Piercing the Fog - Air Force Historical Studies Office

Piercing the Fog
file marked Objective Folder No. 1. Existing air intelligence in Manila consisted
of a few oblique and vertical photographs of the sod airfields in the Philippines
and some file coverage of districts of supposedly military importance. The
Office of the Chief of Air Corps had already begun trying to prepare industrial
target reports covering the Japanese home islands in the fall of 1941 .’
In contrast to photographic or economic intelligence, SIGINT had been
active in the Philippines, albeit with only limited success. When Japanese
military operations began in the Shanghai area in 1932, the U.S. Army Signal
Corps began operating a radio-intercept station at Fort Santiago, Manila. In the
mid-1930s a detachment of the 2d Signal Service Company handled radio
interceptions at Fort McKinley, near Manila. The beginning of a successful era
of radio intelligence dated from the arrival of Maj. Joe R. Sherr as chief of the
detachment in July 1940. Sherr’s station forwarded raw radio intercepts to
Washington; it also discovered and plotted the locations of Japanese radio nets
that were significant to the defense of the Philippines:
The U.S. Navy also operated a signal intelligence unit, code named CAST,
on Corregidor. Under Lt. Comdr. Rudolph J. Fabian, this organization
concentrated on breaking Japanese diplomatic radio traffic, since it possessed
a PURPLE machine. By an agreement of May 1941, Sherr’s Army detachment
did the interceptions while CAST concentrated on decoding or decrypting. With
the establishment of HQ USAF Far East, Lt. Harold W. Brown customarily
carried intercepted messages to MacArthur’s Chief of Staff, Brig. Gen. Richard
K. Sutherland, who scanned them and, if he saw anything that would interest
General MacArthur, directed that they be taken to him. Time proved the
limiting factor in this system. A message intercepted on one day went to
Corregidor on a second; a translation came back from the Navy on a third day,
if it was decodable. Sundays and holidays often delayed deliveries another day,
since the Navy usually took these days off.’
Even before Scherr’s detachment noted a sharp increase in Japanese
diplomatic traffic in early December, American commanders in the Philippines
had become aware of increased Japanese military and naval activity. On the
evening of November 27, G-2 in Manila reported a formation of Japanese
planes flying at high altitude over Central Luzon, presumably detected by one
of the two radar sets operating at Iba (northwest of Clark Field) and at Manila.
Serious defense readiness began on November 28, including sea patrols by
B-17s over waters off Northern Luzon. Unidentified high-flying aircraft were
over Clark Field before dawn on the mornings of December 2 and 3, and early
in the morning hours of December 3 the Iba station plotted radar tracks off the
Luzon coast. It was not definite that the aerial intruders were Japanese, but
American offshore aerial patrols revealed large numbers of Japanese transports
and cargo ships in harbors and at sea, confirming the general assumption that
something impended.*
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