Nha Ky Thuat
The Nha Ky Thuat (Vietnamese) or Strategic Technical Directorate (STD) was the covert action and special reconnaissance organization of the Republic of Vietnam. It paralleled the U.S. MACV-SOG covert operations group,  although U.S. questions about STD security limited the level of joint operations and planning. After MACV-SOG left Vietnam under the Vietnamization program, various STD units continued to operate until the 1975 fall of South Vietnam.
Its lineage goes back to the Lien doi Quan Sat soi 1 (1st Observation Unit), formed in 1957 and, trained by the Central Intelligence Agency and United States Army Special Forces for special reconnaissance and unconventional warfare.
While it was under the Presidential Liaison Office, it was originally based in Nha Trang, and perhaps was comparable to the paramilitary side of the Central Intelligence Agency. It was commanded by a loyalist to President Ngo Dinh Diem, then-LTC Le Quang Tung. 
In various reorganizations, Tung's unit became the Lac Luong Dac Biet, or Vietnamese Special Forces, which variously served in special operations, as a Presidential guard and paramilitary unit, and counterpart to United States Army Special Forces in training irregulars, the Civilian Irregular Defense Group. Tung was shot after the 1963 overthrow of Diem, and the LLDB came under military control. Eventually, the LLDB was dissolved, but its personnel became the basis for the the STD.
Separate from Tung's unit, also by order from Diem, another unit was created in 1959, through a Presidential Order from President Diem, set up and commanded Northern Service, also called Bureau 45B (So Bac, Phong 45B). This was a Vietnamese-CIA operation separate from MACV-SOG. 
Liaison and Technical Services
While part of the LLDB proper was refocused, after the 1963 coup, into being a counterpart organization for the U.S. Army Special Forces training forces for Civilian Irregular Defense Group camps, part of its personnel went to a different military organization, the Liaison Service. This Service, created earlier, was composed of selected ARVN regulars, commanded by a colonel, and had three operational units. In 1962, since its name had been compromised, the So Khai Thac Dia Hinh or Studies and Exploitation Service.
An additional unit, the So Ky Thuat or Technical Service was created, for agent operations into North Vietnam, missions that would run longer than those proposed for the Liaison Service. The level of effectiveness is not known; MACV-SOG had all its long-term agent programs in the north neutralized by security there.
While the degree of actual joint operation between the STD and MACV-SOG is not fully known, an organization called the Liaison Service corresponded to the Ground Studies Branch, with three colocated operational bases were:
- Task Force 1 (Da Nang), matched to MACV-SOG Command and Control North (CCN).
- Task Force 2 (Kontum), paired with Command and Control Central and operating in the triangle among Cambodia, Laos, and South Vietnam.
- Task Force 3 (Ban Me Thuot), working with Command and Control South against southern VC strongholds and operations into Cambodia.
Each Task Force had a Headquarters and Security Company, a Reconnaissance Company of ten teams, and two Mobile Launch Sites with reaction forces.
EARTH ANGELCommunist soldiers who voluntarily came to the southern side were called "ralliers", or Hoi Chanh (Vietnamese for "open arms"). Some were sent back into the Ho Chi Minh trail area as part of a program, run jointly by MACV-SOG, the CIA, and Vietnamese So Cong Tac (Special Mission Service) Earth Angel. Friedman quotes Major “Wick” Zimmer, the MACV-SOG commander of the Earth Angel personnel:
The Earth Angel agent was a product of northern society. They would hold self-criticism sessions at night, just like they had done in the North Vietnamese Army. They never balked at a mission, never gave any disciplinary problems. They were extremely motivated, almost without parallel.
PIKE HILL and CEDAR WALK
PIKE HILL operations were conceptually similar to EARTH ANGEL, but used South Vietnamese citizens of Cambodian heritage, rather than ralliers, for intelligence collection. CEDAR WALK used similar personnel to carry out limited combat operations. 
With the withdrawal of U.S. Army Special Forces under the Vietnamization policy, CIDG and LLDB functions were at an end. Parts of the LLDB became a new 81st Ranger Group, as well as the Studies and Exploration Service/Liaison Service.
At the same time 81 Airborne Ranger Battalion was expanded into 81 Airborne Ranger Group consisting of one Headquarters Company, one Recon Company and seven Exploitation Companies. The Group was put under the direct control of the JGS as a general reserve force.
Lam Son 719
South Vietnamese special operators had conducted reconnaissance before the ill-fated Operation LAM SON 719, an independent ARVN operation into Cambodia.
As American forces left, the STD was enlarged and put under Col. Doan Van Nu, an ARVN paratrooper, and former military attache to Taiwan. The STD now reported directly to the President and to the Chief of the Joint General Staff.
It also was responsible for continuing psychological operations with the radio stations, 'Vietnam Motherland', 'Voice of Liberty', and 'Patriotic Front of the Sacred Sword' clandestine radio stations.
Group 68 ran airborne trained rallier and agent units, including Earth Angels (NVA ralliers) and Pike Hill teams (Cambodian disguised as Khmer Communists). Since Vietnamization did not block U.S. air support, Pike Hill units could call in B-52 strikes, with 48 sorties in one day in November 1972.
- Shultz, Richard H., Jr. (2000), the Secret War against Hanoi: the untold story of spies, saboteurs, and covert warriors in North Vietnam, Harper Collins Perennial, p. 245
- Waggoner, Mark H., Annex B: Studies and Observation Group, Military Assistance Command Vietnam, Command History 1970, SOG-1970
- Conboy, Kenneth & Simon Mccouaig (199`), South-East Asian Special Forces, Osprey Publishing, ISBN 1855321068, pp. 30-31
- Ngo Xuan Hung, History of Liaisons Service, 1st Observation Group, Presidential Palace (1956-1963) and Strategic Technical Directorate, ARVN (1964-1975), A Life for Freedom and Democracy: Special Branch - Northern Service (So Bac) and the Secret War against Hanoi
- SOG-1970, p. B-7
- Friedman, Herbert A., The Ho Chi Minh trail campaign
- SOG-1970, p. B-7 to B-8