FLARM.....for good, or evil??

On 10/28/2010 2:14 PM, Eric Greenwell wrote:
On 10/28/2010 7:29 AM, Mike Schumann wrote:
On 10/27/2010 11:16 PM, Eric Greenwell wrote:


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.
You may be underestimating the value of transmitting the projected path.
When another glider is first detected, your unit has only one position
report and can not determine the flight path from that single point, and
it will take several more precious seconds to determine the flight path
of the potential threat; however, because the projected path is
transmitted every second, your unit immediately knows it.

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.

I'm sure the developers have tested their algorithms with thousands of
simulations using IGC files from gliders in many situations. The Parowan
accident simulation at

http://www.gliderpilot.org/Flarm-Par...dairSimulation

shows what can be done. It would be interesting, informative, and
entertaining if there was a website or application that would let us run
IGC files we select in a simulation like this. I'm curious about how
Flarm would react in a few situations I've encountered. Doing
simulations on a pilot's own files might be more persuasive of the value
of Flarm than even the most well-written explanations, and much more
easily understood than the algorithms themselves.


There's no question that FLARM or ADS-B based systems could have easily
prevented the Parowan mid-air. My question is how much you can rely on
this type of equipment to accurately warn you of collisions when you are
flying in gaggles.

Obviously knowing the relative position of other gliders in the gaggle
is helpful. I would be very skeptical, however, of putting my faith in
FLARM or any other system to accurately warn me of a collision with
another glider that was in the same thermal, near my altitude, that was
in my blind spot.

Transmitting the project path of the aircraft is really only beneficial
if the equipment on board the transmitting aircraft has some added
information that is not available to the receiver on the transmitting
aircraft's intent.

With both FLARM and ADS-B systems, the initial visibility of the other
aircraft occurs way before there is any collision threat, so the
receiver should have no difficulty computing the project path of the
other aircraft. ADS-B actually transmits the category of aircraft (i.e.
glider, balloon, etc...) so the receiver can get a pretty good hint on
the type of maneuvers that can be expected.

--
Mike Schumann

On 10/28/2010 11:32 AM, Mike Schumann wrote:


There's no question that FLARM or ADS-B based systems could have
easily prevented the Parowan mid-air. My question is how much you can
rely on this type of equipment to accurately warn you of collisions
when you are flying in gaggles.

Obviously knowing the relative position of other gliders in the gaggle
is helpful. I would be very skeptical, however, of putting my faith
in FLARM or any other system to accurately warn me of a collision with
another glider that was in the same thermal, near my altitude, that
was in my blind spot.
This situation is addressed at

http://www.gliderpilot.org/FlarmFlig...andPerformance

where it states that the human eye is better than Flarm. I don't think
anyone has claimed Flarm is better in every possible situation, and
users and Flarm itself repeatedly state you must still look outside to
have the best protection; however, I believe Flarm will indicate there
is a glider behind you, something a pilot might not always be aware of,
so it still has value in this situation.

Transmitting the project path of the aircraft is really only
beneficial if the equipment on board the transmitting aircraft has
some added information that is not available to the receiver on the
transmitting aircraft's intent.

With both FLARM and ADS-B systems, the initial visibility of the other
aircraft occurs way before there is any collision threat, so the
receiver should have no difficulty computing the project path of the
other aircraft. ADS-B actually transmits the category of aircraft
(i.e. glider, balloon, etc...) so the receiver can get a pretty good
hint on the type of maneuvers that can be expected.
I can think of three situations where the time involved can be reduced:

1) two gliders approaching head on. At 100 knots each - a 200 knot
closing speed - that's only 18 seconds or so to collision. How many
seconds of warning do you lose while collecting enough points to make a
good estimate of the projected paths - 5 seconds, 10 seconds? I don't
know, but I'd prefer to know sooner than later.

2) Ridge or mountain flying, where the transmissions are blocked by the
terrain. Once they round the corner of the ridge, there may not be
enough time to calculate a projected path.

3) shortened range due to signal blockage by the wings or fuselage.


--

Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm http://tinyurl.com/yb3xywl
- "A Guide to Self-launching Sailplane Operation Mar/2004" Much of what you need to know tinyurl.com/yfs7tnz

On 10/28/2010 3:36 PM, Eric Greenwell wrote:
On 10/28/2010 11:32 AM, Mike Schumann wrote:


There's no question that FLARM or ADS-B based systems could have
easily prevented the Parowan mid-air. My question is how much you can
rely on this type of equipment to accurately warn you of collisions
when you are flying in gaggles.

