Altimeter
From Memory Alpha, the free Star Trek reference
An altimeter is an instrument used to measure the altitude to a fixed level. A radar altimeter makes this determination by measuring the distance between the craft in flight and ground directly below it.
In 2254, the Talosians scanned the library computer banks, including a diagram of the Ranger 5 United States of America space probe. Among the equipment noted on the diagram was the probe's radar altimeter. (TOS: "The Cage")
