TCAS---A wonderful Tool in the Cockpit

What is TCAS, in short words ?
TCAS scans the vicinity by interrogating the transponders of other aircraft. It then uses the received transponder signals to compute distance, bearing and altitude relative to the own aircraft.The evaluated traffic information is displayed as symbols on the ND, the Navigation Display. Note that altitude and vertical motion information is only available if the received signal comes from a mode C or mode S transponder. Otherwise the associated symbol on the ND will have no altitude information and no vertical motion arrow.As TCAS checks the other aircraft’s relative distance permanently in short time intervals, it can, therefore, also calculate the other aircraft’s closure rate relative to the own aircraft. The closure rate is the most important variable and indeed a very fail-safe key to a good collision prediction. Complex, trigonometric calculations of flight paths and ground speeds are unnecessary and would only result in unsharp and unreliable extrapolations. Heading or bearing information is not even required to compute a TCAS alarm. When TCAS detects that an aircraft’s distance and closure rate becomes critical, TCAS generates aural and visual annunciations for the pilots.If necessary, it also computes aural and visual pitch commands to resolve a conflict. If the other aircraft uses TCAS II as well, these pitch commands are coordinated with the other aircraft’s pitch commands so that both aircraft don’t escape to the same direction. Even three aircraft can be coordinated.By the way, TCAS provides only vertical guidance, no lateral guidance. TCAS also ignores performance limitations. In other words, when flying at maximum altitudes TCAS may still generate a climb command.