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Re: SkyEcho 2, SkyDemon and Flarm

PostPosted:Fri Jan 11, 2019 2:11 am
by neilmurg
gaznav wrote:I have never seen a big red circle from a SkyEcho emitting aircraft.
OK, good to have another unbiased data point to add to the knowledge base. You're unbiased right?

Re: SkyEcho 2, SkyDemon and Flarm

PostPosted:Fri Jan 11, 2019 7:45 am
by Dave Phillips
Cheap shot, Neil.

NACp - clearly some combinations of GPS and transceiver/transponder are emitting something that is an issue as far as a reliable traffic awareness system is concerned. Lee, Christian and Tim seem keen to understand the issue. Personally, my portable EC device tells me how well it is performing and that seems to be to an acceptable level. Surely it’s in everyone’s interest to understand why some systems are presenting 10nm rings of uncertainty?

Re: SkyEcho 2, SkyDemon and Flarm

PostPosted:Fri Jan 11, 2019 7:52 am
by Ian Melville
Gaznav, can you confirm that the fitted Nesis is using the internal GPS and not FLARM which from your previous posts will also be fitted. The Nesis can be selected to either if my understanding of the manual is correct.

Re: SkyEcho 2, SkyDemon and Flarm

PostPosted:Fri Jan 11, 2019 8:10 am
by leemoore1966
patowalker wrote:But PAW must have been used to verify the accuracy of the position source, in order to get the mod approved by the LAA.

As far as I am aware the only parameters important for the MOD, are SIL and SDA.
Also NACp is not a static setup value, it is a dynamically derived value which changes over time

Thx
Lee

Re: SkyEcho 2, SkyDemon and Flarm

PostPosted:Fri Jan 11, 2019 8:20 am
by gaznav
The GPS from the Nesis is being used to feed the TRiG transponder. That is what the chap who did the installation told me. Don’t forget that the other aircraft, that overlapped with HMCF was also displaying a 2nm ring - I think that was a Fuji aircraft on EASA so there is definitely something not quite right here. I’m over today to look at what EDGA has as well as a fit - I seem to recall it’s a full RNAV fit, so I am more convinced there is probably a protocol or decode issue here (a ‘1’ or ‘0’ in the wrong place).

You may recall Lee posted a picture of this when he first saw it - this of another aircraft.
Image

He’s also posted some separate received NACp figures in this thread, which would appear to be from a different device to a SkyEcho:
CODE: SELECT ALL
G-BDFR // NACp=2.0nm
G-HMCF // NACp=2.0nm
G-EDGA // NACp=10.0nm

So I think there is definitely something not quite right between some GPS and transponders or they are transmitting on a slightly different protocol?

Re: SkyEcho 2, SkyDemon and Flarm

PostPosted:Fri Jan 11, 2019 8:33 am
by Dave Phillips
leemoore1966 wrote:Also NACp is not a static setup value, it is a dynamically derived value which changes over time

Thx
Lee


Indeed. I have noticed one element that affects NACp is satellite coverage. In that respect this is very similar to RAIM.

Re: SkyEcho 2, SkyDemon and Flarm

PostPosted:Fri Jan 11, 2019 8:49 am
by ls8pilot
This all sounds very odd..... I've been looking at log traces from GPS systems since my first Garmin 55 back in the 1990's. Even with the old kit (which was I think only 5 channel) I've generally never seen deviations of several miles like those being reported - actual accuracy is enough to log an aircraft round a turn with diameter of a couple of hundred metres. Gliding systems commonly use drift over such a turn to estimate wind speed so any lack of accuracy would show up as huge wind shifts and be very noticeable.

I guess it's possible that at higher speeds GPS accuracy would be lower, but I think a lot of the references mentioned are GA so that should'nt be the issue.

So the question is if actual experienced accuracy is generally within a few metres what is causing the systems to generate a trusted accuracy of so much lower. I wonder what the algorithm is for calculating NACp?

Re: SkyEcho 2, SkyDemon and Flarm

PostPosted:Fri Jan 11, 2019 9:21 am
by G-JWTP
So 21 pages in.

