Radio Altimeter Altitude Limit Selector Switch, SA-1/ARN-1 for AN/APN-1
The WWII-era Radio Altimeter Altitude Limit Selector Switch, SA-1/ARN-1 was a component of Radio Set AN/APN-1, used to measure and indicate to the pilot the altitude over the terrain during flight. See image of spec sheet.
The Radio Altimeter was used in a number of aircraft of the US Navy where close monitoring of height over terrain was critical, such as torpedo bomber TBM Avenger (see image from TBM Flight Manual), dive bomber SB2C Helldiver, patrol aircraft PV-1 Ventura, PV-2 Harpoon, PBY Catalina, PBM-5 Mariner, and PB4Y Privateer. It was also common in aircraft of the US Army Air Force such as the C-47 Skytrain transport, as well as Korean War-era aircraft such as the A-1 Skyraider.
According to the description contained in the Graphic Survey of Radar and Radio Equipment, Section Four, Radar Navigation Equipment, dated 1945 (see document under Downloads):
"Radio Set *AN/APN-1 is an airborne frequency modulated radar altimeter designed for installation in aircraft to provide direct indication of altitude above terrain during flight. This equipment is designed to emit, in a downward direction from the transmitter antenna, a radio freguency carrier which is frequency modulated at a rate of 120 cycles per second between the approximate limits of 420 and 460 mc on the low range (0-400 feet) and 443 to 447 mc on the high range (400-4000 feet). The earth's surface reflects some of this radiated carrier, and the reflected signal is received on a separate receiver antenna.
During the time interval required for the signal to travel to earth and return to the aircraft, the transmitter frequency will have changed. The combination of the received signal with a signal obtained directly from the transmitter will result, by process of detection, in an audio frequency signal the average frequency of which is proportional to the altitude of the aircraft above the ground."
The Altitude Selector Switch allowed the pilot to set the lower altitude limit over the terrain. Should the aircraft descend below this limit, a red light would illuminate on the panel. While flying at the selected limit, a yellow light would illuminate. Above that limit, a green light would illuminate.
It is in "experienced" condition. The center knob does turn to set the altitude limit between 50 ft and 300 ft. This artifact has a yellow overhaul and repair sticker from Naval Air Station (NAS) Norfolk (VA) dated Nov 1958. Measures 3.25 inches diameter by ~5 inches deep.