What is the meaning of "aruga aruga'

In Ben Aaronovitch's Foxglove Summer, the main character exclaims this (mentally) upon thinking he's discovered an important clue in an interview. What is the meaning of the phrase?

I felt her mother practically start out of her chair, and then subside again.
'What did it feel like?' I asked.
'Like a big tongue,' said Gaby.
'And was it low down or high up?'
Gaby jumped off her chair and demonstrated by sticking her arm straight out in front of her with her palm turned up. About a metre twenty above floor level, by the way she held her hand, suggested an animal of some kind.
'What kind of animal is Princess Luna?' I asked.
'She's a pony, silly,' said Gaby brightly.
A little klaxon went off in my head.
Aruga aruga, I thought. Set condition one throughout the ship.

Solution 1:

It's an onomatopoea for the sound of an alarm (klaxon). "Aruga," "A-ooga," "Awooga" or other spellings are all ways a writer might represent the sound; onomatopoetic words often don't have a standardized spelling.