A barometric altimeter, used along with a topographic map, can help to verify one's location. G embedded global positioning/inertial navigation system and HG9550 radar altimeter. Suunto "Vector" was the world's first outdoor watch with ABC (altimeter, barometer and compass) functions. The system monitors an aircraft's height above ground as determined by a radar altimeter. It carried an SA-106 topographic camera, a laser altimeter and Doppler apparatus. For altitude it used the Fug 101 radio altimeter. The missile is guided by a gyroscope-stabilized autopilot in conjunction with a radio altimeter. The radar altimeter was checked and reported in normal condition. Some additional inputs include gyros, visual odometry, tilt sensors, altimeter, and hazard detectors. The aircraft have several upgrades including a radar altimeter, DME, ILS and a rescue hoist. One of the instruments, the Lunar Orbiter Laser Altimeter (LOLA), was developed by Goddard. The aircraft was equipped with an older "drum pointer" altimeter. Modern aircraft use a "sensitive altimeter". A lit altimeter (preferably accompanied with an audible altimeter) is a must.