Don't know how reliable they are, but reading through some old engineering notes....
There will be no display of aircraft on the ground if your own aircraft descends below 1700' Rad Alt Height. "Ground aeroplanes" are those considered to be within +/-380 feet of ground level for Mode C aircraft or those with the vertical status set to "Ground" for Mode S.
I don't know why, though, if you're sitting at the gate and watching the TCAS, you see targets flying all the way to the ground (0' relative altitude).