What does ALTM stand for?

Airborne Laser Terrain Mappers

Alternative Postal Delivery (formerly Alternate Marketing Networks)

Altimeter

An instrument used to measure the height above a reference point, such as ground or sea level. An instrument, most usually an aneroid barometer, calibrated in meters and/or feet, to indicate an aircraft's height. A device that measures altitude above the surface of a planet or moon. Spacecraft altimeters work by timing the round trip of radio signals bounced off the surface.