Sonar pulse is a single-use ability that becomes available to Allied commanders when one of their spies infiltrates a Sovietsubmarine pen and hacks into the Soviet computer navigation systems.
When used, the pulse reveals all Soviet submarines operating in the combat zone for a period of time.
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A special ability of the Allies, a Sonar Pulse will temporarily reveal all enemy Submarines on the map for a few seconds. This can be incredibly useful in planning your naval campaigns and devising countermeasures. The Sonar Pulse is acquired when a Spy enters an enemy Sub Pen. - Red Alert manual(src)
Allied Second World War Arsenal 
