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When receiving TIS-B traffic via ADS-B In, I'm curious what mode-C targets are included.

Is it only mode-C traffic with confirmed altitude in contact with ATC services (presumably with a transponder code)?

Or does it include traffic with unconfirmed altitude that's probably squawking 1200? Does it leave off altitude in that case? How would it know if traffic is within +/- 3,500' of your aicraft to give you your "hockey-puck" of traffic?

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The AIM covers most of the operations and limitations of TIS-B on page 4-5-18

TIS−B is the broadcast of ATC derived traffic information to ADS−B equipped (1090ES or UAT) aircraft from ground radio stations. The source of this traffic information is derived from ground−based air traffic surveillance sensors. TIS−B service will be available throughout the NAS where there are both adequate surveillance coverage from ground sensors and adequate broadcast coverage from ADS−B ground radio stations. The quality level of traffic information provided by TIS−B is dependent upon the number and type of ground sensors available as TIS−B sources and the timeliness of the reported data. (See FIG 4−5−8 and FIG 4−5−9.)

According to the AIM all transponder equipped (A/C/S) aircraft are included

  1. Only transponder−equipped targets (i.e., Mode A/C or Mode S transponders) are transmitted through the ATC ground system architecture. Current radar siting may result in limited radar surveillance coverage at lower 10/12/17 AIM Surveillance Systems 4−5−19 altitudes near some airports, with subsequently limited TIS−B service volume coverage. If there is no radar coverage in a given area, then there will be no TIS−B coverage in that area.

section d. TIS-B Limitations offers some more info

The system is not perfect and unconfirmed altitudes may display as shadows

(b) The ADS−B−to−radar association process within the ground system may at times have difficulty correlating an ADS−B report with corresponding radar returns from the same aircraft. When this happens the pilot may see duplicate traffic symbols (i.e., “TIS−B shadows”) on the cockpit display.

More on transponder requirements

(d) The TIS−B system only uplinks data pertaining to transponder−equipped aircraft. Aircraft without a transponder will not be displayed as TIS−B traffic.


So pilot A in his Cherokee with a mode S transponder will show up but pilot B in his cub with no electronics and only a hand held who may be on radar will not show up.

This technical order covers the topic important phrase

TIS-B is the ADS-B In function that provides for the in-cockpit display of other aircraft (without ADS-B) in the vicinity that are being seen via FAA secondary radar. General observations can be made during the flight to verify that known radar targets are appearing in the correct relative position on the ADS-B display(s) in the flight inspection aircraft. Of course, if there are no such targets-of-opportunity in the area during the flight, the opportunity to check TIS-B in this way will not be possible.

So if you are coming up on secondary radar you will be included in the ADS-B/TIS-B broadcast. ATC receives 1200 codes with altitude (encoder provided) so they will show up, and in my recent experience flying in the northeast most VFR traffic does show up. Aircraft with no transponder will not show up (radar target only). Im not sure what happens in the case of no altitude reporting.

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  • $\begingroup$ @StephenS i believe i have added a bit which directly answers the question. If they are transmitting and being received on SSR they are showing up. $\endgroup$ – Dave Dec 13 '19 at 21:45
  • $\begingroup$ It's hard to prove a negative, but showing them consistently not saying something when they could may be the best evidence available in this case. $\endgroup$ – StephenS Dec 13 '19 at 22:50

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