grammar - Is this sentence dangled or not?

When traveling by sea, knowing your exact location can mean the difference between life and death.

This is one of the sentences written in Korean high school English textbook. I spent some years in U.S. and had an opportunity to learn English grammar in a few college classes.

What I learned during class was that when we make a participle construction, the subjects of the main clause and the subordinate clause should be the same. If not, it is dangled, therefore we can't use it.

Korean English text books are usually well written, yet it seems to me, in the sentence above, the subjects of each clause are not exactly the same. I want to hear from someone who knows about the grammar rules associated with the participle construction.