What does Xponder Reply mean in an ADS-B world?

[aldenbj]

I am in the early stages of test flights in my new Skyview equipped RV-12.

I became concerned to see the transponder "Reply" indication apparently "stuck" on... both on the Skyview display as well as the User Data Log. I am accustomed to the "I am not alone" feeling seeing the reply periodically flash indicating that I am in a RADAR coverage area.

Some preliminary research indicated that the transponder might simply be doing its thing in the ASD-B world by Broadcasting my presence. Is this correct?

I noticed in the User Data Log that the Reply is briefly somewhat intermittent (i.e. sometimes 1 sometimes 0) during the transition from Ground to Airborne. Is there a way to tell if I am in a RADAR coverage area?

Thanks,
Alden Johnson

 

[jlakins]

Automatic Dependent Surveillance – Broadcast
•Automatic: it works in the background
•Dependent: depends on other aircraft being equipped
•Surveillance: it’s a technology to track aircraft
•Broadcast: each aircraft broadcasts its position and velocity

Aircraft send latitude, longitude, GPS altitude, pressure altitude, ground track, ground speed, ICAO identifier, N-[ch8208]number, ADS-B In status, and other data once per second

 

[Dynon101]

This is a gee wizz question for those who are very familiar with this technology.

In a typical VHF transmitter only one radio can transmit at a time otherwise the carrier superposed with other radio waves and therefore a receiver gets garbled when two or more radios are transmitting.

When these digital transponders broadcast, it is probable that many airplanes would be broadcasting at the same time so how is it that the system transmission does not get garbled when multiple airplanes are transmitting?

Thanks in advance...

 

[Dynon]

Any radio system that has multiple talkers and listeners rely on some algorithm to schedule or coordinate who talks and responds. This can be accomplished a number of ways, via schemes that break frequencies into different smaller channels to avoid overlap on the same exact radio frequencies, or by scheduling who talks when on a single frequency and having everyone taking turns. This is VAST oversimplification. Here's a very short discussion which sort of alludes to how complicated it is: https://aviation.stackexchange.com/questions/8569/how-does-atc-radar-ads-b-ground-stations-not-get-confused-with-signals-sent-from

 

[Dynon101]

Thanks... now I don't feel so bad that I didn't know it.

 

[Dynon]

I am in the early stages of test flights in my new Skyview equipped RV-12.

I became concerned to see the transponder "Reply" indication apparently "stuck" on... both on the Skyview display as well as the User Data Log.  I am accustomed to the "I am not alone" feeling seeing the reply periodically flash indicating that I am in a RADAR coverage area.

Some preliminary research indicated that the transponder might simply be doing its thing in the ASD-B world by Broadcasting my presence.  Is this correct?

The reply light goes off whenever you're being interrogated by a radar, or another aircraft, or a TCAS, and so on. So if you're in good radar coverage and/or around a lot of other aircraft, seeing the reply light more or less on is normal. The ADS-B pulses are actually automatic, ie they're not responses, they're sent every second no matter what. So they don't affect the reply status or indication.