A Low Cost Remote Sensing System Using PC and Stereo Equipment
  A system using a personal computer, speaker, and a microphone is used to detect objects, and make crude measurements using a carrier modulated by a pseudorandom noise (PN) code. This system can be constructed using a personal computer and audio equipment commonly found in the laboratory or at home, or more sophisticated equipment that can be purchased at reasonable cost. We demonstrate its value as an instructional tool for teaching concepts of remote sensing and digital signal processing. 
Joel F. Campbell, Michael A. Flood, Narasimha S. Prasad, Wade D.   Hodson
physics.ed-ph physics.ins-det