Invert Operations
ORGANIZATION
Detachment 5, 621st TCS operated at Nakhon Phanom RTAFB as a GCI [1] radar facility, using the call-sign Invert from 23 Feb 1966 to 01 October 1975. The unit functioned as a CRP within the Tactical Air Control network in Southeast Asia, under PACAF command. As with similar detachments, they reported to both the CRC, Brigham (Det. 4, 621TCS ) at Udorn RTAFB, and TACC [2] (505TCG) at Tan Son Nhut AB, Saigon. Around March of 1973, HQ 7AF, and consequently the TACC were relocated to Udorn when 7AF left Vietnam.

Within the three-dimensional hemisphere of air-space responsibility, all aspects of air operations were managed simultaneously through integrated teamwork with associated organizations.
The reponsibility for the identification and control of aircraft is known as airspace management. Tactical and Strategic attack, reconnaissance and airlift support aircraft are considered 'weapons' in Air Force parlance, and personnel skilled in manoevering aircraft in a 3-D airspace environment for combat purposes are known as Weapons Controllers, or Weapons Techs. The overall job title encompases various specialized sub-tasks, but the essential difference between Tactical Air Control and what is commonly understood as Air Traffic Control, is that Weapons Controllers bring aircraft together, whereas Air Traffic Controllers try to keep aircraft apart.

Under PACAF doctrine (in descending order of size) CRC's, CRP's and FACP's were charged with managing aircraft missions that included bomber and fighter-bomber ground attack, MIGCAP [3] air interception, 'Wild Weasel' AAA/SAM suppression, Reconnaissance, Trail Interdiction, Forward Air Control, Search-and-Rescue, ELINT missions, cargo transport airlift or parachute drops, in-flight refueling operations, as well as any civil aviation activities flying in, through, or over their assigned airspace. Coordinating U.S. and Allied Air Force missions with Army and Navy aviation - fixed-wing radio intercept operations, helicopter assaults, medevac or artillery operations - mixing aircraft of all types and flight capabilities - was a constant and dynamic 24-hour mission. Everything from piston-engined WW II era A-1 Skyraiders to turbojet CH-53 Jolly Green Giant helicopters, to U-2 and SR-71 Blackbirds were airborne at the same time, with KC-135 Stratotankers always available to refuel them.

Det. 5 was operated and maintained by crews working 24 hours a day, seven days a week, for all of the years of U.S. commitment in Southeast Asia. Radar operations personnel were scheduled to work on shift. Manpower demands fluctuated in response to mission tasking, and workloads caused continuous reorganization of personnel and shifts throughout Invert's operational history. In 1968 there were two shifts - day crews worked 13 hours and night crews worked 11 hours. All 'Ops' personnel attended the daily Crew Briefing, where the Senior Director and Crew Chief decided who and how many would work that day. Radar and Radio Maintenance sections worked a single day shift with one or two technicians on duty at night. Nightime non-routine maintenance events were attended to on a call-out basis from the hootches located across the street. By 1973, unit manning allocations had improved. Operations was staffed with three crews, identified as Alpha, Bravo, and Charlie, working three 10-hour 'days' (6am - 4pm), three 14-hour 'nights' (4pm - 6am) then off on three-day break. Similarly, most Radar and Radio maintenance was performed on 12-hr 'days', with 12-hr night shifts performing scheduled preventative maintenance routines (PMI's) around air mission tasking to avoid conflicts. Maintenance techs accessed the Crypto vault with the shift account custodian, signed for and set up the daily 'frag' keys, reprogrammed and tested the radar and radio crypto equipment each night. These work cycles interrupted daily eating and sleeping patterns for personnel, interfering with circadian rhythms, REM-sleep needs, and eventually producing chronic psychological fatigue. Time off from work was a precious comodity.

