The TCAS computer gets radio altitude from radio altimeter 1 and 2 transceivers although only one input is necessary for TCAS operation. The data is used by the TCAS computer to calculate some sensitivity levels.
At approximately 1700 feet, the TCAS computer uses radio altitude together with barometric altitude to determine intruders that are on the ground and therefore no threat to the TCAS airplane.
At 1000 feet radio altitude, the TCAs computer inhibits
resolution advisories and TA ONLY will show on the NDs.