Radars can be classified as ground based, airborne, spaceborne, or ship based radar systems. They can also be classified into numerous categories based on the specific radar characteristics, such as the frequency band, antenna type, and waveforms utilized. Phased array radars utilize phased array antennas, and are often called multifunction radars. Radars are most often classified by the types of waveforms they use, or by their operating frequency. Considering the waveforms first; radars can be Continuous Wave (CW) or Pulsed Radars (PR). Radars use Doppler frequency to extract target radial velocity, as well as to distinguish between moving and stationary targets or objects such as clutter. In order to accurately predict radar performance, one must account for the earth's shape and atmosphere on the radar signals. Radar electromagnetic waves travel in free space without suffering any energy loss. However, due to gases and water vapor present along the radar wave propagation path, a loss in radar energy occurs.