Very interesting view, Andy!
But, can a Flarm cpu do this? My doubts come from the fact that the
calculation engine in my flight software takes about 25ms every second on a
400mhz ARM cpu. For flight path estimation, some of those calculations would
be necessary too. A very conservative estimation would be 20% of 25ms,
needed by flarm too. So let's say 5ms, but on 8mhz (50 times slower). That
means 250ms.
The available time slot is 1000ms but it cannot be used entirely, because
the data must be transmitted and received within the timeslot itself.
So using the same thumb rule, I would say 250ms out of 750ms are used for
calc, leaving 500ms for the rest.
The "rest" being dealing with the gps, receiving data from the radio,
processing data and compare for each data the collisions. outputting NMEA to
serial port if needed, recording the flight, managing the leds etc.
And we must consider that the ATmel 128 cpu has no floating point, no math,
and even a sin() and cos() function are made from scratch in C language by
the cross compiler.
This is why, although "thumb ruled", I cannot thing that a cone like yours
is in use, but something much more simpler.
Because there is no time to do things like you say, I believe.
What do you think?

"Andy Blackburn" wrote in message
...

I don't know exactly how the Flarm guys do their math to project a flight
path, but I think of it as a cone with it's point at the glider and it's
wide end 15 seconds ahead of the glider. The cone can be straight or curved
and the cone's centerline defines the overall curvature, which can be
defined by a number of factors, but for simplicity's sake lets say that it
is a circle based on the last three datapoints (it could be any kind of
mathematical curve). If the glider is flying straight, the circle has an
infinite radius (a straight line). If the glider is thermaling the circle
has a radius of a few hundred feet.

Now imagine the pilot of the glider has decided that instead of thermalling
to the left he wants to thermal to the right because he thinks the core is
not at the center of the left hand turn but rather at the center of the new
right hand turn he wants to establish.. He puts the stick over to the right
and the glider starts to roll. The bank shallows, passes through wings level
and eventually settles in a steady right hand turn. From start to finish it
takes around 10 seconds. What happens to the projection of his future flight
path? As his circle widens the projection widens until it is straight out in
front of him and then starts curving to the right until it is a cone curved
to the right in a circular arc with a radius of several hundred feet
(whatever his turn radius is.

It is true that because no instrument can read the pilot's mind you cannot
predict that the point 15 second ahead of the glider will flip from being on
a left hand circular arc to a right hand circular arc, but that projected
point and any given moment is the BEST ESTIMATE of where the glider is
headed and as the wide end of the cone arcs through straight ahead the edge
of the cone will intersect any paths on that new circle. It won't be a 15
second warning, more like 7-10 seconds depending on how much the cone
widens. There is no algorithm or system that will read the mind of the pilot
before he even knows his mind, but the Flarm approach certain warns you of a
glider heading your way as soon as it is in fact heading your way - that's
what we want it to do.

You can be more conservative and set the warning to alert for any glider
that is close enough to hit you based on the maximum possible closure rate -
assuming both pilots turn aggressively to go exactly head to head. This
would require a warning for any glider within a mile to a mile and a half
(assuming 300 knot closure rate). Also assume the other glider is in a 25
knot thermal or 25 knots of sink and you are in the opposite to take a worst
case scenario. Anyone plus or minus about 1200 feet and within a mile to a
mile and a half distance should generate a warning. That is the warning
volume to be 99.9999% sure that no matter what you do or the other pilot
does you will be warned at least 15 seconds from possible impact. Oh, except
if the higher glider has speed brakes out, then you need more like a +/-
2500 foot warning height band.

Without some assumption on projected path, collision warning turns into a
traffic alert system if you want 100% certain that no matter what either
pilot does you will be warned at least 15 seconds ahead of time. Getting
continuous warnings on dozens of glider all the time defeat the purpose I'd
like to see served in a collision warning system - that it only warns you
when a collision is a real possibility, not a 0.0001% chance that I would
get warned about a few seconds later anyway if the worst case circumstances
in terms of pilot maneuvering actually came to pass.

If that's what this has been about, it is kind of silly and academic. You
should only give warning on things that are pretty likely to happen - or
like the boy who cried wolf, people will stop listening to you.

9B