In January 1962, the AFSS elements finally arrived in Vietnam. They split up between the two air bases. Two H-1 intercept vans were airlifted from the Philippines to Danang while another four vans, including the HFDF station and the AFSSO operations complex were flown into Tan Son Nhut. A few months later, the intercept mission at Tan Son Nhut was moved to Danang because of the better HF reception there.

At first, the Air Force’s VHF intercept mission at Danang was a bust. The problem was caused by the terrain surrounding the air base: on three sides it was closed in by mountains three to twelve miles away with peaks as high as five thousand feet. Onl ‘ the mouth of the harbor was open [ — ]. The ASA element, which had been training the local ARVN J-7 COMINT personnel manning their own site at the harbor, had found no VHF; even HF was difficult to hear. Any intercept of VHF communications emitted from aircraft flying at medium-to-low altitudes beyond the mountains was almost impossible. The only way to hear anything on the VHF range was to relocate to a better site.

The lack of VHF intercept was addressed in October 1962 [ — ]  authorized a review of the Air Force’s situation in Vietnam. The review aimed at rectifying the initial poor site selection and sought an upgrade of equipment and antennas at its facilities. Another part of the review included a survey of a potential VHF intercept site twelve miles

northeast of Danang, a place known as Monkey Mountain. The survey team, with all of its equipment, arrived at Danang from the Philippines in late October. However, it was the rainy season and the team had to battle its way up muddy trails just to get to their site on the top of Monkey Mountain. The rain had washed out a bridge at the foot of the mountain; a pontoon bridge was constructed to get them across. After climbing up the mountain, they found that their test site was on the other side of an impassible gully; so they got permission to set up their camp next to the Air Control and Warning site. All of this took six days!

The results of the test were less than dramatic, at least to the personnel of the test team: only two potential VHF voice targets were detected, one North Vietnamese. [ — ] For the 6922th, this intercept showed great potential for the Asian problem; [ — ] The 6925th Group, located in the Philippines, was less sanguine. It was mainly interested in establishing an intercept mission in South Vietnam against the DRV’s air force and air defense communications. Despite the uneven results [ — ] ordered work on the site to get started.

Source: Spartans in darkness: American SIGINT and the Indochina War, 1945-1975