An indirect radar echo is caused by a reflection of the main lobe of the radar beam off the observer’s vessel. Which of the following is NOT a characteristic of indirect echoes? （）
1、A frontal thunderstorm is caused by（）.
A frontal thunderstorm is caused by（）.Apronounced local heatingBwind being pushed up a mountainCa warm air mass rising over a cold air massDan increased lapse rate caused by advection of warm surface air
Deviation is caused by（）.Achanges in the earth's magnetic fieldBnearby magnetic land masses or mineral depositsCmagnetic influence inherent to that particular vesselDthe magnetic lines of force not coinciding with the lines of longitude
3、Permanent magnetism is caused by（）.
Permanent magnetism is caused by（）.Aoperation of electrical equipment and generators on board shipBthe earth's magnetic field affecting the s...
4、Deviation in a compass is caused by th
Deviation in a compass is caused by the（）.Avessel's geographic positionBvessel's headingCearth's magnetic fieldDinfluence of the magnetic materials of the vessel
5、A vessel which is being assisted by an
A vessel which is being assisted by an icebreaker would display a flag hoist consisting of the code numeral “4”when the vessel（）.Aspeed begins to dropBstopsCreceives ice damageDbecomes icebound
6、A vessel which is being assisted by an
A vessel which is being assisted by an icebreaker should indicate that she is ready to cast off the towline by（）.Adisplaying code letter Y ...