I’ve been doing a bit of reading on this and I’m not so sure the display of Navigation Accuracy Category - Position (NACp) and in particular the Estimated Position Uncertainty (EPU) with large red circles is a good idea. This is why:
NATS/CAA GA ADS-B trial:
https://nats.aero/blog/wp-content/uploa ... report.pdfThe following are taken from the Executive Summary...
The Navigational Accuracy Code for Position (NACp) quality indicator that reports the expected accuracy of the position reported, was on the whole found to be very conservative in the non- certified fleet; however this is better than over-estimating the accuracy capability.
When assessed against the ESASSP requirements, the mean horizontal position error (HPE) was measured to be 43.94m well within the required 300m and recommended 210m, Overall 99.84% of ADS-B HPE’s recorded by the non-certified fleet were below 300m [which is NACp=7], which was a higher percentage than that of the certified comparison fleet which recorded 99.7% and a mean HPE of 40.35m.
The accuracy of the non-certified GPS position reports was found to be very similar to that of the certified aircraft; however the trial did record several extremely large horizontal position errors. Investigation of these errors determined they were broadcast by one particular airframe and were caused when the reported longitude position swapped from negative to positive for an update. It is not expected that a single large error would lead to significant issues to ATC as a single report would be considered as an outlier by a surveillance tracker and not lead to credible corruption.
Later in the document it states:
Navigation Accuracy Code for position was assigned with 297,435 (87.19%) of the ADS-B reports.
Of these, the most frequent accuracy provided was NACp=3 equating to a reported horizontal positional accuracy of 2NM which was reported in 193,462 reports or 56.7% of all messages. It should be noted that 77,473 (22.71%) of reports received indicated NACp=10 corresponding to an accuracy bound of less than 15m. A summary bar chart of these values is provided in Figure 20 below.
So this is probably why I am seeing lots of 2nm circles from various aircraft if SkyDemon is displaying the NACp ‘circle of uncertainty’. Certainly if it is “the most frequent accuracy provided” as stated in the trial report.
Now here comes the killer statement in my mind (bear with it as it is deep in techno-talk):
The quality indicator NACp=10 indicates that the position broadcast by the airframe should be within 15m of the aircrafts actual position. As this assessment uses radar track data as the baseline ‘truth track’ it is not possible to definitively assess to the reported accuracy to 15m (a DGPS fitted to the airframes would be required for this task), however a plot of the HPE of all airframes reporting NACp=10 would be anticipated all be within the 300m tolerance; given the expected accuracy of the radar track. Figure 21 below shows the cumulative plot of HPE’s for all GA airframes reporting NACp=10 (red) and NACp=3 (green).
It can be seen that 99% of airframes reporting NACp=10 are within a HPE of 150m. However, the largest reported HPE for the NACp=10 sample was 1,126m. The mean HPE for NACP=10 reports was 34.43m, with a standard deviation of 28.94m.
For the airframes reporting NACp=3, which indicates that the aircraft are reporting a position within 2NM of their actual position (see Table 3), the mean HPE was 38.54m with a standard deviation of 29.4m and a maximum HPE of 1,080.76m.
The results and Figure 21 below illustrates that although the error bound NACp=10 is much less than for NACp=3, the overall the magnitude of the HPE’s are very similar suggesting that the for the majority of the time, NACp=3 airframes are extremely conservative at reporting the accuracy capability of the GPS source under fault free conditions.
So a trial that used RADAR to verify position versus the reported “position of uncertainty” found the accuracy reported to be “extremely conservative”. Indeed the uncertified systems reporting NACp = 3 (up to 2nm) were found to be nearly as good as NACp = 10 (up to 10 metres). Therefore, by displaying a circle around the aircraft we may well be misleading what is happening in real life - that is my experience recently, where the aircraft are pretty much pegged to the position they are reporting. Considering that these devices are being used for “Collision Awareness” rather than “Avoidance” (there is a significant difference), then is the displaying of these circles likely to lead to confusion and also missing other things displayed (like a 2nm ATZ or not)?
@leemoore1966 Provided a useful reference to the GDL90 format on “Page 27 in the PDF”. Where it states as a footnote for Multi-Function Displays (MFDs) like SkyDemon:
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.
So @Tim Dawson can we please have a different icon, and not a circle, to depict NACp<=4? Otherwise, when I go to the LAA Rally this year my SkyDemon screen is going to be obliterated up to 10nm out from Sywell with red overlapping circles! If nothing else, could you give us a user selectable option?
I hope this helps?