The radar Operations building was originally a tin-roofed wooden structure, but was replaced by a modular steel-frame and aluminum-skin Butler building in June 1969. Access to the air-conditioned Ops building was through one of two doors, with access controlled by Crypto-brand rocker-switch electric keypads. A roll-up type door was kept locked but available for equipment moves as needed. The Operations building consisted of:
- Dark-room, containing the radar scope consoles, big-board tactical map displays and wall charts;
- Radar maintenance rooms, containing radar and IFF/SIF interrogator equipment;
- Telephone and cryptographic equipment room;
- Classified information storage vault;
- Crew break area with toilets, showers and janitorial facilities;
- Administrative area comprising Radar Quality Control, Operations Training and management offices.

Inside the fenced compound, past the sandbagged entry control guard shack and clustered close to the Ops building, were the corrogated tin-roofed brown wood CEM buildings, supply logistics and long-haul HF radio operations office, squadron Administration building, and various other storage buildings. Maintenance was conducted in accordance with AFM 66-1, Maintenance Management.
The Radio equipment /maintenance shop, or GATR site building and antenna poles were originally located inside their own wooden earth-filled revetment, remote from the main building cluster at the southernmost end of the compound so as not to cause or be affected by RFI (interference) from the radar system. In 1971 the wooden revetment structure was replaced by the Radio maintenance team with an aluminum earth-filled system.
Radar and Radio communications systems had independent diesel-powered emergency electrical generators to maintain operations during base power outages.

Consoles:
The Operations dark-room was arranged similar to a small theater, with four stepped levels. The operator positions were arranged in four rows of three radar scopes, descending from the Senior Director and Crew Chief positions of the Battle Staff section on the highest (back) row, moving down, past the Weapons positions, to the Surveillance positions at building floor-level down front. The third and fourth levels had height-finder scope positions on the ends of their rows.
A building floor-level transit aisle separated the front row consoles from the three higher levels behind. All personnel seated at their 'scopes had a clear view of information posted on the the floor-to-ceiling height self-illuminated 'big-board' - a lexan map presentation of Southeast Asia at the front of the room. There were grid charts on either side of the central map, displaying weather information, tactical, and offensive mission data. Invert was positioned as the center of a series of white concentric rings that were drawn expanding in 25 nm increments. Hanoi showed up close to the top, right-hand corner and Bangkok was down at the bottom. The map detailed all geographical features, like the Mekong river and Vietnam coastline; national borders of Thailand, Laos, Cambodia, the two Vietnams, with the DMZ; the Thai Air Defense buffer zone, designated No-Fly zones, and some of the Route-Pack military areas. All ingress and egress flight corridors, refueling and mission orbits, active air operations and enemy SAM sites were marked. Some areas were identified 'stereo' anchors - stereotypically consistant orbits (aircraft holding locations shaped like racetracks) - that were moved little throughout SEA operations [see Refuelling Tracks below]. Other anchors, like the Crown flights - the HC-130 ABCCC's (airborne command, control, and communications) - were usually stable over Vietnam, Laos and Cambodia, but were moved by 7th AF as mission requirements demanded.

Personnel working behind the big-board plotting real-time mission information from the back side of the board, facing through the lexan into the darkroom. They had to accurately write backwards with fluorescent Chinagraph pencils, color-coded according to the type of information displayed. Communication was by intercom headsets connecting board plotting crews, weapons controllers, battle staff and air intelligence personnel passing tactical data from and to various remote military resources. The Battle staff and Air Intelligence staff consisted of U.S. and Thai Army, Air Force, Navy, Marine and sometimes Coast-Guard liaison members communicating via landline or radio with other forward combat zone, airborne, regional, or ground theater-level facilities, like higher headquarters' CRC and TACC facilities.

Specifications:
	Manufacturer:		Hazeltine
	Display type:		AN/UPA-35 PPI (Planned Position Indicator)
	Indicators:		12-inch radius CRT; 0-250 nautical mile controller; variable surveillance range
	Range Marks:		Fixed 1, 2, 5, 10, 20, 50 nm, plus adjustable

Intercom /telephone consoles:
	Manufacturer:		GTE
	Equipment::		AN/GTA-6A ten-line telephone switch panel
	Indicators:		NE-51 lampfield (48VAC)
	Controls:		Manual ringdown hotline; up/ down toggle selectable.

