Datalink decisions are urgent, but true consensus is lacking

 - May 29, 2008, 7:18 AM

A recent FAA briefing note concluded, “The future of ATC is all about data communications,” and government and industry officials, both in the U.S. and overseas, are in complete agreement with this view. The problem facing aviation is that these same officials seem to be in agreement on precious little else.

There are two separate datalink applications that are being fought over. In both cases, the need for datalink is unquestioned–the controversy is over which technique should be chosen. The first involves the method by which data will be transmitted to support automatic dependent surveillance–broadcast (ADS-B), and the second is a similar concern over the transmission of digital VHF ATC data for controller/pilot datalink communications.

And the very different nature of the messages in each application means that two quite different types of datalink will be required. That is, an ADS-B link wouldn’t be suitable for handling CPDLC traffic, and vice versa, which means that future aircraft will carry both, in any event.

For ADS-B, three different solutions have been assessed in detail in a recently released FAA interim report. While the report made no specific recommendations, it seems clear that the FAA’s position is that the universal access transponder (UAT), developed under FAA sponsorship by the Mitre Corp., offers the greatest potential, with possible drawbacks being a shortfall in its maximum range and the continuing delay in its ICAO standards and recommended practices (SARPS).

In the opposing corner is the Swedish-developed VHF Digital Link, Mode 4 (VDL-4), a sophisticated concept already past the ICAO SARPS hurdle and officially adopted by Russian aviation authorities. However, the FAA report highlighted certain short-range and high-traffic density limitations of VDL-4, which could be critical in terminal procedures.

Sitting between UAT and VDL-4 is what might be called the current incumbent, today’s mode-S transponder, to which would be added a so-called “extended squitter” of additional pulses that would carry the extra data required for ADS-B operations. This variant is known as 1060 ES, where 1060 is its frequency in MHz, and ES stands for extended squitter.

With ICAO SARPS well behind it, and with mode S in widespread use, the 1060 ES concept would be the convenient choice for today’s operators, but it has even more severe limitations than either of the others. In cost terms, all three are reasonably comparable, depending on aircraft type.

The FAA is expected to announce its unilateral UAT decision early this year, but this will undoubtedly be tempered by the notion that while the agency would standardize UAT in the continental U.S., where it is already in use in the Alaska Capstone ADS-B trials and elsewhere, the final solution expected for worldwide use will be a multi-link avionics unit capable of handling at least two of the contending design approaches.

But even then, confusion still reigns. At a recent government/industry meeting, Steve Zaidman, FAA associate administrator for research and acquisitions, expressed his personal, unofficial, opinion that while ADS-B worked well in Alaska, “it has no ‘killer’ application,” and noted that the FAA “is not going to require ADS-B on aircraft.”

Zaidman added that he “saw no need for it on a nationwide basis.” Supporting Zaidman, another agency official noted that pending satisfactory completion of a business case, there was no funding for ADS-B, despite the apparent overseas commitment to the concept.

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