On Friday, May 29, 2015 at 2:13:45 AM UTC-7, Lucas wrote:
you are a control system engineer, therefore you work with deterministic systems. The glider pilot, whose actions depend on atmospheric conditions unknown a priori (i.e. vertical gusts/thermals), is not a deterministic system..

Nope - control systems engineers spend most of their time dealing with the effects of noise and non-determinist effects - no system is perfectly deterministic and you assume so at your peril. The issue is to figure out "how" deterministic a system is . Also, if you make NO assumptions about aircraft dynamics many systems are not accurately observable or controllable. Knowing what the aircraft can do dynamically makes a huge difference in knowing where it is and where it can possibly go. Assuming nothing about aircraft dynamics in your system IS making an assumption - you assume the system will keep doing exactly what it was doing when you measured it last - if it was going up it will keep going up indefinitely, if it was circling you assuming it will follow the instantaneous velocity vector and stop circling instantaneously. The simple model is worse - a lot worse. It is a disservice to the soaring community to assert that the simple model is better because the more sophisticated model is somehow "impossible" - that is bunk. It might be "impossible" to program for someone who doesn't understand aircraft dynamics, but that's not the same as claiming that it is an impossible task. Aircraft have flown on autopilots that use the same basic principles for decades - including ones that can autoland under all kinds of non-deterministic airmass dynamics.

Closure rate is an example of a simple projection model - it is the simplest you can imagine as it understands NOTHING about the dynamics of the underlying systems. Simpler is not better in this case - it is quite useful to know that gliders have energy and stall speeds and minimum turn radii to understand whether a glider getting closer to you is actually on a path to hit you or not.

As an algorithm closure rate implicitly assumes the instantaneous closure rate will continue unabated until zero distance and collision. This is NOT true - hardly ever. In only a small percentage of cases to aircraft getting closer represent a collision threat. Without a more sophisticated algorithm you either have to warn the pilot of ALL of them (hardly useful) or set a threshold on closure rate and distance that delays the warning to reduce the false warnings. You would have to set it pretty tight in terms of time to impact to get the warnings down to a tolerable level and even so you will get a lot.

Thought experiment - imagine you are on a low, flat final glide at 70 kts and a bunch of other gliders are well above you and well to either side of you at 150 knots on fast final glides. None of them represent collision risks but they will have a very high rate of closure with a very short time to impact up until they get nearly abeam of you at which point the closure rate will fall of dramatically. Maybe it would be good not to panic you unless someone actually points their glider close enough to you to be a legitimate risk.

Note: As I understand the FLARM algorithm, it is basically indistinguishable from a closure rate algorithm at close range and for high closure rates because it becomes basically mathematically impossible for the paths not to intersect given the "dynamic radius" that FLARM puts on position estimates. So in the above scenario, if one of those gliders steepend their decent and started turing towards you you would get a warning.

So to summarize - it appears based on your statements that the T-Advisor does use a collision algorithm - it's just a much more simplistic one than FLARM does. This algorithm is based on closure rate and (if I understand you) distance. Depending on the thresholds set for issuing a warning this will generate either more false negatives or (more likely given your statement about 100% probability - a very loose standard since it assumes nothing about what the airplane is physically capable of doing) it will generate a lot more false positive warnings.

I'd be interested to see a cockpit video of one of these systems in flight in a situation with, say 30 other gliders milling about in a thermal in a pre-start situation at a contest.

9B