[gaznav]

I have been told if the ADSB is broadcasting a SIL=0. Then the NACp should not be used
In which case it is indeed a receiver problem, as the receiver is passing NACp over the GDL90 interface for an ADSB source providing SIL=0

Thanks Lee, that is also interesting. I would have thought that knowing if NACp is low whether it is SIL=0 or SIL=1 is useful either way. As @neilmurg states. NACp is related to the number of satellites locked on and the resultant quality of the position report. So surely, this goes back to our debate about SIL=0 and SIL>=1 in that maybe we should have an ability to display the various levels of confidence in the reported position? I’m thinking colours, shades or shapes though and not whopping great rings of uncertainty for those with poor GPS/GNSS fixes for the reasons I stated before!

This is a good journey that we are all going through here and it was made poignant again yesterday when I overflew the area just south of WCO where the tragic mid-air occurred just over 2 years ago. These devices are now starting to see each other much better and flying with a PowerFLARM Portable and a SkyEcho 2 yesterday made me realise that these are great for spotting traffic that I may have missed before. Pretty much all EC devices on the market now (PAW, SE and PFLARM) can either see or be seen by the other, which is really great. I do think we are getting there with GA Electronic Conspicuity and that the convergence is happening - I’m definitely seeing much more GA ADS-B in the UK than I saw even just a year ago. Now with FLARM detection I will see even more. I’m still confident we’ll get over the ‘hump’ of concurrent Mode S use as well - I know this is being discussed at various levels in various organisations to agree the way ahead.

Here’s hoping 2019 sees further significant advances for the good of all.

[Cub]

I am confused now. I thought the depiction of the circle in SkyDemon was as a result of Lee’s request for SD to depict an area of uncertainty in relation to his multilateration activities with PAW. This was being achieved by producing a pseudo NACp value via the GDL90 interface. Is that correct?

This being the case, how is it now intended to indicate a level of uncertainty about an aircraft’s position when derived by PAW multilateration?

The reason for asking is I remain very concerned that when these encounters really matter at close range, I don’t get an indication to look left when I should be looking right.

[Tim Dawson]

What has become clear to me is that the NACp flag is not a very good way for PilotAware to report multilateration-based uncertainty. Not because of anything to do with PilotAware, but because of the apparent way it is output by the transponders it was designed for in the first place.

[GrahamB]

@Tim Dawson Perhaps you could try and depict lower grade targets with a different icon on the SD radar display, whilst leaving the main chart uncluttered by rings of uncertainty.

[Cub]

I think the ‘pseudo’ use of any GDL90 parameter is problamatic and should be avoided.

[Buzz53]

What is "pseudo"? Surely the correct answer is to fix the transponders? Are we really suggesting the ADSB accuracy/integrity mechanisms are worthless?
Alan

[leemoore1966]

I think the ‘pseudo’ use of any GDL90 parameter is problamatic and should be avoided.

What does that mean ?

NACp absolutely is pseudo by its nature, it cannot be measured, only derived

It is dependent upon the Dilution of Precision, to generate Horizontal Field of Merit, which is subsequently ‘massaged’ into a NACp figure

[Dave Phillips]

There’s a bigger issue here. The strategic aspiration was/is to have low cost EC which in some areas, including CAA aspirations, means piggy-backing Modes S transponders to transmit the data. We’re now in the realm of realising that transponders/transmitters don’t necessarily interpret the data presented to them in an equitable fashion.

Am I wrong? I hope so.

[GrahamB]

There’s a bigger issue here. The strategic aspiration was/is to have low cost EC which in some areas, including CAA aspirations, means piggy-backing Modes S transponders to transmit the data. We’re now in the realm of realising that transponders/transmitters don’t necessarily interpret the data presented to them in an equitable fashion.

Am I wrong? I hope so.

Isn't piggy-backing ADS-B on Mode S the preferred solution across the board, whether low-cost or otherwise?

Perhaps the people that wrote the standards should look again and provide guidance on how the data is to be interpreted in the new age of EC for all.

[gaznav]

@Cub

The reason for asking is I remain very concerned that when these encounters really matter at close range, I don’t get an indication to look left when I should be looking right.

