Apparatus for determining a displacement characteristic of an object comprises means (Tx1, Tx2, 3, 4) for transmitting, at a given transmission time, a probe signal towards the object; means (5, 6) for receiving the probe signal returned by the object; means (DL11, DL12, PG28, PG30) for generating a detection timing signal at a delay after the transmission time, corresponding to a selected range for the object; first detecting means (Rx7, Rx8), coupled to the receiving means and responsive to the timing signal, for detecting the returned probe signal occurring at the delay of the timing signal; and second detecting means (Rx19, Rx20) for detecting the relative timing of the detection timing signal and of a signal having a predetermined timing with respect to the transmission time; whereby a measure of the range of the object can be determined from the relative timing of these two signals, from which range measure the object displacement characteristic can be determined. Apparatus for transmitting and receiving data is also disclosed, as further is apparatus for assessing the approach of an object to a specified location. Analogous methods are also disclosed.