Bearing Not Part of Calculation
ITCZ: "Having first detected a likely bearing and range to a target, the TCAS unit then has to decide, using probability mathematics, which of the reported replies are likely to be aeroplanes. In the same way that VOR's may scallop, and surveillance radars might paint flocks of birds or lorries as targets, not every whisper-shout return the transponder reports to TCAS might be an aeroplane at the calculated range, bearing, and relative altitude..."
Those words could be misinterpreted. Relative bearing is not part of the collision avoidance calculation. Range, rate of closure, and altitude difference are used to calculate the CPA, closest point of approach. Bearing is used only to provide a display to the pilot to aid in acquiring traffic visually, and it lacks accuracy, for lots of reasons. Plus or minus 15 degrees is the certification standard in the forward sector. There is no requirement for bearing accuracy behind.
GB