I wholeheartedly agree with this sentiment. However, having re-read the GA ADS-B trial report again, I think this is unlikely given the level of accuracy that the trial reported, even for low NACp values. Granted a 2 mile error could be pretty catastrophic inside 2 miles (the average range that most will see a GA aircraft) in that it could be 22.5 degrees left or right of the nose or 45 degrees away from where you are looking at 2 miles. However, the trial seems to indicate that the reported position was less than 300m from its reported position over 99.8% of the time. Now seeing as ADS-B transmits once or twice every second then on average I would expect the position report to be pretty damn close to where the aircraft/device actually is.

I think @GrahamB and I are the most closely aligned on this - let’s have the same sized symbols for different levels of confidence of their data depicted by either colour, shade or shape? Then if we know there is a lower confidence in the position quality then the pilot will need to expand their scan a bit more?

[GolfHotel]

Were do we find the units for the uncertainty figure? It must be defined somewhere.

If you mean NACp,
https://www.faa.gov/nextgen/programs/ad ... D_RevA.PDF

Page 27 pdf

Thx
Lee

Thanks @leemoore1966

On page 27 of the document Lee linked to there is this comment below the table of values.
MFD Recommendation: Targets with either a NIC or NACp value that is 4 or lower (HPL >= 1.0 NM, or HFOM >= 0.5 NM) should be depicted using an icon that denotes a degraded target.

......

Perhaps the people that wrote the standards should look again and provide guidance on how the data is to be interpreted in the new age of EC for all.

That might be the solution?

GH

[gaznav]

GH

I agree, that is my prefered solution as well at viewtopic.php?p=1664822#p1664822

[ls8pilot]

For the LX8000 Flarm display they have a convention for "lost" contacts which is to display a blinking symbol for a defined number of seconds.

They use (user defined) colour coding to denote the relative altitude of the contact (eg Blue=above, Red=Level, Green=Below), so a blinking red symbol means "there is/was a low confidence contact at your level in this position"

Seems to work OK - might be one idea ?

Typically I've seen this with Flarm contacts at the limits of range, where you get shielding of the aerial (often by carbon-fibre fuselages), so you can get contacts which "ghost" as you move out of range or firm up as you get closer.

From the LX8000 manual:
When a signal from a particular aircraft is lost the aircraft remains blinking on the screen for the duration defined in " Lost device" (default 120 seconds). After that time the aircraft symbol will be removed.

[uAvionix-Ramsey]

I have been told if the ADSB is broadcasting a SIL=0. Then the NACp should not be used
In which case it is indeed a receiver problem, as the receiver is passing NACp over the GDL90 interface for an ADSB source providing SIL=0

I interpret this a slightly different way. By definition a SIL=0 GPS has no integrity (in GPS aviation terms) and therefore the maximum NACp "uncertainty" radius is entirely appropriate when compared to any certified GPS SIL 1-3.

It would be mis-representative to not display a "red circle" for SIL 0 GPS while displaying them as appropriately calculated for everything else. This would give false indication that the SIL 0 GPS position is more accurate/trusted.

A SIL 0 GPS won't even show up on an ATC display or a TCAS display.

[Lefty]

@Cub

I think @GrahamB and I are the most closely aligned on this - let’s have the same sized symbols for different levels of confidence of their data depicted by either colour, shade or shape? Then if we know there is a lower confidence in the position quality then the pilot will need to expand their scan a bit more?

I’m sorry, but I can’t help thinking that all these suggestions of circles, or different coloured, different sized or blinking target, can only serve to force pilots into spending more and more time staring at the SD screen - and almost zero time looking outside. It is fundamentally a bad thing.

Re the alleged poor accuracy being reported by these transponders. Surely for the purpose of EC, we should accept that in 99% of instances, the GPS is actually accurate to within 30-50m and just accept that since the GPS is accurate > 99% of the time, then we shouldn’t add more complexity that simply gives the user (prop 99) incorrect data.

It is clear that a few folks are doing a lot of working trying to perfect EC on SD, which I applaud and thank them for.

However I reiterate my view is that ideally, Traffic Awareness info should NOT be combined with Nav info, but should have it’s own clock face style display that is small enough to be in the pilot’s view whilst he is looking out. (It should also give an audible warning of traffic 2 o’clock 4 miles, 500 ft above). If Tim could offer a facility to have a 2nd phone or similar display device displaying nothing but traffic - I think it would be a winner.