Surveillance Radar
The Search Radar System consisted of two separate AN/MPS-11 units removed from their mobility vans and set inside the building; the antennas remained on mobility towers guyed with concrete anchor footings. They were medium-range search radars operating in the L-band at a frequency of 1280 to 1380 MHz. As a mobile variant of the AN/FPS-8 system, it had been introduced by General Electric in the 1950's. Over 200 units of this model were produced between 1954 and 1958, and MPS-11 ubit S/N 01 was used at Invert. The primary workhorse there was serial number 24, which was shut down at 17:00 hours local time, Friday, September 12, 1975 after the last Olympic Torch mission landed at Utapao AB Thailand.

Specifications: Manufacturer:   General Electric Radar type:   AN/MPS-11, L-Band Radar Frequency:   1280-1350 MHz. Power input:   120/208V, 3 ph, 60 Hz. Power output:   1 MW Indicators:   12 positions in PPI Magnetron:   QK-358 Pulse Rate:   3 us, 360 p/p sec. Antenna:   25' diameter; azimuthal, truncated semi-parabolic reflector. Rotation Rate:   0-10 rpm, adjustable Effective operating range:   220 nm, raw 'paint'; 250 nm with IFF/SIF 'squawk-flash'

Height-finder Radar
The AN/FPS-90 height-finder radar was introduced into service in the late 1950's and served as the principal target height-finder system for several decades. The S-band radar operated at a frequency of 2700 to 2900 MHz. Over 450 units of this model were produced between 1953 and 1960.
The Invert '90-Site' was used for accurate height determination of targets flying between elevation angles of -2 and +32 degrees on any azimuth. Maximum effective range was 220 nm. up to an altitude of 75,000 feet. Data was displayed on two 12 RHI indicators and mechanical counter assemblies. The antenna array was a truncated semi-parabolic reflector that swept in an up-down motion on any assigned heading. It could also be used for very high-power search functions.

Specifications: Manufacturer: General Electric. Radar type: AN/FPS-90 S-Band Search Radar Frequency: 2700-2900 MHz. Power input: 120/208V, 3 ph, 60 Hz. Power output: 5 MW, peak. Indicators: 2 RHI position scopes Magnetron: QK-338A Pulse Rate: 2 us, 400 pps. Antenna: Truncated semi-parabolic reflector. Vertical Coverage: -2 to +32 degrees Vertical Scanning Rate: 20 or 30 cpm Azimuth Resolution: 3.2 deg. Elevation Accuracy: +/- 1000 ft. Horizontal Beam Width: 3.2 deg. Capable range: 300 nm. Effective operating range: 220 nm.

Invert's daily mission involved air support for the Ho Chi Minh Trail interdiction campaign.
Interlaced with, and prempting this would be special mission tasking and various B-52 bombing campaigns, known variously as Arclight strikes under Operations Linebacker, Linebacker II. Most missions usually involved aircraft handoffs from other radar control facilities. The most common missions Invert managed were:
1) Flight-follow for ABCCC; SIGINT; ELINT; FAC, fighter and Trail sensor recon.
2) Linebacker bombing campaign strikes.
3) Special missions; airborne launch /recovery of Buffalo Hunter recon drones, U2 and SR-71 recon flights.
4) KC-135 in-flight refueling missions for almost all other missions.

Refueling Tankers:
In-flight refueling tanker orbits were shaped like racetracks and known as anchors.
Tanker track locations varied slightly over the course of the war but stayed fairly constant. There were three anchor tracks over Thailand.

From roughly overhead at NKP and parallel on a north-south orientation, they extended west over Thailand, each about forty miles apart. They consisted of dual cells oriented line-astern in ascending 500 foot separation; the lowest track starting at altitudes of from 14,000 to 18,000 feet, with the highest track from 24,000 to 28,000 feet.
The altitude could be varied depending on such factors as winds aloft, weather, the condition or the type of the aircraft in question. None of these flight levels were an issue for F-4 Phantoms, but some were a problem for F-105 Thunderchiefs. The tracks were, in order:

Station White,
Station Red,
Station Blue.

