Hi,
In answer to your question about TCAS 'seeing' Mode A replies.
All interrogators operate at 1030 MHz and all replies are at 1090 Mhz.
So TCAS will receive the Mode A reply but will not be able to determine
range from it as the reply is not syncronised to a TCAS interrogation.
Most replies of this type are rejected by the receiving processor in a
section known as the de-fruiter (FRUIT= False Replies Unsynchronised In Time).
The TCAD systems measure the amplitude of all replies regardless of their mode and use that for the determination of range (all transponders having a standard output power). Signal amplitude decays with range in a logarithmic manner. TCAD makes a lot assumptions regarding a clear path to the receiving antenna.
I hope that is of some help.
NTB