Another mid-air (UK)

Whether or not any of this has a thing to do with the original post:
The technology discussed is not perfect, and like a vario is supplementary.
Is your scan 100% perfect? Are your eyes? After 8 hours of flying? Burning from sweat and sunscreen?
Will your vision improve with age? Will you admit it?
Jim

Op donderdag 7 augustus 2014 18:55:16 UTC+2 schreef Stats Watcher:


Error is cumulative, that's how d-GPS and WAAS work. So the guy a mile away from you also has 8 or more meters of error. In exactly the same direction vector as for you. So relative accuracy is virtually +/-0...

In article son_of_flubber writes:
On Thursday, August 7, 2014 11:46:30 AM UTC-4, Ramy wrote:
Even with 8m accuracy, It is more than sufficient for accurate collision=
avoidance, unless someone considers half a wingspan not accurate enough fo=
r collision warning with another glider or obstacle.=20

Prompted to comment here by my Geek Fascination Syndrome (GFS)...

I'd guess that the error on each position fix is not significant. Flarm is =
computing trajectories from multiple position fixes, so a maximum +/- 8M er=
ror for each fix will be smoothed out by the statistical distribution of a =
large number of position fix errors. Some of the errors are + and some are =
-. Average 100 GPS position fixes and you will get a pretty accurate positi=
on.

Not so much as you might expect. The error varies more slowly over
time. If those 100 fixes were taken one every hour or two, then they
would average out much better than fixes taken once per second.

The fact that the error moves more slowly helps the trajectory calculation
as successive samples will have a similar error.

Alan

On Thursday, August 7, 2014 8:49:07 AM UTC-7, Stats Watcher wrote:
At 15:04 07 August 2014, jfitch wrote:



Isn't the Flarm GPS WAAS corrected? The WAAS standard of

performance is

1.6M nominal horizontal (95%). When the NDGPS system is

functional (and GPS

engines take advantage of it) this will be reduced to 15 cm.





Not in Europe as WAAS is US only. A search of the Powerflarm

website returned 0 (zero) occurrences of the term 'WAAS'. So it

seems unlikley.... Same result for NDGPS, so that seems unlikely

too.. If someone finds something different I will obviously be wrong

but until that point it's still +-16m (ie a whole wingspan) at 99%

confidence

From the Flarm.com product page:

"FLARM incorporates a high-precision WAAS 16-channel GPS receiver and an integrated low-power radio transceiver. Static obstacles are included in FLARM's database."

Nevertheless, WAAS is intended to correct for atmospheric and geometric anomalies, to improve absolute position accuracy. For gliders in close proximity, these errors are tracking. The fact is, the Flarm GPS has several times the accuracy required to perform the expected function.

On Thursday, August 7, 2014 12:55:16 PM UTC-4, Stats Watcher wrote:
If you analyse the data rate and bandwidth
requirements of the data link you'll realise what you suggest is
impossible

The error correcting computations that I suggest consume ZERO communication bandwidth between FLARM units. The only thing that needs to be broadcast over the data link is the trajectory vector.

On Thursday, August 7, 2014 1:15:37 PM UTC-7, jfitch wrote:
On Thursday, August 7, 2014 8:49:07 AM UTC-7, Stats Watcher wrote:

At 15:04 07 August 2014, jfitch wrote:







Isn't the Flarm GPS WAAS corrected? The WAAS standard of



performance is



1.6M nominal horizontal (95%). When the NDGPS system is



functional (and GPS



engines take advantage of it) this will be reduced to 15 cm.











Not in Europe as WAAS is US only. A search of the Powerflarm



website returned 0 (zero) occurrences of the term 'WAAS'. So it



seems unlikley.... Same result for NDGPS, so that seems unlikely



too.. If someone finds something different I will obviously be wrong



but until that point it's still +-16m (ie a whole wingspan) at 99%



confidence



From the Flarm.com product page:



"FLARM incorporates a high-precision WAAS 16-channel GPS receiver and an integrated low-power radio transceiver. Static obstacles are included in FLARM's database."



Nevertheless, WAAS is intended to correct for atmospheric and geometric anomalies, to improve absolute position accuracy. For gliders in close proximity, these errors are tracking. The fact is, the Flarm GPS has several times the accuracy required to perform the expected function.

I'll add to it that IIRC altitude separation is based on pressure altitude, not GPS altitude. But this is all academic discussion. Those who experienced flying with Flarm near other pilots can attest to its accuracy. Not once I got close enough to a collision course with another flarm equipped glider without Flarm alerting me.

Ramy

I'll add to it that IIRC altitude separation is based on pressure altitude, not GPS altitude. But this is all academic discussion. Those who experienced flying with Flarm near other pilots can attest to its accuracy. Not once I got close enough to a collision course with another flarm equipped glider without Flarm alerting me.
Ramy

Your memory is correct - from the PF 3.4 manual, page 5:
"If available, PowerFLARM uses the barometric altitude from a Mode S transponder installed on the same aircraft. If not, PowerFLARM uses barometric altitude derived from the built-in pressure sensor. Barometric altitude is used for determining the relative altitude to PCAS targets."
My experience is the same as yours - never had a failure to warn on a flarm-equipped glider. I get about 4.5 km average range. I check my installation occasionally using the range analysis tool.

While I'm not in the market for a new glider, I would not buy one without there being provision for a flarm antenna in the tail with an all-around look, and low-loss coax going to the front; with installations being critical, having to shoe-horn them in (I've installed in an SZD-55 (easy), ASW24 (tight for the antennae), ASW20 (same), and Puchacz (still looking for the right space)) shouldn't be necessary.

On 2014-08-07 15:46:30 +0000, Ramy said:

Even with 8m accuracy, It is more than sufficient for accurate
collision avoidance, unless someone considers half a wingspan not
accurate enough for collision warning with another glider or obstacle.

Even the old 100m accuracy when "selective availability" was turned on
was good enough for avoiding antennas or wires etc. It's not as if you
are going to fly as close as you can to them.

Also, note that the errors are not random. Two GPS receivers in the
same area at the same time will show the same error, in the same
direction (to within 6 or 8 m), thus making their relative locations
accurate enough.