AXP340/322 ADS-B Transponder

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NACv reporting

Printed From: Avidyne
Category: Avidyne General
Forum Name: AXP340/322 ADS-B Transponder
Forum Description: Topics on the Avidyne AXP340/322 ADS-B Transponder
URL: https://avidynelive.com/forum_posts.asp?TID=963
Printed Date: 26 Mar 2026 at 1:01pm
Software Version: Web Wiz Forums 12.07 - https://www.webwizforums.com

Topic: NACv reporting

Posted By: paulr
Subject: NACv reporting
Date Posted: 12 Feb 2016 at 4:53am

So my plane is in the shop to get an IFD540 and AXP340 (and an AMX240 too). I test flew it yesterday, which didn't go well, but that's another story.

After the test flight, I went to www.faa.gov/nextgen/equipadsb and sent them an email requesting ADS-B test results. The report came back this morning with a one-line summary: "Suggest contacting Avidyne for advice." The installed system failed 100% of the horizontal velocity (NACv) reporting latency checks (see FAR 91.227.c.2). Of course I am having the installer figure out what the root cause is, so I expect they will be in touch with Avidyne support, but for my own edification I'd like to understand what the possible causes are for this. I skimmed AC 20-165B; it sets out the acceptable ranges for NACv accuracy and latency but didn't help me understand why the latency would be high.

Replies:

Posted By: AviJake
Date Posted: 12 Feb 2016 at 4:57am

Almost certainly a configuration issue on either the 340 or 540. There is a Service Information Letter and the Install Manual that both cover the proper settings. I can dig those up if you need but your install shop should have both.

If you want to have us sanity check the settings, either send us pictures of the 540 RS-232 page or the 540 Configuration log and pictures of the 340 setup pages.

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Steve Jacobson
sjacobson@avidyne.com

Posted By: paulr
Date Posted: 12 Feb 2016 at 5:04am

Thanks, Steve, for the tip. I skimmed the 340 install manual but didn't see any SILs that looked relevant on your web site. If I had to guess, I would say that 7.5.10 in the AXP340 install manual is at fault. When I was in the plane yesterday I did verify that I could see GPS position coordinates on the AXP340 display, and the PMC report shows position and altitude data being reported correctly.

Posted By: AviJake
Date Posted: 12 Feb 2016 at 5:39am

Right, the IM has the settings to set in 7.5.10. If you're using the 540, then you need to select 10 m/s. Are you saying that is set already at 10 m/s on the 340? If so, we need to look at the protocol setting you have on the 540 serial 232 page.

7.5.10 GPS NACV
Another metric that the ADS-B ground system uses to help it track the aircraft is NACV. NACV is the Navigational Accuracy Category for velocity, and is a design feature of the GPS receiver. It represents the error bound for velocity that the GPS may report in acceleration/deceleration or turning maneuvers. You can find this information from your GPS installation manual.

The following table lists the NACV for the approved GPS systems when connected to AXP340:
GPS Receiver NACV Setting
Avidyne R9 - 10 meters per second

IFD 5XX - 10 meters per second

Garmin 4XX/5XX

NOTE: If using the Garmin ADS-B Plus protocol, the NACv is set dynamically from the GPS data. In this instance, the NACv setting from the setup menu is ignored.

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Steve Jacobson
sjacobson@avidyne.com