Garmin GTS 820 Pilot's Manual page 22

What are the ramifications if GPS data is not available?
Own ship latitude and longitude are not known. Active surveillance still
determines target distance, bearing, and altitude (if an altitude reporter). Passive
surveillance is disabled if GPS position of own aircraft is not available. TAS/TCAS
and traffic alerting remain operational.
Is a Mode C or S transponder required for the GTS 8XX Series
installation?
Not for the GTS 800 TAS low power product, but the absence of an ADS-B
link transmit class of equipment will disallow the ADS-B receive functionality.
ADS-B requires broadcast capability which comes from ADS-B link transmit class
of equipment.
The higher power products, GTS 820 TAS and GTS 850 TCAS I, require
a Mode S transponder that is capable of obtaining the NTA count (number of
TCAS II-equipped aircraft in the active surveillance area). This is due to an inter-
ference limiting requirement.
Does the GTS 8XX Series require a Garmin Extended Squitter
Mode S transponder to support all functions?
The GTS 800 requires a Garmin Extended Squitter Mode S transponder or
other complementary ADS-B link transmit class of equipment for ADS-B opera-
tion.
The GTS 820 TAS and GTS 850 TCAS I require a Garmin Extended Squitter
Mode S transponder for ADS-B operation.
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Garmin GTS 8XX Series Traffic Advisory System 190-00587-02 Rev. A