North American neighbors take different tacks toward ADS-B

 - March 27, 2007, 12:55 PM

Nav Canada last month awarded its national ADS-B program to Syracuse, N.Y.-based Sensis, and installation of the first system ground stations is now under way. The FAA, on the other hand, faces some unexpected pre-contract issues as it moves toward its implementation plan. The system forms the foundation of the two agencies’ future air traffic management system, but the FAA’s goal is to have a nearly exclusive ADS-B environment, while Nav Canada plans to use different systems to meet its operational needs.

Nav Canada will initially install ADS-B stations around Hudson Bay in the north and multilateration (MLat) systems at Vancouver and at the oil exploration center of Fort St. John in the mountainous interior of British Columbia. Sensis ground stations can support either ADS-B or multilateration (a positioning technique aviation authorities worldwide are adopting as an introductory step in the transition to ADS-B).

The Vancouver and Fort St. John installations are due to become operational by late this year. The Hudson Bay systems are not expected to be operational until mid- to late next year because of the shorter weather “window” in Canada’s frigid north.

The Canadian agency, which has options for up to 200 ground stations, has not announced the site of future ADS-B or MLat installations. It is expected that they will be gradually introduced in areas that have specific needs, one way the agency’s plan differs from the FAA’s, which calls for nationwide blanket coverage. The Hudson Bay ADS-B stations, for example, will provide surveillance beyond the reach of shore-based radars, thereby allowing airline traffic to be more closely spaced than current non-radar procedural separations permit. Other areas in Canada’s far north are expected to be high on Nav Canada’s list of future ADS-B installations.

ADS-B and Multilateration
In ADS-B, every aircraft will transmit its identification, altitude, GPS position and other flight details once per second to all other aircraft in the surrounding airspace, providing all pilots with complete “situational awareness” of traffic in the vicinity. The aircraft signals will also be broadcast to a network of ground stations, which instantly pass the information to the nearest ARTCC, where each aircraft is displayed on controllers’ screens. The ATC center can uplink information to the aircraft via the ground stations.

Currently, however, few aircraft carry full ADS-B equipment, and it is in this regard that MLat performs a valuable interim function. In MLat, the ground stations act in groups of “listening posts,” picking up the identification and altitude data transmitted by all current ATC transponders. A central processing unit triangulates these signals to determine each aircraft’s location (few of today’s transponders transmit GPS data), which is then sent to controllers’ screens at the ARTCC. The MLat stations can be grouped around an airport to provide a complete picture of local airborne and airport surface traffic or spread widely to cover much larger areas, out to 300 miles at high altitudes.

MLat transponder monitoring is therefore closely analogous to secondary surveillance radar (SSR), and many feel that it will eventually replace SSR, since it is much less expensive, usually more accurate and requires much less maintenance. Today, MLat’s main advantage is that it can provide a radar-like service in mountainous or other areas, such as an airport surface, where obstructions interfere with conventional radar signals. This is the case at Vancouver’s inner harbor–with its high volume of seaplane and helicopter traffic crossing the airline approaches to the international airport–and at Fort St. John, in the Canadian Rockies, where neighboring SSRs are unable to track aircraft below 3,000 feet.

At such locations, one or more of the local MLat stations can transmit SSR-like interrogation signals to elicit transponder replies. An upcoming joint FAA/State of Colorado project will use MLat for aircraft flying in radar-shadowed areas. Similarly, the UK’s National Air Traffic Services will use a network of MLat stations installed on oil platforms for helicopter surveillance in the North Sea between Scotland and Norway.

The FAA program
The FAA’s nationwide ADS-B program will involve at least 500 ground stations, and is estimated to cost around $1.5 billion. The first installations, covering the Gulf of Mexico, are forecast to be operational in 2009/2010, with all stations expected to be commissioned by 2016.

The contract will be one of the first, and certainly the largest, to follow the agency’s new “performance-based” approach, where the winning bidder will manufacture ground stations of its own design, not the agency’s, and will be solely responsible for their installation, operation and ongoing service support. The contractor will be providing the FAA-certified ADS-B downlink and uplink services and ARTCC connections.

The program must meet the ADS-B needs of general aviation and the airlines, a requirement complicated by the fact that general aviation would use the 978-MHz Universal Access Transponder (UAT) radio frequency, while airlines and corporate aircraft would use the ICAO standard 1090-MHz “extended squitter” (ES) frequency.

UAT was developed as a lower-cost alternative to airline avionics, and it is used in the Alaska Capstone program, at several large flying schools and by government aircraft in the Washington ADIZ. Very few private aircraft have UAT avionics installed. When UAT was developed in the 1990s the FAA hoped that other nations would adopt it for smaller aircraft, but none did.

The UAT and 1090ES signals are incompatible, meaning that while UAT users can “see” all other UAT-equipped aircraft in the vicinity, they cannot see airliners and vice versa. A solution to this problem would be to have dual-frequency ground stations “translating” incoming UAT signals to the 1090ES format and re-transmitting them to airliners and vice versa.
Late last year, several teams submitted proposals to the FAA for ADS-B ground stations. The agency determined that the teams headed separately by Lockheed Martin, ITT and Raytheon would proceed further in the process. Each team included a number of leading technology companies. Many observers believed that Lockheed Martin had the greatest chance of success, followed by ITT and Raytheon.

Raytheon’s Surprise Move

Early last month Raytheon made its preliminary proposal public, and its approach was surprising to industry observers. The company proposed that FAA drop UAT and general aviation adopt 1090ES, since newer technology eliminates the equipment cost and size differences. To provide aviation weather reports and forecasts that UAT carries and 1090ES does not, Raytheon proposed to downlink this information to general aviation pilots via XM Radio satellite, at no charge.

The proposal to eliminate UAT also eliminates the need for the 500 new ground stations to be dual-frequency systems.

This would have certainly caught the FAA’s attention; the ADS-B experts AIN consulted said that single-mode stations could save the agency more than $200 million. 

The FAA made its request for final bids last month, with bidders required to respond within 45 days. Industry sources told AIN that Lockheed Martin and ITT had been expected to propose dual-frequency station networks. Before the FAA’s switch to
performance-based contracting, the agency might have rejected Raytheon’s proposal as non-compliant. Under the new system, however, bidders were invited to offer innovative approaches.

On the other hand, the FAA’s selection–and tacit endorsement– of Raytheon’s simplified, lower-cost concept could cause Lockheed Martin and ITT to protest that Raytheon significantly altered the understood technical approach, and that they should be allowed additional time to revise their bids.

So while performance-based contracting clearly brings the innovative approaches that the FAA sought, in this case it also places the agency in an unexpected dilemma. The FAA had planned to award its ADS-B contract in July, but that date could shift to the right as a result of the unexpected bid from Raytheon.