Electronic Countermeasures Position


The Countermeasures Officer (called the "Raven") was a late addition to the B-29 crew compliment. Their normal function was to identify and counter radar that might be illuminating the squadron for gun laying or searchlight pointing. In the case of the Enola Gay and her sisters in the 509th Composite Group, that function was modified. Because the primary weapon fuse was based on a radar altimeter within the bomb, stray or deliberate radio waves at just the right frequency could potentially set off the bomb as soon as internal timers and a secondary bank of barometric switches had armed it after leaving the bomb bay. The Countermeasures Officer in the aircraft with the weapon (Jacob Beser in the case of both atomic missions) was responsible for scanning the frequencies between 390MHz and 430MHz and notifying the Weaponeer of the frequency and characteristics of any interference. According to Leon Smith, who designed the weaponeer's control station, the weaponeer could then choose between two sets of four fuse frequencies (each of the four APS-13s were tuned to a slightly different frequency to avoid interference with each other). A fuse of the contact type was apparently considered for Little Boy as "salvage fusing" but the gun assembly method was essentially self-assembling on contact with the ground at high velocity. A contact fuse was reportedly installed on the Fat Man weapon, because it was not self assembling due to its spherical construction with multiple fuses.

Enola Gay Countermeasures Position prior to cabling

The primary instrument for detecting these emanations is at bottom right, called an AN/APR-4. The Countermeasures Officer sat on the floor to operate this gear, an uncomfortable positiion to be sure, but the APR-4 was normally configured to automatically scan back and forth across a preset range of frequencies. This would allow the officer to sit on the chair next to the radar operator, or on the toilet seat to the left, listening to the headphones and watching the panoramic adapter screen for any indications of interfering signals. The position itself may seem incomplete and missing a great deal of equipment, but in fact (for the Enola Gay on August 6th, at least) it is shown fully configured before cabling except for the recorder to record interphone conversations, based on documents from the week of the mission. In fact, the AN/ARR-7 receiver will be removed because it has no documented use nor antenna to support it. The empty positions were used when the aircraft was employed in an instrumentation mode, or in some cases for radar jamming equipment when there was an anticipated threat, as there well might have been had it been necessary to continue dropping nuclear weapons on Japan to conclude the war.

The other pieces of equipment seen above are (from top left to right) an AN/APA-11 pulse analyzer, AN/APA-10 panoramic adapter, and an AN/ARR-7 receiver. The panoramic adapter visually displayed a continuous band of frequencies to the left and right of the frequency actually being listened to on any of the three receivers in this bay of equipment. The pulse analyzer was a specialized instrument that permitted detailed characterization of electronic radar signatures. The plywood board on which the ARR-7 receiver is currently mounted was originally the location of a special disk recorder that Jacob Beser reportedly used to record interphone traffic on that day (see "Recorders"). Further research is ongoing.

In the second row is a PP-32/AR power supply, for the AN/ARR-5 receiver to its right, probably installed for listening for the succesful deployment of three instrumentation canisters by the instrumentation aircraft (The Great Artiste on the Hiroshima mission.) (See more on this below.) The aluminum box to the left of the PP-32 is a junction box recently made to the original specs that will be used to rewire the power and audio distribution circuits. It has since been painted the same green color as the racks.


Countermeasures Position, AN/APR-4 receiver




Countermeasures Position, normal B-29 Installation (early)

Standard B-29 countermeasures positions only contained one vertical rack of equipment, shown above. However, a 509th mission was considerably different from those of its brethren. A standard weapon drop envisioned three aircraft over the general target area at one time; the weapon dropping plane, an instrumentation plane, and a photographic plane. Ultimately, any of the 509th aircraft needed to be capable of being configured to serve in any one of those roles. That is the reason that the Enola Gay installation may seem somewhat empty - its spectrum management role on August 5th was limited to the weapon fuse frequencies and cross checks of other roles. By far the most equipment-consuming role was the instrumentation aircraft. In addition to the equipment shown in Enola, it required a total of three VHF receivers to listen to the three air dropped canisters in the vicinity of 50MHz, as well as three recorders to record the overpressure signatures provided by the canister microphones. From this information and estimates of location of each canister with respect to the weapon's detonation, approximate weapon yields could be calculated. As related in a fascinating LAUR (Los Alamos Unclassified Release) monograph by Dr. Glen McDuff, three Hallicrafters S-36 VHF receivers were used in The Great Artiste to receive the overpressure waveform from the parachuted instrumentation canisters dropped prior to bombing, but these required special non-standard wide mounts and were significantly heavier than their repackaged condition in the AN/ARR-5 version. The outputs of these receivers for the Hiroshima mission were connected to three Dumont oscilloscopes of the type shown here, which in turn had three Kodak Cine E 16mm movie cameras attached with light shielding tubes to record the oscilloscope traces. Since this laboratory system was physically large and somewhat unwieldy to set up, future missions (had they been necessary) would most likely have switched to more available countermeasures equipment - three AN/ARR-5 receivers, three AN/APA-11 pulse analyzers, and either three radar film movie cameras made for the APA-11s or possibly three AN/ANQ-1 wire recorders, given the results obtained from the first drop. That suite of equipment types in fact reflects the expanded countermeasures rack configuration capability that exists in Bockscar today, the plane that flew the Nagasaki mission.

Simultaneous recording of three independent signals required a special antenna to be rigged from the belly of the aircraft, through a hole in the bottom of the radar compartment. Below are shown the hatch and antenna. It is very similar to the antenna and procedure used by the Navy for their sonobuoy receivers in locating submarines. The antenna was too long to remain in place for landing. so it was intended to be rigged and removed during flight. You can see it in the deployed position in the "Exterior Antennas - right side" page. On the Hiroshima flight it was deemed necesary to deploy it prior to takeoff, so the antenna was shortened from a half to a quarter wavelength to avoid dragging it on takeoff.


Antenna loading hatch


Special whip antenna for ARR-5 receivers, mounted on ceiling for storage