Radio Altitude Indicator (Altimeter), ID-236/APA-61 for AN/APA-61
The WWII-era Radio Altitude Indicator, ID-236/APA-61 was a component of Radar Bombing Navigational Computer AN/APA-61. The indicator was used to measure and indicate to the pilot the altitude over the terrain during flight. Made by Roller-Smith. This unit looks to be originally of the AN/APN-1 system and was modified for use in the AN/APA-61 system. The nature of the modification is unknown.
The precursor AN/APN-1 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, 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, byprocess of detection, in an audio frequency signal the average frequency of which is proportional to the altitude of the aircraft above the ground. This equipment provides a dual range indicator of 0-400 feet and 400-4000 feet; having an accuracy within plus or minus 6 feet on the 400-foot range and within plus or minus 60 feet on the 4000-foot range."
It is in reasonable condition given its age. The lower left knob switches on the indicator, and the top right knob switches the measured range between 0-400 ft to 400-4000 ft and back (see 2 photos showing range set for each). The dial and glass are intact. Measures 3.25 inches diameter by ~5 inches deep.