KUP 19: Have knowledge of the fundamentals of radar and automatic radar plotting aids (ARPA)

RADAR

RAdio Detection And Ranging

Developed just before WWII

The basic principle behind radar is simple – extremely short bursts of radio energy (travelling at the speed of light) are transmitted, reflected off a target and then returned as an echo. Radar makes use of a phenomenon we have all observed, that of the ECHO PRINCIPLE.

Radar operates by generating a microwave radio frequency pulse and directing it in a narrow beam from a rotating antenna. After the pulse is transmitted, the receiver detects the echoes and amplifies them to produce bright spots or areas on the indicator. The time between the transmitted pulse and echoes is interpreted as distance (range) from own ship. 

As used aboard ships and other surface craft, radar data are displayed on a plan position indicator, a map-like display with its own ship in the center. Other vessels, buoys, and land masses show up as bright spots or areas on a dark background. 

Using a reflection plotter and plotter pencil (similar to a grease pencil), the radar observer can, by successive plots of other vessels, track and calculate the movements of other vessels and assess any risk of collision

ARPA

Automatic Radar Plotting Aid

Developed for marine usage in the 1970’s

Fittings aboard tankers were made mandatory in 1982, or other classes of ships soon after. On an ARPA the radar observer can designate a “target” ship of interest and the ARPA calculates the speed and course of the ship and assesses the risk of collision. Targets may also be “acquired” automatically and tracked. A number of targets can be tracked simultaneously. A “dangerous” target (one having a risk of collision) will be highlighted and an alarm will sound. On any target being tracked, the closest point of approach (CPA) and time to closest point of approach (TCPA) are calculated and displayed.

• Primary items on the radar screen:

Information display on ARPA radars was simple, including ship position, course, speed, bearing, distance, CPA and TCPA. Except for position, course and speed, there were some others in AIS display, such as ship MMSI, name, type and navigational status.

• Fundamental operations of radar and ARPA:

Radar operates by generating a microwave radio frequency pulse and directing it in a narrow beam from a rotating antenna. After the pulse is transmitted, the receiver detects the echoes and amplifies them to produce bright spots or areas on the indicator.

A radar system consists of a transmitter that produces electromagnetic waves, a transmitting and receiving antenna (often the same antenna is used for both), and a receiver and processor to determine the properties of the objects.