The Rockwell Collins TPR-901 ATC/Mode S Transponder is part of the TCAS II Traffic Alert and Collision Avoidance System. The purpose of the TCAS II display and indicate the presence of nearby airborne traffic and advice of maneuvers. The surveillance is completed by computing the closest approach using the range, altitude, bearing and closure rate of other transponder equipped aircraft. The TPR-901 supplies a response to all Air Traffic Control Radar Beacon System and mode S interrogators. The TPR-901 uses two antennas for diversity operations which improve the air to air surveillance of the TCAS. The TPR-901 consists of four principal assemblies; transmitter, receiver, processor and power supply. The TPR-901 includes on board maintenance system compatibility with broadcast menu mode systems and data link capabilities.

The dimension for the TPR-901 are 7.62 inches height, 5.06 inches wide, 15.26 length and the weigh is 13.8 pounds. The power requirement for the TPR-901 is 115 Vac, 400 Hz, 45 Va.