanging voice communications in all set-ups. The formats of the M uplink and Voice headers were verified to be compatible between the units, with supporting documentation in Appendix A. A few minor anomalies were detected during the tested and are described further in Section 3. 2.3 Digital voice recordings were made for the various set-ups. The audio source was recorded Air Traffic Control speech played on a Technics RS-TR262 analog Stereo Cassette Deck, and the audio was recorded on a Sony DTC-A6 Digital Audio Tape (DAT) recorder. For set-ups 1 and 2, the left channel recorded the source audio and the right channel recorded the VDL output audio. For setup 3, the left channel recorded the FAA/MITRE aircraft station audio output and the right channel recorded the NATS aircraft station audio output. 3 Anomalous Behaviour
A few anomalies were detected during the interoperability testing. The following describes those anomalies. 3.1 When first connected, the FAA/MITRE aircraft station detected the synchronization burst, but was unable to decode the contents of the uplink beacon. By using a DSP emulator, it was possible to determine the data being received by the radio. By applying the bit scrambler overlay and determining what data the NATS radio should have been sending, it was possible to determine that the NATS bit scrambler overlay was offset by 1 bit. Once the FAA/MITRE radio was modified with the incorrect overlay, the two units were able to fully interoperate. Considering the bit scrambler
overlay mask is explicitly listed in Figure 12-5 of the Implementation Aspects for VDL Mode 3, the documentation is considered to be sufficient. But for additional clarity, the addition of a Note in Section 6.4.3.3.2.5 of the core SARPs might be considered pointing developers to the abovementioned figure in the Implementation Aspects. 3.2 The NATS radio used Aircraft ID 63 to identify a voice user who has not logged into the system. The Aircraft ID should be 61 for these users as indicated by Section 4.5 of the Implementation Aspects. This has no detrimental effect on the system. While Section 4.5 of the Implementation Aspects for VDL Mode 3 explicitly states that aircraft without assigned local identifiers should use 61, this should probably be stated in the Tech Manual in Section 5.7.2.1.1 in the section on Net Initialization to ensure it is followed. This correction can be accommodated by the SARPs maintenance procedures to be established. 3.3 When the NATS ground station radio was retuned to a different slot of operation, the timing of the system appeared to be modified sufficiently to temporarily confuse the FAA/MITRE aircraft station. It is expected that if the GPS input was utilized for the NATS equipment during the testing, this effect would not have been experienced. Regardless, the FAA/MITRE radio correctly reverted through the timing states and correctly re-acquired timing from the ground station. 4 Conclusions

The FAA/MITRE and NATS VDL Mode 3 systems were able to successfully communicate voice and management information. This validates the physical layer requirements from an interoperability standpoint, as well as the burst structure of the Media Access Control sublayer. The formats of the M uplink and Voice headers were also verified to be compatible between the units.