On Sun, 27 Apr 2008 22:17:44 -0400, Roy Smith wrote in
:

Big John wrote:

However, a full blown mode C transponder may not be
necessary. A radar reflector like they use on weather balloon ought be
sufficient. It is just a piece of foil with a large cross section.

Not really. More like a corner reflector. Take three mutually
perpendicular reflecting planes and you get an interesting and useful
property -- an incoming beam from any direction gets reflected three times
and ends up going out in exactly the same direction it came from. Exactly
what you want to give an artificially large radar profile.

See here for an example:

http://www.landfallnavigation.com/sd152.html


Hey. That looks like just the ticket, and the price is right. Of
course, once it's STCed ...

Here's the corner reflector definition in Federal Standard 1037C:
http://www.its.bldrdoc.gov/fs-1037/dir-009/_1298.htm
Photo: http://www.its.bldrdoc.gov/fs-1037/images/coriverc.gif
That doesn't look nearly as useful as the example you provided.

In any case, primary radar (even with the help of a passive, if efficient,
reflector on the target) only gives you bearing and range. To get
altitude, you need Mode C.

I've had ATC call traffic "altitude unknown" often. I don't see the
lack of altitude information as a real limitation, but then I'm
neither a controller nor engineer.

So while the NTSB in Safety Recommendation A-08-10 through -13 dated
March 31, 2008 is recommending removing the glider transponder
exemption:

The Board notes that, because of the limitations of the
see-and-avoid concept, transponder-initiated collision alerts
(either from ATC or TCAS) provide both VFR and IFR aircraft with a
higher degree of safety in an environment where highspeed closure
rates are possible. Therefore, the Safety Board further concludes
that transponders are critical to alerting pilots and controllers
to the presence of nearby traffic, so that collisions can be
avoided, and that gliders should not be exempt from the
transponder requirements. This is especially important at higher
altitudes, where flight crews may rely more on their TCAS,
expecting that other aircraft, including light aircraft, are in
contact with ATC and/or are transponder-equipped.

Therefore, the Safety Board believes that the FAA should remove
the glider exemptions from the FARs that pertain to transponder
requirements and use.

It would seem that the language the NTSB used leaves room for
equipping gliders with a simple passive corner reflector installed
within the composite or other non-metallic skin of the glider, thus
overcoming the lack of electrical system, and providing ATC with a
useable target for potential traffic conflicts.