Azimuth and Ground Mode
IBTheseus:
I am surprised that you are able to reliably identify other aircraft with your weather radar, but certainly it is true that if you have on-board radar that can paint another aircraft it will be more accurate in relative bearing than the TCAS receiver. TCAS depends on receiving the reply on a multipole antenna and then doing some maths to figure out an approximate bearing. The TCAS antennae are generally on the fuselage and at certain angles the local path distortion from the airframe will make the bearing "off" by a good chunk. That's why TCAS RA's are vertical modes - the accuracy of deciding a turn direction is not good enough.
Captain Cautious:
Mode S has a specified ground mode behaviour - briefly, when on the ground a Mode S transponder ignores Mode A/C interrogations and Mode S All-call interrogations, but still emits acquisition squitters and replies to directed interrogations. Part of the Mode S datalink protocol is to do with on-ground or in-air status. It is an important part of the ground safety picture and a useful defence against runway incursions. Providing your installation knows when it is on the ground or in flight (see later), it will not cause any clutter or other interference. The airports who have spent gazillions of euro installing modern ground radar installations are right to chide you for switching the thing off as you taxi clear of the runway!
Unfortunately there have been different styles of aircraft installations, and different manufacturers implementations, so SOP's have not always caught up with the equipment.
Some installations, like one of the aircraft I fly, have a squat switch input to the tranponders. Our SOP is therefore to just leave the thing switched on all the time. On the ground it is in ground mode (and has a little annunciation that says so) and at wheels up, it goes into airborne mode. On landing it goes back into ground mode. No pilot intervention required!
I have also seen installations where the transponder has a specific GND setting, between SBY and ON. That's for people who don't have the squat switch wiring, so the pilot has to tell it whether you are on the ground or in the air. I suspect that has proven to be too challenging for some pilots, because there have been further proposals in the industry to add features like (1) always respond to ground acquisition and directed messages, even when switched to SBY and (2) if the airspeed is greater than 90 knots go into airborne mode, even if the switch is set to GND, and (3) always go to GND when the ground speed is less than 10 knots, even when the switch is ON. Some or all of these features may have been implemented in some boxes, but off-hand I'm not sure what the current state of play is with the particular vendors.
Since ATC have complained, it sounds like your installation does not respond to ground interrogations when the switch is in the SBY position. If there is no obvious ON GND / IN AIR switch on the control panel, it is likely that your transponder has a squat switch input, and should switch modes automatically. In which case - just leave it on whilst taxying.