Notice of Proposed Rulemaking (NPRM), Performance Requirements of ADS-B to Support Air Traffic Contr

Wed, Nov 28, 2007 — David Evans


5 October 2007
FR Doc 07-4938 – Docket No. FAA-2007-29305

This 32-page document lays out the FAA case for ADS-B, or at least a partial case, as it only addresses ‘ADS-B Out,’ that is messages broadcast from the aircraft, and does not address ‘ADS-B In,’ which is to say messages broadcast from air traffic control on the ground to the aircraft.

As the FAA says, ADS-B is a “key element” in realizing the goals of the Net Generation air traffic control system (NextGen). Although there are many components to the NextGen architecture, the deployment of ADS-B is regarded as a major step.

“The current approach to air transportation, where ground based radars track flights along congested airways, and pass information among the control centers, is becoming operationally obsolete,” with inefficiencies and delays the norm, according to the FAA.

ADS-B is seen as a way to break this logjam. The acronym ADS-B stands for “automatic dependent surveillance – broadcast.”

It is automatic in the sense that the airplane avionics do not have to be queried by radar. Rather, the system on the aircraft relies on navigation signals from global positioning system (GPS) satellites in space, and other on-board avionics, to automatically generate key elements of the airplane’s location, altitude, speed, and so forth.

It is dependent, in the sense that ground stations rely on the aircraft systems to reliably broadcast the airplane’s navigation solution and other identifying parameters. Many of these factors, such as the aircraft’s horizontal (e.g., lateral/longitudinal) position have heretofore been determined by ground radar.

The term surveillance refers to the need for ground control to know where aircraft are in relation to each other.

The term broadcast refers to the airplane’s new role in providing this information, making the airplane a much more active, as opposed to traditionally passive, participant in the air traffic “solution,” as it were.

Presently, when an airplane transponder is queried by ground radar, it sends a short, pulsed message indicating its identity and altitude. ADS-B Out, the broadcast from the aircraft, is envisioned to provide much more information:

1. The length and width of the aircraft, allowing ATC more information for handling aircraft in close proximity to each other, both in the air and on the ground.
2. The aircraft’s lateral and longitudinal position. As the FAA rightfully says, “This information is critical to the safe and efficient separation of aircraft.”
3. The airplane’s barometric pressure altitude.
4. The aircraft’s airspeed.
5. An indication that the airplane has an Air Collision Advisory System (e.g., TCAS).
6. If it has such a system, whether or not a resolution advisory is in progress to avoid a mid-air collision.
7. An indication if ATC services are requested.
8. A squawk of the aircraft’s unique four-digit code.
9. A broadcast of the airplane’s radio call sign.
10. An indication of an emergency (as input by the flight crew).

Actually, the notice lists 15 parameters to be broadcast by ADS-B. Unlike today’s ATC system where en route position accuracy must be determined to within 0.3 nautical miles, ‘ADS-B Out’ is to provide horizontal accuracy to 0.016 nautical miles and vertical accuracy to within 45 meters. This greater accuracy is to enable aircraft to be spaced closer together in the air routes.

‘ADS-B Out’ will be required for aircraft cruising above 24,000 feet, and it will be required for all aircraft operating in the airport environment, from 0-10,000 feet.

The FAA is not requiring ‘ADS-B In’ capability at this time, which would enable an ADS-B equipped aircraft to receive information from other aircraft and from the airport. As Rep. Jerry Costello (D – Ill.) said at the 17 October hearing, “Some capabilities require ‘ADS-B In’ are not addressed by the NPRM.

Costs for an ‘ADS-B Out’ capability are estimated as follows:
General aviation (GA) aircraft: $4,300 — $7,200
Turboprop aircraft: $12,900 — $460,000
Turbojet aircraft: $3,800 — $135,000

No reason was given for the high range of costs for turboprop aircraft. The cost ranges also differ from those presented at the 17 October Congressional hearing on ADS-B (see story above).

The FAA calculates a positive cost-benefit, based on the following:

$1.9 billion to $6.3 billion in costs to deploy ADS-B

$10 billion in benefits, “primarily from fuel, operating cost and time savings from more efficient flights”, according to the NPRM.

Although NextGen and ADS-B are touted for their role in enhancing safety, the cost of incidents or accidents avoided is not part of the cost-benefit presented by the FAA.

Of interest, the NPRM does go into some detail regarding ADS-B’s positive impact on global warming. The more efficient operations possible with ADS-B are estimated to avoid excess emissions and some 318 million tons of carbon dioxide (CO2) production over about a 20-year period. CO2 is considered a “greenhouse” gas that contributes to global warming.

Operators have until 1 January 2020 to equip their airplanes with ‘ADS-B Out’ technology. ‘ADS-B In’ requirements will be addressed separately.

The NPRM does not address the vulnerability of ADS-B to jamming of the GPS signals. It has been demonstrated that a 1 watt jammer will block GPS signals for many miles around an airport, disrupting ADS-B’s role in descent and landing under the NextGen concept.

In addition, there is the issue of networked data security. The evolution of air traffic control and other FAA operational networks away from stand-alone systems has made them more vulnerable to network threats. The FAA CSIRC (Cyber Security Incident Response Center) is tasked to provide more protection. As the FAA’s air traffic control system transitions toward NextGen and becomes more software-centric, it is vital that software integrity become as secure as Ft. Knox. Radar outages, screens going blank and other crumbles seen in recent months and years would pale in comparison to a future nation-wide failure in ADS-B’s underlying protocols and software-driven networked processes. The existence of CSIRC is not well known, but its role seems sure to grow alongside and underpin the dynamic solution to crowded skies known as NextGen.

Regarding the role of ‘ADS-B Out’ in that solution, comments to the NPRM are due 3 January 2008.

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