Introduction

1.1 In 2002, High Frequency Datalink (HFDL) trials started in the North Atlantic (NAT) Region to evaluate the use of HF as a data link medium for ATS communications, as the SATCOM coverage (Inmarsat service) is currently limited to 80 degrees north and south of the Equator.

1.2 This paper gives a historical background on the development of HFDL, its current operational status and some technical information that is necessary in defining future ATS operations based on HFDL.

Discussion

1.2.1 In the 1990s, the airline industry sought ways to improve HF communications. A successor to a military design, the HFDL was developed as a low-cost alternative to the aeronautical mobile satellite service (AMSS) for wide area coverage (5000NM/site). HFDL first entered commercial service in 1998 to support airline operational communications (AOC). By Marchv ’02, all standards and documents (for ATS) had been finalized, including the ICAO Manual on HF Data Link (Doc 9741, 1st Ed., 2000).

1.2.2 HFDL uses the same frequency band (2.85-22 MHz) and equipment (HFDR) as HF voice. It operates in the same 3 KHz channels with a single side band (SSB) digital modulation, providing up to 2.4 Kbits per second data throughput. HFDL is less susceptible to varying propagation conditions than HF voice, because it is automatically adaptive both in radio frequency and data rate. It also includes extensive error detection and correction algorithms in the protocol. HFDL uses a bit-oriented data protocol designed to function as a sub-network of the aeronautical telecommunications network (ATN). In order to be compatible with present airline data communications infrastructure, it also supports characteroriented ACARS messages. HFDL provides an average transfer delay of 75 seconds and a delivery time of 200 seconds for 95% of all messages.

1.2.3 HFDL is used for AOC worldwide, and in pre-operational trials for ATS applications in the North Atlantic and North Pacific. Once the ground station at Las Palmas becomes operational in Oct ’03, ARINC will have global coverage with 14 HFDL ground stations (HGS). It must be noted that the North Pole routes are provided with triple-redundant coverage (with stations at Reykjavik, Krasnoyarsk and Barrow). Currently, there are over 300 HFDL-equipped aircraft.

2.2 Technical Background

2.2.1 HFDL Configurations

There is only one HF antenna installed on the aircraft to support both HF voice and HFDL. This being said, there are three types of configurations as regards aircraft architectures to sustain HFDL services. Long-range aircraft such as A330/340 are all based on the mono-HFDL architecture: two HFDR transceivers are installed one on each side of the pilot and co-pilot, but the HFDL function is activated on one side only (captain side).

2.2.2 HF Voice Preemption Over HF Data

When HFDR 1 is in data mode HFDR 2 is in voice mode. HFDR 1 monitors the R/T operation of HFDR 2 and will stop transmitting data immediately if HFDR 2 is used for voice transmission (ARINC 753 rule). When HFDR 2 returns into reception mode, data transmission through HFDR 1 remains inhibited for a period of time set between 30 and 90 seconds (long enough period for the crew to hear any acknowledgements). In addition, as there is only one antenna, data reception is not possible during voice transmission.

2.2.3 ARINC HFDL Network

Use of the ARINC HFDL network has revealed the following behaviour: whenever the last AOC message has been downlinked via HFDL, then any further uplink (be it ATS or AOC) will be routed through HFDL, thus giving a data link performance (i.e. times) inferior to VHF and SATCOM.

2.3 NAT Trials Of FMC WPR Using HFDL

2.3.1 The CFRS (Centralised FMC waypoint Reporting Service) receives airline operational position reports, converts them into ATC WPR format and delivers them to ATS service provider ground systems. This service enables non-FANS aircraft on transatlantic flights to make WPR to NavCanada, NavPortugal and UK NATS via data link. Pre-operational trials were performed in the NAT Region during the 3rd quarter of 2002 to evaluate HFDL as a delivery medium for FMC WPR.

2.3.2 These trials showed HFDL performance to be substantially better than HF voice, especially when crews were not transmitting any HF voice reports that could interfere with HFDL. More than 75% of HFDL reports reached their destination within 3 minutes, which well exceeds the success criterion (50%) that the North Atlantic Systems Planning Group (NATSPG) set for other data link position reports. However, the portion of reports delivered within 5 minutes was between 85-92%, which does not quite meet the 95% level that the Group has set. Before a recommendation can be made regarding the applicability of HFDL to NAT WPR, additional data must be collected, without voice interference, over several months at least, to confirm the results under a more complete range of HF propagation conditions.

2.4 HFDL And FANS

2.4.1 The standards for FANS communications were developed and published in the South Pacific Operations Manual (now incorporated in the ICAO POM) long before the HFDL SARPs were developed. Based on performance data obtained from FANS-1 AMSS-equipped aircraft, the current POM system performance requirements call for 95% of the downlinks (one-way) to be delivered in 60 seconds. Unfortunately, the architecture and data link protocols of HFDL cannot, by definition, meet the current standards of FANS. This situation makes it very difficult for HFDL equipped aircraft to maintain communications with an ATS service provider, since only 60% of HDFL messages will arrive before the ATC ground systems will time out (in 60 seconds). As a result, part of the industry is currently campaigning to have the standards modified to accommodate HFDL.

Conclusions

3.1 HFDL performance appears to be substantially better than voice performance. However, in order to complete definitive trials of HFDL performance, we must move from existing pre-operational ATS data link trials to operational trials (no parallel voice WPR), to remove interference from HF voice.

3.2 To determine whether or not HFDL is a suitable means to support FANS-1/A it is necessary to modify current FANS (POM) standards.

3.3 Aircraft normally get better performance from SATCOM than HFDL so the only motivation for aircraft with both avionics to use HF while in Inmarsat coverage would be the lower price. Consequently, these aircraft will probably use HFDL when outside SATCOM coverage, or as a backup when there is satellite outage. As there is currently a small number of flights operating over the North Pole, the main source of HF data link traffic will be HFDL equipped aircraft that fly outside VHF coverage but do not have SATCOM avionics.

Recommendations

It is recommended that;

4.1 This paper is accepted as information material.