Letters

ADS-B Concerns

Read your Counter-MANPADS article (Avionics, July 2007, p. 26). While it mentions the pluses of the system, there are some issues that need to be resolved before the concept is widely accepted. My ADS-B concerns are genuine and I feel others should be aware.

ADS-B broadcasts the position and trend vector of a specific aircraft, making it the ideal target for a home-brew terrorist GPS-guided missile. Unlike expensive, infrared-guided missiles that only work in VFR conditions, cheap ADS-B missiles can hit an ADS-B aircraft in IFR conditions without being detected by the aircraft’s onboard infrared countermeasures. Furthermore, the laser gun or decoys will be ineffective against non-infrared ADS-B guided missiles.

Because ADS-B broadcasts the aircraft ID (found on the FAA data base) the terrorists can target specific aircraft like those of large corporation CEOs or high government officials, particularly when overflying high-risk countries with ADS-B ATC. In current Mode A/C/S surveillance radar systems, only ATC knows who you are and where you are.

Also, a terrorist on an aircraft (with a bomb) can fool ATC by broadcasting on ADS-B a false flight path when in fact he is headed for a crash on a nuclear power plant. Mode A/C/S radars are very difficult to fool, especially when traffic is observed from two different radar sites.

Even though the GPS constellation is robust, the signal level at the aircraft is very weak when compared with other ground based navigation systems. This leaves the signal susceptible to disruption by either solar flares or balloon jammers. An ADS-B based ATC could suffer a nationwide breakdown in the event of a solar flare or GPS signal disruption (sabotage or software bug). This single point of failure does not exist with current mode A/C/S radar system. Avoidance of single point of failure is something that the FAA and the NTSB strongly advocate for.

Will ADS-B position data be based on GPS only? Why not Russian GLONASS or European Galileo? Will airlines need to equip with GPS, GLONASS and Galileo when flying overseas? I do not think leaders of these nations will embrace GPS-based ADS-B, especially when GPS is controlled by the U.S. Department of Defense. This would imply U.S. DoD having some control over foreign ATC infrastructures. Current Mode A/C/S is an internationally accepted system defined by ICAO with none of the above issues.

The ADS-B benefits? Maybe some cost savings for developing nations. And even those can use a low-cost multilateration surveillance system compatible with current Mode A/C/S transponders.

Air traffic flow is mostly affected by weather and airport capacity than by current radar surveillance performance. Do the savings (if any) on ADS-B ‘Out’ outweigh ATC susceptibility to terrorist attacks or nationwide system breakdown?

Jose J. Monroy

Coral Springs, Fla.

Radar Correlation

I have some comments regarding your cover story, "Multilateration: Radar’s Replacement?" (Avionics, April 2007, p. 30). The president and CEO of Era Corp., Dave Ellison, said in the article "multilateration provides the same level of fleet coverage as traditional SSR, and with ADS-B support, it’s a future-proofed solution." This statement is probably overstated. He cannot rule out the vulnerability of ADS-B systems.

GPS-based ADS-B, along with the support of multilateration, is a fundamental departure in the architecture of air-traffic control systems. Specifically, air-traffic controllers on the ground give up their initiative to pilots in the cockpits. Flight information displayed in the air-traffic control centers becomes passively derived from messages relayed by pilots.

The degradation in a single message will drastically affect the integrity of flight information. RF powers emitted by a GPS satellite are low and its spread spectrum codes are clear. In other words, GPS is vulnerable to hostile jamming and deception. A malfunction or bad reception of a GPS receiver will make an aircraft invisible. The probability of midair collisions suddenly increases. Air-traffic controllers will hesitate to issue their flight orders. Will aircraft in flight be able to tolerate such hesitation?

We have witnessed Oklahoma City, the Pan Am Flight 103 bombings and the 9-11 terrorist attacks. We cannot assure that the GPS vulnerability will not be exploited by terrorists. A hostile action will render satellite signals useless. All in flight information will be lost. Air-traffic controllers will not know the exact locations of aircraft in flight. Pilots of flying aircraft will not know where they are in relation to others.

We have known that a sudden loss of visibility due to heavy fog or a sand storm can lead to highway chain collisions. A sudden loss of satellite-based enroute aviation surveillance systems could lead to similar accidents and the loss of thousands of human lives.

The notion of using ADS-B for future air-traffic control is that air traffic will choke the aging radar grid in the next decade. The cause of such choking is due to inaccuracy of current radar systems, which cannot locate an airplane as accurately as satellite-based systems. Increasing the accuracy will permit the reduction of aircraft separation and the effective use of airspace.

The causes of inaccuracy in radar systems are due to the super heterodyne-based radar receivers and monostatic radar systems. As the technology evolves, these causes will disappear. We can have true correlation receivers based on optical fiber recirculation loops and an optical radar network as described in my recent paper on "The Use of Optical RF Network for Target Detection and Tracking," published in the Proceedings of SPIE, Vol. 6567 (May 2007).

The true correlation receiver has been referred to as an interferoceiver, which with the help of an optical radar network can provide the same location accuracy as that from ADS-B systems. Furthermore, they will provide accurate information on aircraft velocity and weather. It is weather that dictates when and where a plane can fly, and when a plane will crash.

We must reconsider replacing radar for future air-traffic control.

Ming-Chiang Li

Mitchellville, Md.

Display Clutter

"SafeRoute (Avionics, June 2007, p. 32) may work for UPS with a limited number of aircraft on approach, but I’d like to see anyone try that stunt when the scope is cluttered with targets coming from all directions, such as in a New York or any other high-density terminal area.

Karl Kettler

Princeton, N.J.

Avionics welcomes signed letters regarding topics covered in the magazine. Letters, including the writer’s contact information, should be emailed to Bill Carey, Editor in Chief, at bcarey@accessintel.com.