Just as a pleasant request
Please select mode C or A for that matter on being cleared onto the runway.
With TCAS mode S has on ground mode which comunicates with other mode S transponders so that all the bells and whistles don't go off.
Mode C and A itself doesn't and if you turn it on while sitting at the hold point it triggers the TCAS on aircraft fitted on approach. If we are below 1100ft it will be a "traffic traffic" and above that possibly a Resolution advisory.
It gives you a bit of a jump and pulls your attention away from the approach.
Even if it does go off while you are lining up its doing its job warning us that there is a chance of collision. Also as well some say to turn it off in the circuit. Please don't it has allowed me to avoid situations occuring with light aircraft that could have got quite close if the TCAS hadn't triggered alerting us to previously unseen traffic.
Anyway this isn't a rant just a request with reasons for that request.
Also There are some airports with ground radar that needs mode A to work, off course in this situation please comply with local regulations.