[leemoore1966]

It was supposed to be a question. I meant you can but you may not achieve any meaningful accuracy.

Hmm, not sure I agree with that statement.
FLARM have been using GPS altitudes for comparison in their system for many years, and you have to agree this has made a very positive impact on safety between closely located aircraft in thermals.

its all about using common references for the comparison.

Thx
Lee

[Marvin]

My GPS altitude as given by the WAAS/EGNOS GPS receiver on the PilotAware, and reported from Sky Demon, is always within 20ft of my altimeter.

I have seen huge differences on extreme high/low pressure days, just imagine different days of 990mb or 1030mb (not particularly extreme), this is a variation of 40mb, which equates to a barometric height difference using 27ft/mb, of :-
40mb * 27ft/mb = 1080ft !!!

So now imagine that GPS altitude remains (relatively) constant, but Pressure altitude varies, the variation of altitude measured between GPS and Palt can be huge.

This is why it is crucial to use the same metric when comparing altitude, either
GPS -> GPS, or
Palt -> Palt

but NEVER
Palt <-> GPS

Thx
Lee

Lee to clarify please?

Are you assuming that the pressure altitude has not been zeroed according to the published QNH and is remaining on standard ISO of 1013.25 HPa?

The documentation for my Power FLARM claims that it first refers to the altitude of my transponder through reception of the mode C response and, if not available, to the internal pressure sensor. This is at odds with what your are saying about comparison for FLARM altitude based on GPS unless I've missed a document somewhere.

All the instrument approaches in the U.K. Publish the difference between threshold altitude based on amsl and the Geoid ref used by WSG84. The difference, although significant, is in a few 10s of feet and not the ranges you are inferring.

Thanks.

[leemoore1966]

Hi Marvin

Lee to clarify please?

Are you assuming that the pressure altitude has not been zeroed according to the published QNH and is remaining on standard ISO of 1013.25 HPa?

Yes, apologies for not being totally clear. I was referring to a barometric pressure sensor which is sent from a Transponder, whose reference will be 1013. The altitide reported by this compared to GPS will deviate, depending upon the actual air pressure on that day

The documentation for my Power FLARM claims that it first refers to the altitude of my transponder through reception of the mode C response and, if not available, to the internal pressure sensor. This is at odds with what your are saying about comparison for FLARM altitude based on GPS unless I've missed a document somewhere.

I was referring to FLARM not PowerFLARM. When comparing FLARM to FLARM, GPS Altitude is used, I think you are referring to PowerFLARM which will also compare to Mode-C/Mode-S/ADS-B, so uses the same technique as PilotAware described in an earlier post.

All the instrument approaches in the U.K. Publish the difference between threshold altitude based on amsl and the Geoid ref used by WSG84. The difference, although significant, is in a few 10s of feet and not the ranges you are inferring.

I think you have totally misunderstood my point, I am not referring to altimeter settings, I am referring to Altitude encoding as per a transponder whose reference can be GPS or QNE.

You can only (reliably) compare upon a reference based upon
GPS <-> GPS, or QNE <-> QNE
but should not compare
GPS <-> QNE

Thx
Lee

[leemoore1966]

I was just curious how someone might use a PAW in order to arrive at a QFE. If using the difference in GPS altitude that doesn't sound very good, as it's quite inaccurate sometimes, but if it's using the barometer then it would appear to be quite a good idea.

As Paul_Sengupta mentions in another posting the WAAS/EGNOS GPS is pretty good, and we are only talking here about relative altitude differences - not absolute.
if we were to compare to Mode-C then the absolute maximum accuracy can only be to within 100ft, this is all the encoding allows for a 12 bit code (shared with Mode-A), and even Mode-S is within 25ft

That said, PilotAware has the ability to provide both the barometer or the GPS.
I have to say, I do like your idea of using the barometer, and in fact you may recall a while back, I did try to convince/encourage you to have an icon for a GroundStation which I could use for the OGN-R

If we had a GroundStation icon, I would definitely use this (with barometric comparison) for the OGN-R, this could then be used to calculate relative height based upon the Palt rather than the GPS alt, although I am still not convinced this is more accurate over WAAS/EGNOS. Happy for others to educate me otherwise

edit: Found a nice posting here regarding WAAS
https://en.wikipedia.org/wiki/Wide_Area ... ion_System
skip to the part on 'Accuracy'

Thx
Lee

[joe-fbs]

In answer to the earlier question from Wilma about 2020, I had a brief look into this for work recently and the answer I thought was clear, but I may have misunderstood EASA speak, that ADS-B with extended squitter (where a Mode S transponder additionally broadcasts an aircraft’s unique ID, GPS position and related information) is mandatory in Europe for large IFR aircraft from 2020 i.e. not for all of us.

I found this article very helpful as well (note to self, join PPL / IR):

https://www.pplir.org/using-the-rating/ ... s-feb-2014

[Marvin]

Hi Marvin

Lee to clarify please?

Are you assuming that the pressure altitude has not been zeroed according to the published QNH and is remaining on standard ISO of 1013.25 HPa?

Yes, apologies for not being totally clear. I was referring to a barometric pressure sensor which is sent from a Transponder, whose reference will be 1013. The altitide reported by this compared to GPS will deviate, depending upon the actual air pressure on that day

The documentation for my Power FLARM claims that it first refers to the altitude of my transponder through reception of the mode C response and, if not available, to the internal pressure sensor. This is at odds with what your are saying about comparison for FLARM altitude based on GPS unless I've missed a document somewhere.

