A method for determining at least one of the distance to and the speed of an object is discussed. The method comprises determining an indication of whether the object is approaching or moving away and generating an interrogation signal comprising a sequence consisting of segments at constant frequency and segments of varying frequency, wherein if the determining step indicates the object is approaching then the varying frequency segments have decreasing frequency and if the determining step indicates that the object is moving away then the varying frequency segments have increasing frequency. The interrogation signal is transmitted and a version of the interrogation signal reflected from the object is detected. At least one of the distance to and speed of the object is then determined using a combination of the interrogation signal and the reflected version of the interrogation signal.