TCAS works by plotting successive responses to interrogation to determine a track and a potential threat and estimates a closet point of approach. Altitude information is key in that solution, hence only TA when no Mode C/S. It is as I said about relativity and co-operation.
Regarding TSOs:
Extended Squitter Automatic Dependent Surveillance - Broadcast (ADS-B) and Traffic Information Service - Broadcast (TIS-B) Equipment Operating on the Radio Frequency of 1090 Megahertz (MHz)
TSO-C166b PURPOSE. This technical standard order (TSO) is for manufacturers applying for a TSO authorization (TSOA) or letter of design approval (LODA). In it, we (the Federal Aviation Administration, or FAA) tell you what minimum performance standards (MPS) your 1090 MHz ADS-B and TIS-B equipment must first meet for approval and identification with the applicable TSO marking.
Manufacturers means Eqpt manufacturers such as Honeywell or Garmin, they can get eqpt assessed as meeting the TSO and it then is a commercially viable product. Aircraft designers, or other DOs then use this eqpt to install the systems in the aircraft they design and use the credit from the TSO rather than starting from scratch but they must get Certification (Airworthiness Approval).
From AC 20-165 Airworthiness Approval of Automatic Dependent Surveillance - Broadcast (ADS-B) Out Systems - 2-1. ADS-B System Approval Process.
a. This AC addresses the initial airworthiness approval through the type certification or supplemental type certification process of an ADS-B Out system that meets the equipment requirements of 14 CFR § 91.227.
c. ADS-B Out System Components. The ADS-B system is depicted in figure 1 and includes the ADS-B equipment, a position source, a barometric altitude source, an air-ground status source, a TCAS II source if the aircraft is equipped with TCAS II, an optional heading source, and all associated antennas and displays.
Para 3.7 (b) ADS-B Function Failure. The ADS-B system depends on a position source to provide the data to populate the ADS-B messages and reports. This position source or interface may fail and prevent the system from providing pertinent information to the ADS-B equipment. In this case, the ADS-B system cannot function, but there is not a failure of the ADS-B equipment. TSO-C166b and TSO-C154c require this condition to be annunciated. The ADS-B system should indicate this position source or interface failure independently of the ADS-B equipment failure annunciation. The flight manual must describe the means to interpret the difference between the device failure and function failure annunciations if the annunciations are not unique. The ADS-B function failure must not cause a TCAS II system failure.
As you recognise the intent of hybrid surveillance is to reduce the TCAS interrogation rate on 1030 Mhz, observed above as a problem with many gliders, through the judicious use of the ADS-B Out data provided via the Mode S extended squitter to discriminate no potential threat aircraft, without any degradation of the safety and effectiveness of the TCAS.
So degraded ADS-B out should not impact upon TCAS II operation as the fall back is the active interrogation it just means there is less automatic discrimination and greater number of interrogations
There are loads of resources if you look.
https://www.faa.gov/documentLibrary/...AC_20-151C.pdf
https://www.faa.gov/documentLibrary/...%20booklet.pdf
https://sesarju.eu/sites/default/fil...rveillance.pdf