In the quest to build drones that can enable companies such as Amazon to make door-to-door deliveries, engineers are racing to overcome a fundamental challenge: helping unmanned, suitcase-sized aircraft see where they're going.

The answer is developing sensors that are smart enough to keep the drones from smacking into buildings, people and anything else that would impede travel — yet small and light enough that the machines can stay aloft.

Startups around the United States, eager for a slice of a market projected by Teal Group to more than double to $11.6 billion by 2023, are responding. Aurora Flight Sciences Corp. is testing echo location, a method that mimics how bats navigate in the dark, while engineers at 3D Robotics Inc. are turning to optic flow sensors, which detect objects by examining video pixels.