Flutterby™! : 757 and F117 in near miss 2000-09-22 23:14:14+00

757 and F117 in near miss

2000-09-22 23:14:14+00 by Dan Lyke 8 comments

757 and F117 in near miss incident, the interesting bit is that the U.S. military spent how many gazillions making the F117 undetectable, and yet "The Boeing 757 was flying at about 10,800 feet when its on-board radar equipment detected an approaching aircraft." Think about this as those morons in high places are trying to push breaking ABM treaties in favor of a system that's failed every time it's been tested.

"The F-117, which is able to evade radar, was broadcasting its position publicly when the incident occurred, the Air Force said." Or so they say...

Uhhhh... Yeah. Right. I might be able to believe that they've got a transponder that'll detect inbound radar and broadcast a reflected pulse, and I can imagine that for a system which uses doppler shift to detect speed, but position info seems a little harder to switch on and off...

I thought all sizable aircraft carried a transponder which broadcast a 'this is me, I'm over here' signal. The military get to turn them off when they need to of course... But I could be completely wrong...

They all carry a "this is me" transponder, but that associates a given position with a number. Actually echoing a position sounds a little more difficult, but maybe my understanding of radar is just outdated.

I believe the transponders are mostly tied into GPS now. I could be wrong, though... Plus, an aircraft that would be invisible to radar from a mile away might not be so invisible at 1/4 mile or less.

All current stealth planes currently in use or rumored to be in testing work on 2 principals: absorption and indirect reflection. The f117 is the most basic of the stealth designs. At something like 1 mile, I could easily see it being radar visiable. Which isn't a big deal because that works for all its mission specs. It was designed to get to the target, drop down, drop bombs, get back. It is visible at end of drop down and during bomb drop. The issue is can it get there undetected and get out without being shot down.

My impression on the F117 is that all those flat surfaces are to make the aircraft radar visible from only a few discrete angles. No curves means that radar gets reflected in a direction other than the transmitter most of the time. And yes, the point of the F117 is to get to the target with detection ranges sub 10 miles, slow down enough to lose its payload, and get the heck back out. I think it is important to remember that when talking about distances, especially between aircraft of these classes, 1/4 mile is just barely above the human reaction threshold if the pilots are alert, if the aircraft are closing head-to-head a mile could easily be under 5 seconds, the F117 might be able to react in that time (ie: Enough time for those involved to say the "Oh" part of "Oh shit!"), the 757 certainly couldn't. And visual on an aircraft the size of the F117 at a mile is not trivial. A while back I linked to an article on GPS accuracy that had some really interesting notes about visual contact and reaction and evasion issues. (Dunno what my point was there, mainly just rambling, I guess.)

Dan, all military planes have an IFF (identification friend or foe) transponder. It may be turned off during certain combat operations (e.g. bombing an Iraqi radar site), but certainly should have been active during routine flight in civilian airspace. The IFF system was designed to be compatible with both air traffic control systems and the TCAS (traffic collision avoidance system) that is increasingly found on commercial aircraft. (Raytheon describes their IFF system.) TCAS (tea-cass) is the system that activates a cockpit alarm and, in the case of two planes, tells each which maneuver to take (e.g. "Pull Up! Pull Up!"). Note that while IFF is required on all military aircraft, TCAS is not (which led to a tragic collision a couple years ago). IFF became a DOD threshold requirement after our pilots shot down our own helicopters ferrying a diplomatic entourage in Kurdish Iraq. One ongoing problem is that not all IFF conforms to the standard that will activate TCAS. Upgrades are in place, spurred by that collision, but my reading suggests they're not complete. But it looks in this instance like this all came to light because the TCAS system was activated by the transponder. I don't think this has any technical bearing on the stealth technology.