Wailupe Naval Station
Call sign NPM.
"Radio Wailupe wasn't an ordinary naval radio station--it was an OP-20-G intercept site--located east of Honolulu, and well away from both Pearl Harbor and Wahiawa.
"Wailupe's staff was all OP-20-G people, who weren't supposed to mix with ordinary navy radio folk, and they weren't supposed to talk about what they were doing. The radiomen were specially trained in Washington to receive Japanese morse code and enough rudiments of the Japanese radio equivalent of brevity code get some sense of what they might be picking up."
"The Navy radio receiving and control station (NPM) was located at Wailupe until the Pearl Harbor attack -- after which it was moved to Wahiawa. The OP-20-G group (nowadays known as the Naval Security Group) was co-located with NPM at Wailupe until 1933, when their operation was relocated to Heeia -- on the eastern shore of Oahu at Kaneohe Bay -- to get better reception. The Pan Am HFDF site at Mokapu Point was about 2 miles east of Heeia, on the peninsula that forms Kaneohe Bay. It's probably no coincidence that both sites were in that area -- the only commercial activity in the area was a large taro farm, so there was minimal electrical equipment to generate interfering radio noise."
Brandenburg's clarification of the relationship between the OP-20-G group and NPM suggests that by 1937 Wailupe was not part of the Navy's radio-intercept operations.