AVmail: February 17, 2003

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Reader mail this week about GPS jamming, missile jamming, D.C-area pilot jamming and more.


The Beginning of the End

Could it be that the FAA will cease to exist when many more functions continue to be contracted out, and the TSA and National Park Service assume additional responsibilities and pick up what they haven't already?

Ron Saeger


TWA 800 Conspiracy

For those interested in an exhaustive review of the evidence that TWA 800 was downed by a missile (most likely fired by the U.S. Navy in a botched exercise), get a copy of (ex) CBS investigative reporter Kristina Borjesson's article published in "Into the Buzzsaw." I can promise that her story and evidence will convince you that the official explanation is a fabrication.

Steve D. Mullins


GPS and Beyond: The SatNav Transition

I have just read this article by Nav Canada's Ross Bowie in your avionics section.

The article is most informative even today, given that it is dated 1997. Perhaps you could get an update from the author. However, I have a more mundane question. In this article, like many others on the subject, we see statements such as:

"In each case, performance met Category I (200 ft DH) accuracy standards. In fact, the system was accurate to within 3 meters horizontally and 4.5 meters vertically."

I'm curious as to what is the reference for these measurements. Perhaps radar?

Thanks for any input.

Andrew Dawe

AVweb responds ...

In the early '90s when the FAA and others started measuring GPS, LAAS and WAAS performance, the only "truth" system available was a laser tracker. The system used radar to track the test aircraft and to aim a laser beam at it. A reflector installed on the nosewheel door reflected the beam back to the tracker. Subsequent analysis of recorded data established true aircraft position to within a few metres. These systems were available at only a few airports, including Atlantic City and Crows Landing, Calif.

In 1994 we took part in an FAA WAAS trial at Crows Landing, where we tried out a new truth system based on post-processed GPS data. This technique had been used by surveyors for years to achieve centimetre accuracy. It is based on using two high-quality GPS receivers, one at a carefully surveyed location on the ground, the other on the aircraft. By processing the data from the two receivers together after the flight, aircraft position can be established within a metre or so. We compared this system with the laser tracker results and proved that the new system was at least as good. The obvious advantage was that we could use it at any airport.

I admit that, at first, it seems like we are using GPS to check GPS, but we are not falling into a circular logic trap -- the truth system does provide an independent assessment of performance because the post-processing reports on the statistical validity of its results. If errors were being observed in the GPS signals that could not be removed by the post-processing, then we would get an indication in the report. This allows us to have confidence in the data and use this technique as a truth system.

Ross Bowie
NAV CANADA


GPS Jamming

I just read the article about GPS Jamming. If you read the article you make it sound as if GPS is so unreliable that it should never be used under any circumstances. Your concerns about terrorists jamming may be somewhat acurate, but do you think GPS signal are the only thing they could jam if they wanted to? While I only have a few hundred hours flying, I have never experienced a complete GPS failure, but have had numerous VOR/ADF outages/inaccurate indications. I think you should have balanced your article a bit more to the fact that all forms of navigation have weaknesses and can be inacurate if not used correctly or if there is a system failure. How many people have been killed over the years because they failed to identify the VOR/ADF they where tracking correctly? As you pointed out a IFR GPS should issue a RAIM warning if it does not have enough data for a safe approach. You must know how to operate any piece to equipment in order to do it safely and knowing what a RAIM error is and what to do if you get one while flying an approach is critial. For all its failings I believe that GPS is the best, most acurate system available to GA today, I will still trust my GPS and continue to fly by it, and to always keep a prudent eye crosschecking the other instruments as well.

Douglas Hicks

AVweb responds ...

If you already crosscheck with other navigation systems and never push on with GPS when it gives you a RAIM warning, then you're following the suggestions of the article. This was not a full review of the pluses and minuses of all forms of navigation: It was intended as a reminder, especially to those using VFR GPS, to trust but verify. You do raise an important point about VOR/ADF, though, which don't have an automated warning to tell you when they cannot be trusted.

