#1. WOW Switch. TCAS systems are disabled on the ground so they don't respond to aircraft flying overhead. So for you to carry out a ground test you must defeat the WOW or Weight on Wheels System. On older systems it may mean a shim or removal of an undercarriage switch. On newer systems it may mean pulling a PSEU (Prox Sensor Elec Unit) breaker. Again consult the ship's data. #2. Setup Test Set and Cockpit per Procedure. Transponders have lots of functions so all the little knobs and the sequences of the tests are important. Be very sure to set up the test equipment and the aircraft control heads as called out in the procedures. Any little thing can prevent operation.