This document represents a significant improvement to existing Selective Calling (SELCAL) standards used with HF and VHF voice communications. The goal is to eliminate the possibility that two or more aircraft could respond to the same radio call. The Mark 4 SELCAL system definition provides the following: " Guidance for integrated and federated audio systems " 32 audio tones total (16+16) yielding up to 215,760 unique codes " SELCAL form, fit, & function definition " Interwiring and installation details " Defines SELCAL decoding requirements in the presence of significant signal degradation