After watching Rocky gleefully play with a measuring tape, seemingly unaware of the flat markings on it (and therefore missing the scientific significance of the tool), Ryland realizes that Eridians must “see” with sound 11.051.

Echolocation is a biological form of active sensing in which animals emit sounds and then use the echoes of those sounds to discern properties of the environment, such as the distance of other organisms or objects. Ryland recalls that bats and dolphins use echolocation, but it is also used by all modern toothed whales, some shrews, and a handful of bird species including the oilbird. Even some humans are skilled echolocators.

What sets Rocky apart from these prior examples is that he and his fellow Eridians seem to use a form of passive sensing: “They use ambient sound waves to resolve their environment instead of making a specific noise to track prey.” 11.052 Whereas classic echolocation is most comparable to human-made active sonar, the emitted signals of which can be detrimental to marine life, Rocky’s sensing is closer to the passive sonar used by many scientific missions as well as vessels that wish to avoid detection. Rocky later explains that he hears sound using tiny receptors all over his body that “report back to brain. Like touch.” 14.202

NASA has created a story about a character called Echo the Bat which compares the way bats sense their environments to different examples of active and passive sensing satellites.