Obviously knowing the relative position of other gliders in the gaggle
is helpful. I would be very skeptical, however, of putting my faith in
FLARM or any other system to accurately warn me of a collision with
another glider that was in the same thermal, near my altitude, that
was in my blind spot.
This situation is addressed at

http://www.gliderpilot.org/FlarmFlig...andPerformance

where it states that the human eye is better than Flarm. I don't think
anyone has claimed Flarm is better in every possible situation, and
users and Flarm itself repeatedly state you must still look outside to
have the best protection; however, I believe Flarm will indicate there
is a glider behind you, something a pilot might not always be aware of,
so it still has value in this situation.

Transmitting the project path of the aircraft is really only
beneficial if the equipment on board the transmitting aircraft has
some added information that is not available to the receiver on the
transmitting aircraft's intent.

With both FLARM and ADS-B systems, the initial visibility of the other
aircraft occurs way before there is any collision threat, so the
receiver should have no difficulty computing the project path of the
other aircraft. ADS-B actually transmits the category of aircraft
(i.e. glider, balloon, etc...) so the receiver can get a pretty good
hint on the type of maneuvers that can be expected.
I can think of three situations where the time involved can be reduced:

1) two gliders approaching head on. At 100 knots each - a 200 knot
closing speed - that's only 18 seconds or so to collision. How many
seconds of warning do you lose while collecting enough points to make a
good estimate of the projected paths - 5 seconds, 10 seconds? I don't
know, but I'd prefer to know sooner than later.

2) Ridge or mountain flying, where the transmissions are blocked by the
terrain. Once they round the corner of the ridge, there may not be
enough time to calculate a projected path.

3) shortened range due to signal blockage by the wings or fuselage.



The proper logic on unexpectedly seeing a new target close by without
have a chance to compute trajectory is to use a worse case scenario.
Granted, having the trajectory as part of the transmission would be
helpful in this instance.

--
Mike Schumann

On 10/28/2010 4:03 PM, Mike Schumann wrote:
On 10/28/2010 3:36 PM, Eric Greenwell wrote:
I can think of three situations where the time involved can be reduced:

1) two gliders approaching head on. At 100 knots each - a 200 knot
closing speed - that's only 18 seconds or so to collision. How many
seconds of warning do you lose while collecting enough points to make a
good estimate of the projected paths - 5 seconds, 10 seconds? I don't
know, but I'd prefer to know sooner than later.

2) Ridge or mountain flying, where the transmissions are blocked by the
terrain. Once they round the corner of the ridge, there may not be
enough time to calculate a projected path.

3) shortened range due to signal blockage by the wings or fuselage.
The proper logic on unexpectedly seeing a new target close by without
have a chance to compute trajectory is to use a worse case scenario.
Granted, having the trajectory as part of the transmission would be
helpful in this instance.
And which way do you turn, when you don't know where the threat is going?

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)

On 10/28/2010 10:38 PM, Eric Greenwell wrote:
On 10/28/2010 4:03 PM, Mike Schumann wrote:
On 10/28/2010 3:36 PM, Eric Greenwell wrote:
I can think of three situations where the time involved can be reduced:

1) two gliders approaching head on. At 100 knots each - a 200 knot
closing speed - that's only 18 seconds or so to collision. How many
seconds of warning do you lose while collecting enough points to make a
good estimate of the projected paths - 5 seconds, 10 seconds? I don't
know, but I'd prefer to know sooner than later.

2) Ridge or mountain flying, where the transmissions are blocked by the
terrain. Once they round the corner of the ridge, there may not be
enough time to calculate a projected path.

3) shortened range due to signal blockage by the wings or fuselage.
The proper logic on unexpectedly seeing a new target close by without
have a chance to compute trajectory is to use a worse case scenario.
Granted, having the trajectory as part of the transmission would be
helpful in this instance.
And which way do you turn, when you don't know where the threat is going?


It appears that ADS-B does transmit the location as well as the
instantaneous velocity vector each second, so if you have a graphical
display, you will be able to see the orientation and physical location
of the aircraft immediately. It doesn't currently transmit any turn
rate info. I don't know if it gives you any data on rate of climb /
descent.

A basic FLARM unit, without a graphical display, won't give you any
hints of which direction the target is moving in. It will only give you
a rough idea of where the threat target is, so you have to use your eyes
and judgment to take evasive action.

I suspect that a graphical display is primarily useful to identify the
location and course info for aircraft in your area before you get into
an alarm condition. Once the alarm goes off, you should probably be
looking outside. (Not having flown with these types of units, others
probably have better insights into this).

--
Mike Schumann

Mike Schumann wrote:
My question is how much you can rely on
this type of equipment to accurately warn you of collisions when you are
flying in gaggles.

In gaggles: Not at all. Period. Simply not possible - and not
necessairy, either.