On 05/07/2005, at 11:03 AM, Geoff Kidd wrote:
Double negatives aside ............ how do you rate the double
positive of a warning that DOES occur when you have no idea that
the other ship is aiming for you HEAD-ON.
Sigh.
There are four possible modes that a system like this can be working in.
* True Negative (Unit is silent, no threats nearby)
This is where the typical FLARM installation will spend almost
all of its life; the proverbial NULL state. As long as you have
some way of testing whether it's still working, this is where you
WANT it to spend its entire life.
* True Positive (Unit is alerting, threat is actually nearby)
This is where the FLARM is correctly alerting you to a potential
midair.
* False positive (Unit is alerting, no threats are actually nearby).
If FLARM units do this more than a handful of times, they will
produce an extremely powerful psychological reaction in the pilot,
who will say, "The unit is dodgy, it doesn't work, I'm turning it
off." At that point the FLARM becomes completely useless to
everybody.
* False negative (Unit silent, actual threat nearby)
This is at least as dangerous as having no FLARM unit at all,
possibly more so if the pilot has formed the idea that he can
compromise his lookout if the FLARM is going to tell him about
midairs anyway.
For a FLARM to be 100% reliable, there would need to be 0 incidences
of the last two categories.
Currently we can probably guess that the false positive category
is pretty low, because anecdotal evidence about the usefulness of
FLARM seems positive, and if the people telling the anecdotes had
been pissed off by errors and turned it off they wouldn't be raving
about its virtues.
But we presently have no data whatsoever about the false negative
category.
Would FLARM be worth installing if the possibility of a false negative
was greater than the possibility of a true positive? I'd think not.
Mike B reckons the ATSB is comfortable with ADS-B having a 25% false
negative rate (because it's only expected to prevent 75% of midairs).
Do we have any data whatsoever on whether FLARM's false negative
rate exceeds that of ADS-B ?
We need that data to evaluate it. The data isn't just one of those
things that's nice to have, it's absolutely essential to determine
whether FLARM is an inferior offering to what CASA has in mind for
us.
- mark
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I tried an internal modem, [EMAIL PROTECTED]
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