On 10/28/2010 2:14 PM, Eric Greenwell wrote:
On 10/28/2010 7:29 AM, Mike Schumann wrote:
On 10/27/2010 11:16 PM, Eric Greenwell wrote:


This projected path is a key element to the system working properly.
Without it, each FLARM unit would have to calculate the path of every
nearby glider; with it, each unit only has to calculate one path - it's
own. Potentially, it could be using a much higher position rate than
once a second to calculate it's projected path. In any case, the result
is much better than you might think for a system that transmits once a
second.

Does ADS-B transmit a projected path, or just position?

I'm not an expert on either FLARM or ADS-B. I believe that ADS-B
currently only transmits absolute position. Future enhancements might
transmit trajectory, which would be most useful for aircraft with
Flight Management Systems where the trajectory is well defined and
could be used by the ATC system for airspace management.

Regardless of whether or not the trajectory is transmitted, a
sophisticated receiving system (either FLARM or ADS-B based), can
remember each aircraft's position data and compute it's current
trajectory. While a glider might be moving 75 ft / sec, this is
obviously in a relatively forward direction.
You may be underestimating the value of transmitting the projected path.
When another glider is first detected, your unit has only one position
report and can not determine the flight path from that single point, and
it will take several more precious seconds to determine the flight path
of the potential threat; however, because the projected path is
transmitted every second, your unit immediately knows it.

Neither the transmitting nor the receiving FLARM or ADS-B system can
predict an abrupt change in course that a pilot flying manually might
command. However, every aircraft has physical limits on roll rates,
etc. that restrain the potential change in direction that can occur
within the one second update interval of these systems. As a result,
the systems can, theoretically, compute a pear shaped threat envelope
for each aircraft and limit collision warnings to those situations
where these envelopes intersect.

It would be interesting to get more detailed information on the exact
algorithms that FLARM uses in it's collision threat analysis and
compare this to the actual unit performance in situations where
gliders are flying at close distances in formation or in gaggles. This
could also help pilots understand the limitations of these systems so
they don't develop a false sense of security in situations where these
systems are not reliable.

I'm sure the developers have tested their algorithms with thousands of
simulations using IGC files from gliders in many situations. The Parowan
accident simulation at

http://www.gliderpilot.org/Flarm-Par...dairSimulation

shows what can be done. It would be interesting, informative, and
entertaining if there was a website or application that would let us run
IGC files we select in a simulation like this. I'm curious about how
Flarm would react in a few situations I've encountered. Doing
simulations on a pilot's own files might be more persuasive of the value
of Flarm than even the most well-written explanations, and much more
easily understood than the algorithms themselves.


There's no question that FLARM or ADS-B based systems could have easily
prevented the Parowan mid-air. My question is how much you can rely on
this type of equipment to accurately warn you of collisions when you are
flying in gaggles.

Obviously knowing the relative position of other gliders in the gaggle
is helpful. I would be very skeptical, however, of putting my faith in
FLARM or any other system to accurately warn me of a collision with
another glider that was in the same thermal, near my altitude, that was
in my blind spot.

Transmitting the project path of the aircraft is really only beneficial
if the equipment on board the transmitting aircraft has some added
information that is not available to the receiver on the transmitting
aircraft's intent.

With both FLARM and ADS-B systems, the initial visibility of the other
aircraft occurs way before there is any collision threat, so the
receiver should have no difficulty computing the project path of the
other aircraft. ADS-B actually transmits the category of aircraft (i.e.
glider, balloon, etc...) so the receiver can get a pretty good hint on
the type of maneuvers that can be expected.

--
Mike Schumann