An electronic navigation aid using radio waves sent out as a narrow beam to detect solid objects around the ship. Any solid object of a reasonable size reflects the beam and is detected on the radar screen as a bright spot. Radar systems for ship navigation operate at wavelengths of 3 and 10 cm. Two systems are usually required for larger ships, one main S-band and one auxiliary X-band. A system consists of a transmitter-modulator, receiver, a rotatable directional antenna (scanner), required power supplies, a master indicator, and remote indicators (when desired). The radar video presentation displays relative motion data in Plan Position Indicator (PPI) form on either a 12- or 16-in. cathode ray tube. The display is a map presentation of an area of 360 deg around the ship.