NXT-800 ATC Transponder

From  AOPA  : WHAT YOU NEED TO KNOW ABOUT ADS-B
  
Automatic Dependent Surveillance-Broadcast is a primary technology supporting the FAA’s Next Generation Air Transportation System, or NextGen, which shifts aircraft separation and air traffic control from ground-based radar to satellite-derived positions. ADS-B Out broadcasts an aircraft’s WAAS-enhanced GPS position to the ground, where it is displayed to air traffic controllers. It’s also transmitted to aircraft with ADS-B receivers, either directly or relayed by ground stations, increasing the pilot’s situational awareness.
In the continental United States, ADS-B Out has been required since January 2, 2020, for flight in:

• Class A, B, and C airspace;
• Class E airspace at or above 10,000 feet msl, excluding airspace at and below 2,500 feet agl;
• Within 30 nautical miles of a Class B primary airport (the Mode C veil);
• Above the ceiling and within the lateral boundaries of Class B or Class C airspace up to 10,000 feet (note that ADS-B is not required below a Class B or Class C airspace shelf, if it is outside of a Mode C veil);
• Class E airspace over the Gulf of Mexico, at and above 3,000 feet msl, within 12 nm of the U.S. coast.

One piece of equipment that fulfills the ADS-B requirements is the NXT-800 Transponder.
  
     
  
ADS-B ATC Transponders require a valid GPS position input. On some aircraft I've seen, a separate GPS receiver (usually feeding the Flight Management System (FMS) can be used to input position information to the transponders.

The NXT-800 manufactured by ACSS  use a standalone GPS receiver NXG-900.

We recently had a system that was showing as failed when it was selected on the cockpit control panel. The front of the transponder showed a "ADS-B" fault light on the front of the unit.
  
     
  
The system manual states that this failure is related to the GPS position input. We replaced the NXG-900 in the cabin ceiling which corrected the problem.
  
     
  
We resistance checked the antenna while we were there. Our reading was 27M Ohms. This value needs to be verified on another aircraft before we can state that this is proper for this system.