 The unit may use this information to establish base altitude for use in step 4.The PCAS unit computes range (maximum 6 miles) based on the amplitude of the received signal, the altitude code is decoded, and the signal angle-of-arrival is determined to a resolution of "quadrants" (ahead, behind, left, or right) using a directional antenna array.XRX will recognize interrogations from TCAS, Skywatch, and any other "active" system, military protocols, and Mode S transmissions.