The blanking out or obscuring of weak radio signals by a stronger signal

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A flag, sound, light, or radio signal meaning a vessel is threatened by grave and imminent danger and requires immediate assistance.


A radio signal (GMDSS) from a distressed vessel automatically directed to a RCC giving position, identification, course and speed of the vessel as well as the nature of distress


Any form of radar which detects radial motion of a distant object relative to a radar apparatus by means of the change of the radio frequency of the echo signal due to motion.


A radio transmitting station which emits a distinctive or characteristic signal so a navigator can determine the direction of the source using a radio direction finder, providing a line of posi- tion. The most common type of marine radiobeacon transmits radio waves of approximately uniform strength in all directions. These omnidirectional beacons are called circular radiobeacons. A radio- beacon some or all of the emissions of which are directional so that the signal characteristic changes according to the vessel’s bearing from the beacon is called a directional radiobeacon. A radiobeacon all or part of the emissions of which is concentrated in a beam which rotates is called a rotating radiobeacon.


Coarse Acquisition - the radio signal on the L band frequency of 1575.42 MHz that civilian GPS receivers use. As opposed to the P code used by the US military.


Radio reception in which the incoming radio-frequency signal is combined with a locally generated rf signal of different frequency, followed by detection. the center of the exterior surface to permit tightening with a spanner.


The region in space within which the difference in amplitude of two radio signals (usually emitted by a signal station) is indistinguishable


  1. Radar which transmits a SIGNAL and receives the incident energy reflected from an object to detect the object. 2. As defined by the International Telecommunications Union (ITU), a radiodetermination system based on the comparison of reference signals with radio signals reflected from a position to be deter- mined.


An attended light station or lightship emitting simultaneous radio and sound signals as a means of determining distance from the source of sound, by measuring the difference in the time of reception of the signals. The sound may be transmitted through either air or water or both and either from the same location as the radio signal or a location remote from it. Very few remain in use.


A visual time signal in the form of a ball. Before the widespread use of radio time signals, time balls were dropped, usually at local noon, from conspicuously-located masts in various ports. The accuracy of the signal was usually controlled by a telegraphic time signal from an observatory.

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