Questions for avionics techies

[Bennett]

MOVED TO AVIONICS FORUM FOR RELEVANCY 

 

I have been extolling the virtues of traffic on ADS-B in/out, but I do have some technical questions that I am sure some of gurus on this forum can answer.

One: I assume that all participating "out" aircraft report their altitude via the blind encoders (29.92") as opposed to their own barometric altitude, and that ATC corrects the encoder altitude by their local barometric models. If this is the case, then all participating aircraft can believe the relative altitude differences. Is this true? If true, then aircraft that have set their barometric altitude incorrectly (below 18,000') are not a problem as to relative altitude.

Two: The GTN 650/750 units have the ability to set their map orientation to Heading, Track, or North. I prefer Heading. Suppose I am showing a Heading of 300 degrees (magnetic), and the strong winds give me a track of 320 degrees. The traffic display on my GTN 750 Traffic page or overlay shows an aircraft 4 miles away, at the same altitude, and about 20 degrees off the nose to the left. I assume that the ADS-B in/out reports the aircraft's track in magnetic degrees, and my magnetic track to him. Where should I be looking to spot that other aircraft? Would I be better off using Track up for the GTNs for traffic? Does it make a difference if I select Relative to Absolute on the Traffic page as to his azimuth relative to me?

Three: Is it true that NOAA and National Weather report wind directions in True, and not magnetic? What about the wind barbs in NextGen via ADS-B?

Thank you for the replies.

[tony]

you will report your GPS altitude not your pressure altitude,

[smccray]

MOVED TO AVIONICS FORUM FOR RELEVANCY 

 

One: I assume that all participating "out" aircraft report their altitude via the blind encoders (29.92") as opposed to their own barometric altitude, and that ATC corrects the encoder altitude by their local barometric models. If this is the case, then all participating aircraft can believe the relative altitude differences. Is this true? If true, then aircraft that have set their barometric altitude incorrectly (below 18,000') are not a problem as to relative.

I asked a similar question to Garmin specifically about the GDL39. Here is the response:

ADS-B Out equipment transmit both pressure and geometric (GPS) altitudes. The relative altitude of these ‘Out’ targets can be computed by the 39 from the geometric altitude. Traffic uplinked from GBTs do not have geometric altitude. These targets will not have relative altitude but should show up on the traffic page (assuming you have TIS-B uplink from a GBT).

Also, the GDL 39 has an internal pressure setting for determining its own altitude. The pilot can tell the GDL 39 if it is being used in a pressurized aircraft. Setting the Aircraft Type to ‘Pressurized’ will tell the 39 to not use the pressure sensor reading. This will disable the display of relative altitude and traffic alerting. If the Aircraft Type is set to Not Pressurized’ and the GDL39 is used in a pressurized aircraft, the relative altitude will be inaccurate.