Marine radar is an instrument of near perfect synchronisation. Ranges and bearings are obtained by the fact that each component does a specific job at the correct time. Marine radar consists mainly of 4 units- the transmitter, the aerial or scanner, the receiver and the display unit.
The transmitter sends out short powerful bursts of electromagnetic energy, called pulses, through the scanner at a specific number of times per second, called the Pulse Repetition Frequency (PRF) or Pulse Recurrence Rate (PRR). These pulses travel at the speed of light and when they strike any object (target) in their path, they are reflected back to the scanner as echoes. The receiver processes each echo and causes it to show up visually as a bright spot, called a blip, on the screen of the display unit.The display unit has a circular screen representing on a scale, an actual area around the ship and is called the Plan Position Indicator (PPI) because it gives a bird’s eye view (or plan) of the positions of targets. The distance represented by the radius of the screen is called the range scale in use and this can be varied, by a switch, as desired by the observer.
The underside of the Screen is Coated with a phosphor compound that glows when struck by a stream of electrons and then fades off slowly. This slow fading off, of the spots on the screen, is called the persistence or after-glow of the screen and helps the observer to view the screen at leisure. A thin stream of electrons is made to strike the underside of the screen and move radially across from the centre to the edge of the screen, at a number of times per second equal to the PRF thereby creating one radial line (called the trace) on the screen for every pulse sent out through the scanner. When an echo is received and processed by the receiver. a sudden brightening and fattening of the tracing spot occurs momentarily. Though the tracing spot is moving continuousy during this time, the blip created by it (refered to as the paint) remains stationary and visible due to the persistence of the screen. This is how radar detects the presence ot a target in the vicinity.
Ref : Shipborne Radar & Arpa book by Capt H Subramanian
