Apparatus, methods, and computer program products for detecting or locating a viable data carrier in a downstream channel by a cable modem. The channel is formed between an aerodynamic channel ring supported in a smooth annular recess formed in a compressor housing and the separate inlet member. The method includes determining: a difference between the first time and the first corrected time; a first round trip time comprising a difference between the first and second times; and a second time correction amount comprising a sum of the difference between the first time and the first corrected time and half of the first round trip time.