ADS-B drop-outs

I have an early Navworx box that's been upgraded a couple of times, and the last FAA performance report came in all good. However ever since I got it one thing has dogged me and that is that I DON'T SEE ALL THE ADS-B TRAFFIC.

Specifically I will be flying in radar airspace (class C) with RADAR GOOD on the display and ATC is calling out multiple airliner traffic within 5 miles, and I can SEE the airliners - but many if not most of them don't show up on the screen.

Other times I'll be flying where I know other aircraft are that have ADS-B out and some will show up, others not. Most notably, close-by aircraft (within5 miles) with ADS-B out will not be there or will suddenly disappear.

Just today I was entering class D airspace (KHIO) and there were 3 ADS-B OUT targets on within 5nm on the screen and showing N numbers, and they all disappeared at once. The dropout seemed to coincide with my descending below radar coverage, but why would I lose them if they're transmitting ADS-B out? Meantime other targets further away still showed up, and all status indicators showed green.

My performance reports are showing good with the exception that there are usually Mode-3A partial failures, which coincide with being partially outside or on the edge of full coverage areas. For example on today's flight showed was a 27% Mode 3/a failure which makes sense since I started from the coast where there's little to no coverage below 3000', and flew into an area of good coverage within the first 6 minutes / 20 miles.

Too many variables to answer your question.
Are you affected by the NavWorx A/D?
Do you have a 1090 MHz receiver, or just 978 MHz?
Are you using the Transmon device to get pressure altitude and squawk code?
If you have both receivers, then I'd look at your antenna location.
If you only have 978, then you're dependent on ground stations to send up 1090 traffic. The whole AD mess was over Navwork setting some parameter to 3 (which gets you replies from the ground station) while the FAA saying it should have been "0" (which will generate no replies, so you don't see 1090 traffic). If you are using a Transmon - which needs your transponder to be interrogated - I have no idea how the system reacts if you drop below radar coverage and no nearby airliners interrogate you either. Again, maybe the ground stations drop you. Personally I have never understood how a Transmon type device was ever allowed.

If you are seeing traffic disappear when descending, can you level off or climb to see if traffic is reaquired? Are receiving on 1090 and /or 978? You can only transmit on one, but you can receive on both if your equipment allows.

The received ADSB signal is very line of sight and it is possible your wing or other aircraft structure is blocking the signal.

If your received GPS signal goes wonkey then your position information may not get transmitted and the ADSB ground station will not know where you are and therefore may not transmit near by traffic information.

Thanks for the replies. More details:

• Navworx 200-0012 unit broadcasting SIL=3, AD applies
• TransMonSPE and KT-76A transponder
• UAT only (978 MHz)
• GPS antenna on top, UAT ant. on bottom

Yes this is a Navworx box that's affected by the AD. However my understanding of that issue (admittedly not complete) is that it should still be receiving ADS-B -- UNLESS they're still intentionally not retransmitting to Navworx boxes broadcasting SIL=3 -- are they?

The KT-76A / TransMonSPE is soon to be replaced with a GTX-327. But still the comment makes me wonder, what's wrong with the TransMON? I guess I can search forums to see if there's more on that.

Regarding dropout from ground stations, yeah I suppose if I lost the ground station I'd miss all the 1090 aircraft (but should still get the 978s that are close by, right?) Just weird that I'd lose signal from 3 nearby aircraft all at once -- again, unless they're only broadcasting 1090. I can check with the flight school (whose planes those are all likely to be) to see what their aircraft are equipped with. That would also make sense with regard to the airliners, except that in those cases I'm flying in class C and right in the middle of a high ADS-B coverage area, should be well within line of sight of ground transmitters, but not getting most of the airliners at all.

rv6n6r said:

The KT-76A / TransMonSPE is soon to be replaced with a GTX-327. But still the comment makes me wonder, what's wrong with the TransMON? I guess I can search forums to see if there's more on that.

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This is simply my opinion, nothing more. I don't see how the FAA ever approved the Transmon, because it won't furnish the pressure altitude data if the mode C transponder doesn't squawk. And it won't squawk if it isn't "pinged". For example, my home airport (KLVK) is within the SFO mode C veil. ADSB will be required in 2020, right down to the ground. But LVK is in a valley, and transponders are not pinged by ATC at all until you get 1200' AGL or so. So low flying aircraft, required to have ADSB, won't be transmitting altitude data unless they happen to be pinged by an airliner's TCAS system. I don't think that's the way it's supposed to work. If you're upgrading your transponder, feed the digital signal via hard wire and ditch the Transmon (again, my opinion).

I haven't kept up with this as I'm not affected, but: Doesn't the AD require you to remove the box immediately, or at least turn it off? I know the initial proposal did.

Edit: I looked up the AD. You have until January 11, 2018 to turn it off.

BobTurner said:

I haven't kept up with this as I'm not affected, but: Doesn't the AD require you to remove the box immediately, or at least turn it off? I know the initial proposal did.

Edit: I looked up the AD. You have until January 11, 2018 to turn it off.

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Believe me I'm keenly aware of the AD and the conditions and dates and all the rest. But thanks.

BobTurner said:

I don't see how the FAA ever approved the Transmon, because it won't furnish the pressure altitude data if the mode C transponder doesn't squawk. And it won't squawk if it isn't "pinged".

