My Trafficscope has always been extremely accurate in range. I think
their engineering took into account all of the factors you mentined.
All I can say is that if anyone who is conered with surrounding
traffic, the VRX trafficscope is the most accurate and dependable
device on the market today.


"John Giddy" wrote in message ...
Dave Martin wrote:
As one of those who is sceptical about such devices
for glider use, I have looked the following web page
which gives details of the device

http://www.avionix.com/collis.html

To try to keep my comment brief I have cut and pasted
relevent bits, to get the full descrition look at the
web pages

'TrafficScope is a completely self-contained passive
collision avoidance system designed to indicate and
alert traffic around your aircraft. TrafficScope VRX
displays the three most threatening aircraft with digitally-precis
e range'

'The system listens for transponder replies from aircraft
in the area and derives altitude and range information
on-the-fly. When target aircraft are interrogated by
ground radar or any other active system, such as TCAS,
the transponder responds with range and altitude (Mode
C) information.'
snipped

Mode C transponders do NOT transmit "Range" information. The range in
normal operation is measured by the secondary radar which interrogates
the device.
The only information in the transponder transmission is the Code (set
by the pilot on the switches on the transponder) and the Altitude,
taken from an altitude encoder.
AFAIK, these collision warning gadjets use a measure of the received
signal strength as an indication of how far away the transponder is.
This is a rather inaccurate measurement, as it depends on the actual
power output of the transponder, the orientation and possible
shielding of the antennas, as well as the distance.
That being said, I guess some warning is better than no warning, but
don't put too much faith in the "distance" figure.
Cheers, John G.