Primarily for general aviation discussion, but other aviation topics are also welcome.
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uAvionix-Ramsey wrote:Interesting, do these logs also capture the SIL value? If so, certainly seems like something is wrong if SIL >=1 and NACp=10.

They capture
version, nic, nacp, sil, sils

When I did this anslysis, I may have filtered the search by only looking at the NACp figures when SIL=0
I cant recall definitely

I would need to rerun the searches, I cannot do that till Monday...
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By Buzz53
Dave Phillips wrote:From CAP 1391 (my bolds)
I think Christian has no option but to report a NACp=0 if he is to comply with CAP1391.

Surely the table entry for NACp says "ZERO/depends on HFOM"? Which is pretty ambiguous but lacking clarification it would seem that NACp may more usefully be set from the GNSS HFOM as usual.
By Buzz53
I get your drift, but then why does the entry for NACp in Table 4 not just say ZERO as for all the other parameters on COTS systems? The table seems to have been created specifically for CAP1391. Is it a typo? Seems unlikely as it sticks out like a sore thumb. What was the author's intent in offering the option for "depends on HOFM"?
Prompted by posts by @leemoore1966, @uAvionix-Ramsey and others re NACp values, I decided to carry out some research of my own. This involved a fairly ‘subjective’ analysis of records carried out by trawling through 3- years’ worth of screengrabs from my PAW Testing - comprising in the region of 3000 records.

Whilst I concentrated particularly on records showing my own aircraft transmissions (124 separate records - which only covers a miniscule proportion of the flying I have done during the period, but is what I have available), I took careful note of all recorded instances of NCAp, including both CAT and GA aircraft. For the record my own aircraft is fitted with a Trig TT-21, transmitting Mode S-ES by way of GPS data supplied by a dedicated uncertified GPS source - an American ‘Byonics GPS4’ module and set to SDA and SIL both = 0.

The results for my own aircraft comprise a total of 124 records, 88 of which indicate an NACp = 9; - 33 an NACp = 10; - and 3 an NACp = 8, with NO indications of any lower value ever having been transmitted.

Examination of records from other known regularly observed aircraft also indicated very similar figures. In fact I counted less than 20 aircraft over a 3 year period indicating a NACp of lower than 8, and in almost all of these cases these could be attributed to a small handful of aircraft (6) seen retrospectively to have been regularly transmitting NACp values of 1 or 3, which would seem to suggest a fault in their installations. Strangely, I have NO records showing an NACp of level 2. or any showing NACp between 4 to 7 inclusive. There are a few (CAT) records indicating NACp = 11. There were also a few records showing NACp = 0 - i.e. ‘Unknown’, but these numbers were relatively low.

Whilst this is merely a subjective analysis of my own screengrabs, very few of these records would have resulted in an ‘ambiguity circle’ of any concern.

Hope this helps inform the debate.

Oh and over this same period, I can say with conviction that I have had very few doubts as to the position of aircraft seen in relation to their indicated screen position


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By neilmurg
leemoore1966 wrote:...To be honest I think once the uncertainty exceeds possibly 2nm, it would be better to either exclude these ADSB targets or represent them as bearingless...
but available evidence (AFAIK) suggests that despite the crappy NACp, they are actually where they think they are. So, maybe just a nominal uncertainty circle/indicator? That's much better info than bearingless (IMO)
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By neilmurg
exfirepro wrote:Prompted by posts by @leemoore1966, @uAvionix-Ramsey and others re NACp values, I decided to carry out some research of my own. This involved a fairly ‘subjective’ analysis of records carried out by trawling through 3- years’ worth of screengrabs from my PAW Testing - comprising in the region of 3000 records.

You're providing exactly the kind of open, raw data that's needed. Thankyou.
I worry that it may be distorted by other vested interests, but I hope that other parties will release their unedited data and we'll all learn from it.
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By exfirepro
@neilmurg @Tim Dawson

I agree with that principle. I have for a while now been involved in testing PilotAware Mode S-3D ‘positional’ signals, transmitted from my own Ground Station in West Edinburgh. Testing to collect specific data on positional accuracy is ongoing and now on a more scientific footing.

With few exceptions, the actual aircraft positions (from direct observation and comparison with other aircraft sites such as FR24) have complied fairly accurately with their indicated Mode S-3D positions, with the SD ambiguity circles generally indicating ‘time since last positional update’ rather than positional inaccuracy per se, - the position starting as a straightforward aircraft symbol with no ambiguity circle, then with a small circle appearing around it and expanding usually in 3 stages up to about +/- 1 NM then resetting back to a point as fresh position data is established.

Rather than reverting aircraft with NACp beyond this point to a Bearingless Target, I would suggest either keeping the initial (small) ambiguity circles and changing to a different ‘symbol’ to indicate any further reduction in ‘alleged’ reliability or replacing the ambiguity circles completely, with, for example colour, coded aircraft symbols to indicate the reliability (or otherwise) of positional accuracy.

It is important to remember that what we are trying to achieve here is to indicate position for the approximately 80% * of GA (and other) aircraft in the UK who are still running pure Mode S Transponders and hence currently ‘Bearingless’, - i.e. with no known position, so being able to show an accurate position of these aircraft with a small degree of ‘uncertainty’ is surely significantly better than what we have at present.

Remembering of course that the previous trials and ongoing testing indicate that the reported aircraft positions are highly accurate in the vast majority of cases. * *



* Figure taken from a survey into EC device traffic levels obtained as a preamble to the ongoing CAA / FASVIG sponsored HIAL Dundee Trials.

* * Reference the NATS GA ADSB / GPS Trials Report ... report.pdf
Last edited by exfirepro on Sun Jan 13, 2019 9:34 am, edited 3 times in total.
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By neilmurg
Lefty wrote:
gaznav wrote:@Cub
...Traffic Awareness info should NOT be combined with Nav info, but should have it’s own clock face...
That's an installation option. OK, yes, you can choose to do that.
By FlyingEagle
Thought I'd briefly add my personal experience of Sky Echo 2 and the company Uavionix.

Avoid like the plague. This product is no where near ready!

I'm now on my second unit that still dosen't work and I'm expected to start reprogramming it myself! Thats not what you pay nearly £500 quid for.

I'd avoid the SkyEcho 2 as it clearly has been rushed to market not ready.

A shame as I'd put down a deposit at AeroExpo last year (which wasn't taken off my purchase price either!)

Only my personal experience, but AVOID Uavionix!

I've been trying to return the two units and get a full refund including all Fed Ex and duty charges, but to no avail. Very poor.

If anyone wants to purchase either non working version, please let me know!

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By gaznav


Sorry to hear that, but my experience couldn’t be any more different. If you need help reprogramming the unit then PM me and I’ll talk you through it with some pictures - it took me about 5 minutes with my iPad and that was my first time.

Best, Gaz
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By stevelup
FE, to distil the problem down to one sentence - you want to return them because they need a firmware update and you're not willing to do it yourself?

Is that the long and short of it? Or are they actually faulty?
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I think if I had put down a deposit for something, received it (but no deposit refunded) and it didnt work, then return, and finally received a second one which also didnt work, I dont think I would be very impressed either and would possibly want to wash my hands of the thing... but maybe thats just me...

This isnt a solution knocked up by enthusiasts "at cost", this is a commercial product which has been much flaunted (on here especially) as the solution which is supposed to work "out of the box".

As others have not had issues, maybe this company's chaps will see the post above and do something in response?

Regards, SD..
Last edited by skydriller on Sun Jan 13, 2019 3:51 pm, edited 1 time in total.
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