Transponders

Mike Raisler wrote:

TCAS installed on all transport category aircraft can "read" mode "c" or "s"
and determine if a climb or descent is needed to avoid the other aircraft.
The glider does not need to have TCAS installed, only a transponder, in
order for the other aircafts TCAS system alert for an avoidance manuever.


Ok, but the advice emitted by the TCAS in the other aircraft is based on the
assumption that the glider will keep its flying level and this is not true.

"Robert Ehrlich" wrote:
Mike Raisler wrote:

TCAS installed on all transport category aircraft can "read" mode "c" or
"s"
and determine if a climb or descent is needed to avoid the other
aircraft.
The glider does not need to have TCAS installed, only a transponder, in
order for the other aircafts TCAS system alert for an avoidance
manuever.


Ok, but the advice emitted by the TCAS in the other aircraft is based on
the
assumption that the glider will keep its flying level and this is not
true.

It is far better than no warning at all. The pilots of the TCAS equipped
aircraft will at least get their heads out of the cockpit and take a look
out of the windows.

Marc

Robert Ehrlich wrote:

TCAS installed on all transport category aircraft can "read" mode "c" or "s"
and determine if a climb or descent is needed to avoid the other aircraft.
The glider does not need to have TCAS installed, only a transponder, in
order for the other aircafts TCAS system alert for an avoidance manuever.



Ok, but the advice emitted by the TCAS in the other aircraft is based on the
assumption that the glider will keep its flying level and this is not true.

It's my understanding that the conflict resolution algorithm is based on
much more realistic assumptions, so that climbing and turning flight of
the potential threats is included. The simple assumption of straight
flight might have been used in the very beginning, but no longer. If you
have a recent reference that suggests otherwise, I'd like to know about it.

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Eric Greenwell
Washington State
USA

Eric Greenwell skrev den Tue, 03 Feb 2004
12:15:57 -0800:
It's my understanding that the conflict resolution algorithm is based on
much more realistic assumptions, so that climbing and turning flight of
the potential threats is included. The simple assumption of straight
flight might have been used in the very beginning, but no longer. If you
have a recent reference that suggests otherwise, I'd like to know about
it.

The algorithm is to look at a number of consecutive returns to determine a
rate of closure and then divide the distance with this rate to give a
'tau' value, of time to impact should there be a collision. When this
value goes below an [altitude dependant] threshold, you have a traffic
advisory. Another lower [also altitude dependant] threshold, and the TCAS
begins working on a resolution advisory.

In other words, more or less straight but not level flight is assumed.

Cheers,
Fred

Fredrik Thörnell wrote:
Eric Greenwell skrev den Tue, 03 Feb 2004
12:15:57 -0800:

It's my understanding that the conflict resolution algorithm is based
on much more realistic assumptions, so that climbing and turning
flight of the potential threats is included. The simple assumption of
straight flight might have been used in the very beginning, but no
longer. If you have a recent reference that suggests otherwise, I'd
like to know about it.


The algorithm is to look at a number of consecutive returns to determine
a rate of closure and then divide the distance with this rate to give a
'tau' value, of time to impact should there be a collision. When this
value goes below an [altitude dependant] threshold, you have a traffic
advisory. Another lower [also altitude dependant] threshold, and the
TCAS begins working on a resolution advisory.

After looking at an "Introduction to TCAS II version 7" more closely, I
have to agree that Fred's description is a good one. It says (page 7):

"In particular, it is
dependent on the accuracy of the threat
aircraft's reported altitude and on the
expectation that the threat aircraft will not
make an abrupt maneuver that defeats the
TCAS RA."

I'm guessing the relatively slow speeds (compared to the airliner) and
low G turns of a glider (compared to a fighter, for example) would still
allow the TCAS to sort things out to the benefit of all concerned. At
the least, the TCAS is providing range, bearing, and altitude to the
glider, a big improvement over a pair of eyeballs looking out a window
going 300 knots.

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Eric Greenwell
Washington State
USA

Eric Greenwell wrote:
...
I'm guessing the relatively slow speeds (compared to the airliner) and
low G turns of a glider (compared to a fighter, for example) would still
allow the TCAS to sort things out to the benefit of all concerned. At
the least, the TCAS is providing bearing, and altitude to the
glider, a big improvement over a pair of eyeballs looking out a window
going 300 knots.


I understand clearly how the TCAS can determine range and altitude (by
timing the return and decoding the encoded altitude), but how can it
determine the bearing? directional antennas coupled with compass information?

Robert Ehrlich skrev den Wed, 04 Feb 2004
18:21:08 +0000:

I understand clearly how the TCAS can determine range and altitude (by
timing the return and decoding the encoded altitude), but how can it
determine the bearing? directional antennas coupled with compass
information?

Directional receiving antenna was correct, compass information was not.
You con't care about the bearing, only the direction. Which won't be too
exact anyway.

Cheers,
Fred

Robert Ehrlich wrote:

Eric Greenwell wrote:

...
I'm guessing the relatively slow speeds (compared to the airliner) and
low G turns of a glider (compared to a fighter, for example) would still
allow the TCAS to sort things out to the benefit of all concerned. At
the least, the TCAS is providing bearing, and altitude to the
glider, a big improvement over a pair of eyeballs looking out a window
going 300 knots.


I understand clearly how the TCAS can determine range and altitude (by
timing the return and decoding the encoded altitude), but how can it
determine the bearing? directional antennas coupled with compass information?

I'm not sure, either. Here's a quote from "Introduction to TCAS II
version 7":

"TCAS interrogates
ICAO-compliant transponders of all aircraft
in the vicinity and based on the replies
received, tracks the slant range, altitude
(when it is included in the reply message),
and bearing of surrounding traffic."

It also uses the term "relative bearing" about half the time, so it's
not clear to me which they mean. I suspect it's actually relative
bearing. In any case, the bearing info appears to be determined by using
two or more antennas.
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Eric Greenwell
Washington State
USA

Eric Greenwell wrote:
...
In any case, the bearing info appears to be determined by using
two or more antennas.
...

More than 2 are necessary. With 2 antennas, you can only measure
the time difference between the 2 received signals. This time
difference can be translated into a distance difference. The points from
where a given difference is observed are on an hyperbola, which
can be considered as the same as its 2 asymptotes, as the distance
to the antennas is high compared to their mutual distance. But
to determine which of the both asymptotes is the correct one, you
need some more information, i.e. another antenna.

Robert Ehrlich wrote:

Eric Greenwell wrote:

...
In any case, the bearing info appears to be determined by using
two or more antennas.
...


More than 2 are necessary. With 2 antennas, you can only measure
the time difference between the 2 received signals. This time
difference can be translated into a distance difference. The points from
where a given difference is observed are on an hyperbola, which
can be considered as the same as its 2 asymptotes, as the distance
to the antennas is high compared to their mutual distance. But
to determine which of the both asymptotes is the correct one, you
need some more information, i.e. another antenna.

A more careful reading of the document shows only two antennas are used:
one bottom mounted omni-directional and one top mounted directional. It
also shows the omni-directional antenna can optionally be a directional
antenna.

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Eric Greenwell
Washington State
USA