Garmin GTS 820 TAS Pilot's Manual page 12

TIS-B coverage is available when the aircraft is within ground station coverage,
in Secondary Surveillance Radar coverage, and the other aircraft is in secondary
surveillance radar coverage and transmitting altitude.
ATC radar tracks for other aircraft within ±3,500 feet and 15 NM of the
participant is provided by the ground station, to include altitude, position,
ground speed, and ground track. TIS-B data is broadcast once every three to
thirteen seconds depending on the characteristics of the FAA radar ground
station.
The GTS is capable of active surveillance and will directly track most
targets provided via TIS-B. If the GTS is not able to track a target using active
surveillance (e.g. it is at a range too far to track reliably), it can still be displayed
if provided via TIS-B.
NOTE: Even if the display is showing TIS-B traffic, unless the TIS-B
participant indication is present (see Table 1) TIS-B services are not being
provided specific to your aircraft.
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Figure 3 TIS-B Operation - FAA Illustration
Garmin GTS Traffic Systems 190-00587-02 Rev. E