On Oct 28, 1:11*am, Andy wrote:
On Oct 27, 8:16*pm, Eric Greenwell wrote:





On 10/27/2010 5:33 PM, Mike Schumann wrote:

On 10/27/2010 3:48 PM, Eric Greenwell wrote:
On 10/27/2010 7:20 AM, wrote:
in "awareness mode", of course a led will indicate where the closer
glider is, but still alarms will only be triggered when a deviation
makes the collision a real possibility.

aldo cernezzi
Since the GPS accuracy is probably much poorer than 10 feet, when a
glider comes within 10 feet, I figure the FLARM should consider that a
collision, no?
I don't know what Flarm does in that case, but my guess is the
*relative* accuracy is much better than the *absolute* accuracy. If
that's true, then each glider might have position errors of much more
than 10 feet, but they'll have nearly the same errors, giving a more
accurate separation distance.

Maybe someone more familiar with GPS in this situation can jump in here
and tell us?

Even if the relative GPS position computed by each glider has 0 error,
you still have the problem that at 50 Knots, each aircraft is moving ~
75 ft / second. *With FLARM (or ADS-B) only transmitting positions
every second, you can't rely on these technologies to protect you from
random course changes that the systems can't possibly predict, if you
are in close proximity.

FLARM does more than transmit positions: it transmits the projected path
of it's glider. Here's what I understand will happen: when the pilot
makes a course change, a new path is calculated and compared to the
paths Flarm has received from nearby gliders. If this new path puts it
on a collision course with any of them, the pilot is warned
"immediately", meaning it does not have to wait one second. The new path
will be transmitted within one second, so the nearby gliders can update
the other glider's path in their database. It might actually be more
sophisticated than that, such as transmitting a new path sooner if the
amount of change is "large", but I don't know what the algorithms are.

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?

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

That's my understanding as well - the Flarm algorithm looks at a range
of potential paths that the glider might maneuver to and calculates
potential collisions on the basis of all potential paths within that
maneuvering envelope. *I expect within a couple of seconds the path
assumes a fair amount of potential maneuvering while over a longer
period of time it would limit extreme maneuvering assumptions to be
closer to an extrapolation of the current path/turn rate. Because it
understands your trun rate it works well in thermals.

I have not heard anything about similar capabilities for ADS-B and I
am confident that it doesn't extrapolate with a glider performance
envelope in mind, since it isn't designed explicitly for gliders. I
would think ADS-B would be particularly challenged in predicting
likely collisions in thermals - much more so than Flarm.

9B- Hide quoted text -

- Show quoted text -

I hadn't really considered how the projected flight path system works
before. But after thinking about it for a bit it has a lot of
potentional. How much the Flarm actually uses I do not know. But it
would be possible for the Flarm to actually learn the possible flight
paths for a given glider and dynamically adjust it's algorithm for
where the glider might be able to go from that. Even without that I
can see that gliders could get very close but have potential flight
paths that would make colliding impossible and as a result would not
create a collision alarm.

Brian