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'academic' ADSB UAT question

R

rv7charlie

Well Known Member
An academic question for you ADSB experts:

If a plane has one of the UAT units that 'sniff' the plane's existing mode C transponder output to transmit ADSB traffic info to the world, What happens when the plane isn't being painted by a radar? Does the plane become invisible to ADSB receivers in the area that would otherwise see its ADSB data?

Hope the question makes sense; if not, I'll try to rephrase the question.

Charlie
 
my experience

If you are in a non radar environment, your "sniffer" UAT will show an altitude of zero. On my summer trip to Alaska with two other RVs this summer, they could "see" me all the time,
but the altitude was zero whenever we were not in radar.
 
I have asked the same question. The FARs say adsb should send pressure altitude out. I don?t see any exceptions. And since the FAA?s long term goal is to get rid of radar, what then of ?sniffers??
 
More than once I have seen aircraft with my eyes before they showed up on ADS-B.

Then a few minutes later they magically appear on the SkyView.

ADS-B isn't the panacea safety improvement the FAA thinks it will be.

That said, Saturday morning, it saved my life when some idiot in a C182 did a straight in approach and I came within 100 feet of him and never saw him. (We "chatted" on the radio, so I know it wasn't a shadow.)
 
I believe the UAT will revert to GPS altitude when not in a radar environment. And the latest additional TSO for uAvionix wing tip units contain a baro sensor so it is not reliant on the transponder. The FAA is well aware that accurate baro altitude is not available in some cases when the transponder is not replying.
So far, this has been an acceptable solution. By the way, don't hold your breath for radar to go away.
 
I doubt that GPS altitude will be acceptable to the FAA at this time due to the possible large differences in pressure altitude currently in use.

However, I can foresee a day when we use GPS altitude and GPS track instead of pressure altitude and heading.
 
Sounds like Tom has empirical data. :) Sounds like other a/c in the area will get a target, but without altitude data; not unlike a non-mode C a/c would appear to a radar: target with ID, but no altitude. Sound right?
 
Trying to delete this response.
I now know that an ADS B-Out "transponder" transmits by itself (uninterrogated) every second.

Finn
 
Last edited:
rv7charlie said:
An academic question for you ADSB experts:

If a plane has one of the UAT units that 'sniff' the plane's existing mode C transponder output to transmit ADSB traffic info to the world, What happens when the plane isn't being painted by a radar? Does the plane become invisible to ADSB receivers in the area that would otherwise see its ADSB data?

Hope the question makes sense; if not, I'll try to rephrase the question.

Charlie
Click to expand...

Hello Charlie,

This question makes perfect sense.

You shouldn't see any approved ADS-B Out devices that operate this way. It wouldn't be acceptable for the altitude/squawk code data to get "old" or go missing.

As others have said, the Garmin GDL 82 uses a patented means to interrogate the transponder, when necessary, so that the ADS-B Out data is just as good when the aircraft is out of any radar coverage area and the transponder is not being interrogated.

Thanks,
Steve
 
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