On Saturday, May 30, 2015 at 9:29:15 AM UTC-7, pcool wrote:
Then we all agree on the fact that we dont want a monopoly.
I am not by any mean pushing one or another technology, I exactly want what
you write, too.
I cant subscribe a petition like the one in this thread, I think it is
worthless, to be gentle.

Concerning the strawman, it is an english term and it is not in use commonly
although the meaning is clear.
In this case it is abused, I think, and let me say it sounds offensive .
The argumentation about "where is a glider in 30 seconds" made by a 8mhz
device with no math computing power, leaves no doubts to me.
This is why I keep calling it projection, and Andy did explain what I think
better than I can do myself.

Whatever, we all agree it works.


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