Related Clues:
- Submarine tracker
- Speed checker
- Sub detector
- Underwater detection system
- Submarine-detecting system
- Salvager's equipment
- Air-traffic controller's device
- Salvaging aid, perhaps
- Deep-sea fishing aid
- Echolocation method
- Bass boat feature, sometimes
- Scientific breakthrough of the '30s
- Sub locator
- Plane-tracking device
- Pitching coach's aid