Gertrude

Would you give me the meaning of "gertrude" used in the following sentence from a novel?
"They were lashing her pretty hard with active sonar, then a 'can queried her on a gertrude."
(Hunt for Red October)
context: Russian vessels are lashing a submarine hard with active sonar and their another vessel
is questioning her on a gertrude.
my guess: f rom the above, i figure gertrude is a communication devise, but no definition fitting in my dictionaries.
I appreciate your help in advance.