I was referring to FLARM not PowerFLARM. When comparing FLARM to FLARM, GPS Altitude is used, I think you are referring to PowerFLARM which will also compare to Mode-C/Mode-S/ADS-B, so uses the same technique as PilotAware described in an earlier post.

All the instrument approaches in the U.K. Publish the difference between threshold altitude based on amsl and the Geoid ref used by WSG84. The difference, although significant, is in a few 10s of feet and not the ranges you are inferring.

I think you have totally misunderstood my point, I am not referring to altimeter settings, I am referring to Altitude encoding as per a transponder whose reference can be GPS or QNE.

You can only (reliably) compare upon a reference based upon
GPS <-> GPS, or QNE <-> QNE
but should not compare
GPS <-> QNE

Thx
Lee

Thank you. Now understood


Sent from my iPhone using Tapatalk

[Paul_Sengupta]

Finally - I'm pretty sure I read somewhere that in 2020 GA planes will need to have ADSB out anyway (??).

Only in the US.

[T67M]

Finally - I'm pretty sure I read somewhere that in 2020 GA planes will need to have ADSB out anyway (??).

Only in the US.

And even in the USA, most light aircraft are installing the cheaper, not internationally standardised UAT rather than standard "ADS-B Out" since UAT offers quite a few extra features for free (weather, TFRs, NOTAMs and radar-derived traffic position for non-ADSB aircraft to name but four).

[loopylune]

Wow stirred up a hornets nest here ref the QFE with a pilot aware.
The Pilot aware has a barometer built in. It sits in our club house at the end of 26 at Tatenhill.
I use it rather than getting the atis from East Midlands as its much more convenient and not 30 minutes out of date, its live.
I have checked my altimeter after adjustment and at the threshold its just about crack on.
Back to the original point.
The trial.
Its a no brainer the system is there already in Pilotaware, (its in the name) its low cost, very reliable picks up more traffic than ADSB ever will and is in full use and proven a success.
These guys at NATS need to WAKE UP and smell the roses, they are trying to re-invent the wheel again, they are so out of touch.
Pilots and airfields are looking for a low cost reliable method of detect and display.
We already have it, we are happy to take part in any trial at Tatenhill as long as its sensible and includes a pilot aware unit as an optional piece of kit, oh by the way it has the ability to detect gliders also.
What more can I say.
Remember the big guys who wield the stick are often incorrect, look at Mode S !!
I have no affiliation to the Pilotaware guys, I'm just a very satisfied user and customer.
I rest my case m'lud.

[davef77]

Is that like WTF?

No, that is what you say just after you see a conflicting aircraft, not what you use to avoid it

[BEX]

loopylune wrote:

These guys at NATS need to WAKE UP and smell the roses, they are trying to re-invent the wheel again, they are so out of touch.

Sorry, I must have missed something. Where did NATS come into this?

?BEX

[Mike Tango]

What BEX said. If you are going to present and then rest a case at least try and get the facts right first. Less ranty helps as well.

Wow stirred up a hornets nest here ref the QFE with a pilot aware.
The Pilot aware has a barometer built in. It sits in our club house at the end of 26 at Tatenhill.
I use it rather than getting the atis from East Midlands as its much more convenient and not 30 minutes out of date, its live.
I have checked my altimeter after adjustment and at the threshold its just about crack on.
Back to the original point.
The trial.
Its a no brainer the system is there already in Pilotaware, (its in the name) its low cost, very reliable picks up more traffic than ADSB ever will and is in full use and proven a success.
These guys at NATS need to WAKE UP and smell the roses, they are trying to re-invent the wheel again, they are so out of touch.
Pilots and airfields are looking for a low cost reliable method of detect and display.
We already have it, we are happy to take part in any trial at Tatenhill as long as its sensible and includes a pilot aware unit as an optional piece of kit, oh by the way it has the ability to detect gliders also.
What more can I say.
Remember the big guys who wield the stick are often incorrect, look at Mode S !!
I have no affiliation to the Pilotaware guys, I'm just a very satisfied user and customer.
I rest my case m'lud.

[Cub]

I think all will become clear who is doing what over the next few days but I am not sure why anybody would think NATS are delivering ADS-B transceivers at GA airfields or why they would consider them anything other than completely proactive with GA given their work on AWARE, SkyDemon Light and Project EVA (CAP 1391).

[shuttle]

Correct, BEX & MT,
NATS are not running this particular trial.

It was all in the announcement.... FASVIG are sponsoring the trial under the supervision of the CAA, and abiding by the Concept of Operations being developed by the CAA.

The aim of the trial is to gather evidence to allow the CAA to assess the possibility of authorising use of ADS-B Traffic Displays by GA Airfield ATS Units.

This next bit is rather key and people need to recognise this......... 1090MHz ES ADS-B operates on a regulated frequency protected for aviation use. The trial is progressing on the basis that the CAA are not going to authorise ATS traffic displays based on non-protected non-regulated non-aviation frequencies.

If ADS-B Traffic Displays are eventually authorised by the CAA for use by GA Airfield ATS units (subject to approved safety cases) then those airfields will be able to incorporate use of ADS-B Traffic Displays into their formal operating procedures.

If we get to this point then who knows how ATS regulations may evolve in future to maximise the safety benefits of this technology.

[loopylune]

I stand corrected for a one word error.
silly me.