Kevin Lane-Cummings
Features Editor


GPS Jamming

In some of the early GPSs even a broadcast radio station antenna could put them out to lunch. They had a problem designing the early GPS Comms, as the R.F. energy from its own transmitter would cause inteference. Good article.

Jack Silva


GPS Jamming

There is a large difference between jamming and spoofing. In jamming, a noise signal of sufficient strength to hide the actual signal is broadcast to deny use of a signal. Noise is easy to detect.

Spoofing is the generation of a false signal that mimics the true signal but delivers false information. It is much much harder to spoof a receiver than to jam one.

A GPS receiver that is jammed should present a "Navigation Flagged" message if it encounters sufficient noise to cause it to lose 2D lock-on.

It would be quite challenging to build a spoofer that could keep track of the satellite almanac and know what signals to present to cause an error in position determination without causing a flag.

Rayford K. Brown


DC-3 Hardest Hit, Again

I read your article relative to pilots at the DC3 airports, which indicated that even if the pilots remain aloft their entire flight they must return to Lee Airport prior to returning to their home airport. There may be some misunderstanding: The current procedures indicate that once the pilot takes off, if the pilot remains aloft without an intermediate stop, the pilot may then return to their home airport without first landing at Lee Airport. Please call if you have any questions or concerns relative to the procedures implemented at the DC3 Airports under OHS condition Orange.

Thank you.

Bruce Landry
TSA,A/Assistant Director for General Aviation Operatons
571-227-2221


DC-3 Hardest Hit, Again

Will there even be an end to nonsensical rules promulgated by dysfunctional bureaucrats in our government? I believe the sun has set forever on Constitutional Law in our beloved country. I don't even think the dysfunctional bureaucrats in Washington have good intentions anymore with their endless blather about situations in which they have no clue.

For example, the day will never come that any thinking person cannot bust any security system at ANY airport. Anybody think this fact is lost on a terrorist?

The day will never come that a thinking terrorist would utilize a substandard-capability mode of transportation, such as a single-engine airplane, to accomplish major damage in a terrorist attack.

But our all-knowing, all-seeing, incompetent bureaucracy (elected and unelected) in Washington continues with their uneducated blather about how safe they are making everything.

Anyone besides me notice that, with the Enhanced Fluorescent Orange Warning Level, the Federal Government took immediate steps to protect itself and not the general populace? I mean NObody else.

Who else in the USA but dysfunctional bureaucrats take every holiday off from work these days? Talk about the privileged class ...

Steve McDonald


Missile "Jammer" Bill Introduced

Regards the countermeasures for heat-seeker missiles -- I doubt the plane could even get off the ground, even without passagers, with $1.5 million worth of flares on board.

Victor V. Kirilloff


Missile "Jammer" Bill Introduced

Regarding this article, one of your statements may be misleading.

"Shumer and Israel reportedly told the news conference that the systems they propose work by steering the missiles away from planes by jamming their guidance systems. The shoulder-launched missiles we've heard of are heat-seekers, so we don't understand how those can be "jammed" per se, but for $1.5 million each, who knows?"

I have worked on two projects by different manufacturers that utilize turret mounted lasers to confuse or prematurely detonate "Heat Seeking" missiles. These IRCM projects have been successfully tested on fixed-wing and rotorcraft, and scheduled for deployment.

Perhaps you might let the readers know that this is not just a speculative answer to a potential terrorist problem.

I am a faithful reader of AVweb and recommend it highly to my colleagues.

Larry L. Gray

AVweb responds ...

I guess when I think of jamming, I think of using some kind of electronic interference to disable the guidance system. Of course, heat-source decoys, like flares, have been used to divert heat-seeking missiles since the 50s. The modern laser and lamp-based systems seem to be updates on that basic premise. Is it "jamming"? Not in my opinion, but then I don't design and make those systems for a living, either.

Russ Niles
Newswriter