On 10/27/2010 11:16 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?

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.

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.

--
Mike Schumann