ADS-B may have problems in the US - What about Europe? Curt Lewis

FAA Worried About ADS-B, 1090 MHz Interference

Radio frequency congestion is a side effect of ADS-B the FAA is working to alleviate

With Automatic Dependent Surveillance-Broadcast (ADS-B) equipage on the rise and a wholesale mandate for the NextGen surveillance technology less than four years away, the FAA is getting serious about how to deal with a downside of the upgrade-radio frequency interference.
The issue involves the 1090 MHz frequency and the potential ill effects of too many signals occupying too small a volume of airspace. That frequency is used not only by ADS-B transmitters but also by FAA air traffic control radars, airborne traffic-alert and collision warning systems (TCAS) and military identification friend-or-foe systems.

Potential mitigations include replacing terminal radars (those typically covering a 60-nm radius around larger airports) with wide-area multilateration (WAM) systems, modifications to ground-based radars and adoption of hybrid TCAS variants.

"It is like being at a concert-the louder the music, the more 1090 MHz traffic, the harder it is to hear the person next to you," says Rob Strain, a senior principal engineer with The Mitre Corp.'s FAA-sponsored Center for Advanced Aviation System Development. Mitre, a not-for-profit organization, is part of an industry group tasked by the FAA with analyzing the issue and suggesting potential fixes.

The FAA is considering replacing terminal radar systems with multilateration systems in an effort to reduce 1090 MHz frequency congestion.

Of most concern is that in a congested environment, the 1090 MHz signals an aircraft depends on to ensure self-separation or to provide position information to air traffic control (ATC) will be blocked, making it temporarily invisible to the ground or other aircraft.

Strain emphasizes that there is no evidence of problems today but that mitigations will be needed to preserve the spectrum for the next 20 years or more. "I don't think the sky is falling here," he says.

TCAS operates on two frequencies: An "interrogator" broadcasts queries on 1,030 MHz; transponder-equipped aircraft in the vicinity respond with a 1090 MHz message. Ground-based secondary surveillance radars (SSR) also use the same two frequencies to query airborne transponders (on 1,030 MHz) and receive aircraft position and identification information (on 1090 MHz).

However, unlike legacy airborne transponder systems, in which a transponder does not broadcast its 1090 MHz signal unless it is queried, ADS-B transmitters continuously broadcast messages on 1090 MHz at more than five times per second, increasing the potential for interference as more aircraft are equipped.

While ADS-B in theory could replace SSRs (since ADS-B automatically broadcasts aircraft position), helping to reduce 1090 MHz congestion-particularly in the terminal area-in reality a significant number of SSRs will have to be kept operational in part to track aircraft not required to carry ADS-B and "noncooperating" targets.

The FAA first started analyzing the potential interference problem in 2008 and again in 2010 as the agency began building a ground network to receive the 1090 MHz ADS-B messages from aircraft. The network sends those messages to ATC, which then retransmits composite traffic updates (including the position of general aviation aircraft that are using an alternate ADS-B frequency) to the aircraft on 1,090 MHz. ADS-B downlink messages contain highly accurate GPS-derived position and velocity information, along with a variety of other identification and metrics from onboard systems.

Study results show airborne ADS-B and TCAS systems would fail to meet performance requirements as the number of ADS-B-equipped aircraft grows. The FAA identified two potential software modifications to SSR to reduce frequency congestion in the near term (through 2020), both of which the agency is now in the process of implementing.

Beyond 2020, the FAA is considering a broader slate of changes. For SSRs, potential mitigations include Mode S range limitations, or lockouts, beyond which no interrogations are sent, and using ADS-B information from the FAA's ground network to help the radar system acquire targets. In the terminal area, the FAA could replace SSRs with WAM systems, quasipassive networks that receive transponder signals and provide surveillance information.

Solutions for the airborne segment include installing TCAS hybrid surveillance systems, which take advantage of ADS-B functions. "With ADS-B, you have the opportunity for TCAS to passively listen to the ADS-B traffic and only go active and start interrogating when that traffic becomes more of a close-in threat," says Strain.

The FAA, in a March 11 status update, noted that the WAM and hybrid TCAS alternative "appears to provide the greatest improvements" from a cost-benefit standpoint, although further analyses are underway.