Could someone precis what we've learnt so far?

I'm completely lost! :roll:

RAIM/SIL/SDA/EGDA/NACp/HMCF.

All I rearly want to know is, where is the local traffic and am I going to hit anyone when flying ?

G-JWTP

Re: SkyEcho 2, SkyDemon and Flarm

PostPosted:Fri Jan 11, 2019 9:26 am
by Cub
On the basis I have now seen lots and lots of emissions from aircraft both big and small, using my SE2 and SkyDemon combination but never yet noted any circles of death, do you think I have been lucky, have I not selected something or are Gazzer's examples genuinely exceptional?

Re: SkyEcho 2, SkyDemon and Flarm

PostPosted:Fri Jan 11, 2019 9:33 am
by GolfHotel
G-JWTP wrote:....

All I rearly want to know is, where is the local traffic and am I going to hit anyone when flying ?

G-JWTP


I haven’t been following all the fascinating posts on here. But I can tell you you are very unlikely to hit another aircraft. The chances of loss of control or CFIT are much higher. But seems to attract much less interest or emotional reaction.


Edit : I made this post while completely unaware of the tragedy in Spain yesterday. I hope no one takes offence. But I don’t intend to modify the post above. Any mod who thinks I should drop me a line. Or simple remove it.

Re: SkyEcho 2, SkyDemon and Flarm

PostPosted:Fri Jan 11, 2019 9:52 am
by G-JWTP
As yesterday showed.

G-JWTP

Re: SkyEcho 2, SkyDemon and Flarm

PostPosted:Fri Jan 11, 2019 10:01 am
by Shoestring Flyer
OK...So maybe the chances of hitting another aircraft are small compared to CFIT etc. There are times though when if you had some extra knowledge, i.e. whereabouts of other aircraft in close' ish proximity to you from EC that your visual scan has missed, that things just might be a bit more comfortable and you can manage the situation better. :)

Re: SkyEcho 2, SkyDemon and Flarm

PostPosted:Fri Jan 11, 2019 12:37 pm
by dewidaniels
I agree that it looks as if something's wrong somewhere. Either some aircraft are transmitting the wrong (or at least excessively pessimistic) NACp or the SkyEcho/SkyDemon combination is decoding the ADS-B messages incorrectly.

I configured my SkyEcho, which arrived yesterday. It was claiming NACp = 9 (< 30 m), and that was indoors.

Most GPS errors tend to be a constant offset rather than random fluctuations, You can see this if you look at a hand-held GPS while in a motor vehicle. The reported position may drift off to one side or other of the road (devices like TomTom snap the position to the nearest road). Even if a transceiver is reporting NACp = 3 (< 2 nm), that doesn't mean the reported position is jumping around within a 2 nm radius circle. The reported position is likely to be relatively stable, but may be offset by up to 2 nm from the true position.

My recollection from working on EGNOS many years ago was that GPS errors are mainly caused by factors such as ionospheric delay, multipath effects and clock errors (in the GPS satellites). Two GPS receivers in close proximity will see the same errors, so we can be more confident of their positions relative to each other than of their absolute positions. This is why differential GPS works.

I'm unsure that it's helpful for SkyDemon to display a circle based on the NCAp. Because of the way GPS works, I think that the likelihood that the distance and bearing to the other aircraft is correct is much higher than would be suggested by the NCAp. This is not true of multilaterated positions, which are not based on GPS, so that the relative positions and absolute positions are both uncertain to a similar degree.

Re: SkyEcho 2, SkyDemon and Flarm

PostPosted:Fri Jan 11, 2019 5:32 pm
by Nick
Could this error be caused by the OGN re broadcast signal causing a time delay, so you are getting one true position, then the previous position? Just a thought.

Nick

Re: SkyEcho 2, SkyDemon and Flarm

PostPosted:Fri Jan 11, 2019 6:20 pm
by Ian Melville
SkyEcho cannot receive OGN/R rebroadcasts. Also, Gaznav has clearly stated that they are ADS-B.

OGN/R rebroadcasts Mode-S and FLARM on the PAW frequency, not ADSB on 1090, so is not a factor.