Collision avoidance equipment

I have been reading about several low cost alternatives which provide
collision avoidance from Aviation Consumer. These devices seem to give
a rough estimate of range and the seperation altitude, but not
direction. Does anyone know of a portable system that give direction as
well?

samcapt wrote:
I have been reading about several low cost alternatives which provide
collision avoidance from Aviation Consumer. These devices seem to give
a rough estimate of range and the seperation altitude, but not
direction. Does anyone know of a portable system that give direction as
well?

No, that would be rather difficult. These devices listen
omnidirectionally for other aircraft's Mode C returns. The
built in units use multiple directional antennas to do this
which would require a very large portable unit or precise
setting of the remote antennas neither of which was practical.

What might be possible is to sniff the TIS signal off a mode
S datalink (provided there is a mode S transponder somewhere
nearby), but I don't know enough about how that works.

The problem with a TIS "sniffer" is that it is designed to be a "login"
type thing. It could probably be done, but the information supplied is
specific to each aircraft request. And without knowing the location of
the requesting aircraft... I have much greater hope for TIS-B
(essentially the transistion plan from existing ATC radar to ADS-B.
But, of course, it's only available "elsewhere." Of course, TIS isn't
much better down where I fly... probably less than 1/10th of 1%
coverage.

It *is* possible to build an antenna (suite) that estimates direction
from differential time of arrival, coupled with relative signal
stength. We use something similar in some military applications, at
almost the same frequency as the xpndr. But it's not cheap, and the
bigger the antennas (and more spread out) the better. Distance would
still be a very crude estimate, of course, unless you make the array
active (interrogate like TCAS does) - and that is a big certification
issue as an intentional radiator.