TCAS & ADS-B question

[MachFly]

I'm trying to figure out if ADS-B can receive information from other transponders that were just interrogated by a TCAS?
In other words say there are three aircraft in the area, one with an ADS-B, one with a mode C transponder (and nothing else), and another with a TCAS. When the TCAS interrogates the aircraft with a mode C, would the aircraft with the ADS-B receive the location of the aircraft with the mode C?

I was able to find some stuff on this online but none of those sources there reliable. I also don't really want to go out and test this as it might take a long time for me to find those two aircraft in the same area that is not covered by a ground radar.


Thanks

 

[Piloto]

On the scenario you describe the mode C and the ADS-B out aircraft both will reply to the TCAS mode C interrogation. An aircraft with ADS-B in only will not acknowledge a Mode C only aircraft. However a mode C only TCAS system will detect both. ADS-B transponders are required to have mode C reply capability also.

José

 

[MachFly]

On the scenario you describe the mode C and the ADS-B out aircraft both will reply to the TCAS mode C interrogation. An aircraft with ADS-B in only will not acknowledge a Mode C only aircraft. However a mode C only TCAS system will detect both. ADS-B transponders are required to have mode C capability also.

José

Right, but when TCAS interrogates a mode C, the mode C sends out a signal with it's location. Will the ADS-B nearby be able to receive that signal and know that aircraft's position as well?

 

[Piloto]

Right, but when TCAS interrogates a mode C, the mode C sends out a signal with it's location. Will the ADS-B nearby be able to receive that signal and know that aircraft's position as well?

The mode C only aircraft only sends out baro altitude information. An ADS-B in only receiver can not determine relative position because there is no LAT/LONG data transmitted by the mode C only transponder.

José

 

[MachFly]

As far as I know TCAS equipped aircraft generally have more than one receivers so they can pinpoint the location of the transponder, kinda like human ears. Can TCAS equipped aircraft transmit the location of the traffic?

 

[Piloto]

Typical TCAS systems employs directional antennas connected to a processor that contains 4 channels receivers, thus the four cables between the antenna and the processor. The relative phase/amplitude of each channel is compared against each other for bearing determination. However the Monroy Traffic-Watch employs a modern digital phase processing technique that only requires one antenna cable.

TCAS processors do not transmit aircraft location. Only the transponders can do that.

José

 

[Jim Logajan]

I know that the Zaon PCAS units estimate distance based on the strength of the signal. The absolute number isn't as much value as the trending information (a shrinking number gets your attention more than one that is staying the same or increasing.)

 

[Bob Noel]

to pick a few nits, the "Mode C" transponder will respond to a Mode A interrogation with the 4 digit octal code. It will respond to a Mode C interrogation with the baro altitude. The interrogator has to use the timing and direction of the separate Mode A and C replies to correlate the transponder code and baro altitude and ultimately to determine range and relative bearing of the aircraft.

 

[poadeleted20]

In other words say there are three aircraft in the area, one with an ADS-B [#1], one with a mode C transponder (and nothing else) [#2], and another with a TCAS [#3]. When the TCAS interrogates the aircraft with a mode C, would the aircraft with the ADS-B receive the location of the aircraft with the mode C?

To answer the original question directly, airborne ADS-B equipment cannot not locate other aircraft directly from the other aircraft's Mode 3A/C transponder no matter how that other aircraft's transponder is interrogated, either by ground SSR or airborne TCAS. So, in the posited situation, the ADS-B system in aircraft #1 will not get any information from the "standard" transponder on aircraft #2 being interrogated by the TCAS in aircraft #3. The only way for #1 to know where #2 is would be for ground-based radar to locate #2 and then datalink that information up via ADS-B, and the use of TCAS by #3 is irrelevant.

 

[flyingron]

As far as I know TCAS equipped aircraft generally have more than one receivers so they can pinpoint the location of the transponder, kinda like human ears. Can TCAS equipped aircraft transmit the location of the traffic?

It's actually one receiver multiplexing multiple antennas.

 

[MachFly]

Thanks for explaining.