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Oh I see. Yeah that makes sense. Well good, another reason I'm glad to be finally updating to a modern transpoder!

PS. Know anyone who wants to buy a good used TransMonSPE?

noelf said:

If you are seeing traffic disappear when descending, can you level off or climb to see if traffic is reaquired? Are receiving on 1090 and /or 978? You can only transmit on one, but you can receive on both if your equipment allows.

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I only receive 978, which would explain why I get 3 aircraft dropping out at once, while others are still showing up -- if the lost ones are transmitting 1090ES and the others are 978, and I'm getting below ground station coverage. However it doesn't explain why I can go all the way through PDX Class C well above anything that would obstruct line of sight and not get retransmission of many if not most of the presumably 1090-equipped airliners (or even just retransmit of transponder-equipped a/c) the whole time? I s'pose I could ask them if they're getting a good ADS-B out from me... how would one go about asking that?

The received ADSB signal is very line of sight and it is possible your wing or other aircraft structure is blocking the signal.

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Maybe, but this condition occurs (and persists) when I'm well inside good ADS-B coverage, flying level, high enough that line-of-sight shouldn't be an issue.

If your received GPS signal goes wonkey then your position information may not get transmitted and the ADSB ground station will not know where you are and therefore may not transmit near by traffic information.

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I suppose so but in that case I would expect to see some red marks on my public performance "report card".

Anyway all good input, I appreciate it and other ideas please let me know.

You're aware that ground stations do not send up all traffic - only those within a hockey puck shaped region around you (IIRC 3500' up/down, 15 mile radius (or diameter?)). If you're descending are any of the "disappearing" aircraft moving to more than 3500' above you? Also most displays let you set your own filtering parameters, even tighter than what the ground stations send up, if you want.

OR, maybe the FAA is blocking ground stations up to you (How do they know what equipment you have??) and you're seeing ground station replies intended for other aircraft. As they move out of range the signals stop coming.

G327 transponders are crazy cheap on the used market right now - like $400. People going to mode S-ES have flooded the market with 327s. Of course it will cost half that much more to have it re-certified.

Just talked to the nearby aero-academy and confirmed that their planes all have GTX-345s which output 1090ES. So that explains why they'd all drop out at once as I fly below (presumably) ground station coverage.

However I'm still mystified why I wouldn't get retransmit of the airliners' positions when flying in what should be very good line-of-sight from ground stations in busy class-C PDX airspace. Tomorrow I'll be making a flight through there, maybe I'll ask ATC how they're reading my own OUT signal, and also get another report card afterwards.

BobTurner said:

You're aware that ground stations do not send up all traffic - only those within a hockey puck shaped region around you (IIRC 3500' up/down, 15 mile radius (or diameter?)). If you're descending are any of the "disappearing" aircraft moving to more than 3500' above you? Also most displays let you set your own filtering parameters, even tighter than what the ground stations send up, if you want.

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I've had some targets called out by ATC (so they're on radar) and I can see out my window and identify the airline by the paint scheme, and nothing on my screen.

OR, maybe the FAA is blocking ground stations up to you (How do they know what equipment you have??) and you're seeing ground station replies intended for other aircraft. As they move out of range the signals stop coming.

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I don't remember for sure how but I know for at least a while FAA stopped retransmitting to these boxes (see other posts re. Navworx). But I assumed (probably dumb, I know) that they would have started transmitting again once the AD came out.

G327 transponders are crazy cheap on the used market right now - like $400. People going to mode S-ES have flooded the market with 327s. Of course it will cost half that much more to have it re-certified.

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Yeah, I just got a used one. My KT-76A is old and tired and costs extra $$ to re-tune every time I get my txpdr check anyway, so it's definitely worth it for me to replace and pay for an IFR check which I'm due for anyhow.

One of the menu setup items in my Appareo Stratus ESG ADS-B Out transponder is "Select the ADS-B-In capability of the aircraft, installed or portable." The choices are:
1. UAT
2. 1090 ES
3. UAT and 1090 ES
4. None
I assume that a ground station will not send traffic info to an aircraft that does not have ADS-B In. And I assume that the ground station will not transmit traffic info if your aircraft can receive that info directly from other traffic.

Mich48041 said:

One of the menu setup items in my Appareo Stratus ESG ADS-B Out transponder is "Select the ADS-B-In capability of the aircraft, installed or portable." The choices are:
1. UAT
2. 1090 ES
3. UAT and 1090 ES
4. None
I assume that a ground station will not send traffic info to an aircraft that does not have ADS-B In. And I assume that the ground station will not transmit traffic info if your aircraft can receive that info directly from other traffic.

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My Navworx box is ADS-B In/Out, so there are no such choices with this device. Interesting comment however about the ADS-B in, or rather UAT in, in my case. Makes sense that I wouldn't get rebroadcast of those since I can sense directly but I don't know? Anyway on my latest flight over PDX the results were consistent with what I'd expect based on this discussion, that is some targets (presumably transmitting 1090ES) drop out when I'm not line-of sight with ground stations, while others (presumably UAT) come in more reliably since I'm receiving those directly.

Now I wish I'd held out for a dual band receiver (and never mind the other issues with Navworx). Oh well, it's still a huge improvement over what I used to have for traffic display, i.e. nothing.

Thanks for the responses, this has been very helpful.