Nearly Mid-Air Collision at Estrella

Fredrik Thörnell wrote:
Ian Johnston skrev den 3 Feb 2004 04:55:56 -0800:

I am planning to fit a marine aluminium corner cube into the fuselage
of the Pirat. It's about 12" on each side, very light, and can be
assembled in situ. There more compact reflectors for marine use which
are claimed to have larger radar cross sections, but the emphasis has
to be on "claimed" there.


It'd be interesting to find out what the speed threshold for the doppler
radars typically is. Unfortunately, that is not a piece of information
they go out of their way to make available. The question is, above
or below the speed of a glider in a thermal?

Cheers,
Fred

In an air surveillance radar the doppler information (instantaneous
radial velocity) is used primarily to reject stationary targets that
creep in via the sidelobes (as opposed to a weather radar which is
really interested in all the IRV data it can gather). In my experience
the doppler clutter threshold was set very low - say 1 or 2 knots. My
experience is military, where the cutoff was set as low as possible to
counter the postulated "spiral in at low radial velocity" attack, but I
believe ATC radar would also have compelling reasons to keep the doppler
cutoff very low (don't want tangential targets disappearing on you).

Where gliders are likely to disappear is in the processing of multiple
radar returns into tracks. A variety of clutter rejection algorithms
can be used, and we never used anything quite as simple as "under X
knots, throw it out". We did, however, try very hard to eliminate bird
tracks, and glider flight patterns obviously have much in common with
certain birds. The distinction is between "visible to ATC" and "tracked
by ATC" - and we'd really like to be tracked.

In short, I don't think there is any simple answer to at what speed a
glider will be tracked by ATC. And while I think using a corner
reflector to provide a great big RCS is a great idea, I can still
imagine the ATC software thinking, "Hmmm, that must be a GREAT BIG hawk
out there...".

IMHO,
Dave