Snorkel: mixed blessing?

[Jay]

It's Oct., 1943, in the Mediterranean. Allied aircraft are making surface travel (even at night) increasingly dangerous. My radar and/or radar detector have given me just enough warning of approaching planes that I 've been able (so far) to avoid serious damage by crash diving.

I've just equipped my VIIB with a snorkel so I can spend more time submerged. However, I'm finding out that when I use the snorkel, enemy planes still can spot me. When this happens, I have no way of detecting their approach. I only find out that planes are attacking me when the S.O. yells "depth charges in the water!"

Is there a way to detect approaching aircraft when using the snorkel, other than continuously sweeping the sky with the observational periscope (something that would be quite tiring)? How do other captains deal with this problem?