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.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm http://tinyurl.com/yb3xywl
- "A Guide to Self-launching Sailplane Operation Mar/2004" Much of what you need to know tinyurl.com/yfs7tnz