There were two tracks over Laos, east of Vientiane, known as:

Station Green,
Station Orange.

There were two tracks out over the Gulf of Tonkin. Oriented the same way again, in parallel and stacked line-astern, they were:

Station Purple,
Station Tan.

After Operation Linebacker II, two new tracks were established running aast-west over Southern Laos and Cambodia. They were in order:

Station Chestnut,
Station Hickory.

Support of B-52 bomber missions:

B-52 bombing missions were refered to as Arc-light strikes, usually a flight of three B-52s delivering 750 lb. bombs to any requesting force ubder heavy attack.
Contrasting the Arc-light strikes were the heavily planned special bombing campaigns, known generally as the Linebacker operations.

During the holiday season of 1972, a typical Linebacker II mission frag. (Fragmentary Air Tasking Order) would commit a hundred refueling tankers and over three hundred fighters at a time. Invert would routinely handle these refueling task orders seven days a week for the duration of the campaign. For the two to three hours a Linebacker mission was in progress all other air operations except SAR missions, were generally put on hold. After a Linebacker mission ended, the Laotian Trail interdiction campaign would resume, and that involved another whole set of tanker refueling anchor tracks.
Pre-strike mission events:
A typical Linebacker II pre-strike refueling scenario went as follows:
Two cells, each with 6 Boeing KC-135 tankers were stacked one behind, and above, the other in a 'line-astern' (trailing) formation. Each tanker followed the other some 500 feet higher in altitude, and a 1/2 mile behind the one in front. Each of the five tankers had a pre-strike task to refuel a four-aircraft flight formation except for the last (sixth) tanker, remaining in attendance as a reserve or emergency contingency bird.
The pre-strike refueling mission tasking was usually a pretty predictable and orderly affair.
A given pre-strike control track like anchor White would consist of 12 tankers and 10 flights of four fighters or fighter/bombers in the same orbit accomplishing all refueling within a 10-15 minute timeframe.
The Drop-Off (tanker disengagement) point for the strike push into North Vietnam was The Fish's Mouth (look at a map of N. Vietnam and see what resembles a fish's mouth).
This was really getting pretty risqué for the unarmed KC-135 tankers, because it was very close to the North Vietnamese MiG-21 intercept airfield of Quang Lang. Fortunately, the air superiority reputation for the MIGCAP protection force was so well established that the MiG's hardly ever effected the decisive threat that they should have been, though it was still a very tense maneuver for the crews.

Post-strike mission events:
The post-strike refueling mission tasking was very unpredictable at best. Things got very busy, very fast, because the returning aircraft were no longer in tidy flights of four. They would come back as singles, pairs, or whatever, in different states of fuel reserve; some with battle damage, some with mechanical problems, etc. and have to be handled as each situation arose. /Lynn Zerull, Tucson, Arizona.

Fighter-bomber strike mission events:
One combat fighter pilot recounts the following:
Strike missions could either approach and leave their targets by the same route, or keep moving past their targets, and leave out of the other side of Vietnamese airspace.
On a Bac Giang, Kep, or Bac Ninh mission, one could be fragg'ed inbound from over Laos, and return west the same route;
or go in from Laos and out to the Gulf for post-strike;
or one could go in from the Gulf and back out the same route;
or go in from the Gulf and depart westbound, over Laos, and transit (cross) the US Navy's Route-Pack A-6/B mission corridor in the process.
The over-land refueling tanker orbits stayed over Thailand, up to about 18 deg. 30 min. North Latitude. Then at a pre-determined time, the tankers would "extend" up, to 20 degrees N. Latitude, at which time the aircraft would rapidly cycle on the tanker boom to top-off their tanks. Both the over-land and over-water tracks extended only as far as 20 N. Latitude - a long way from the "North-West of Yen Bai" scenario usually cited. /Ed Rasimus, USAF (Retd.).

See the Invert Crew Roster page...

Notes:

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This web site is developed and presented by a U.S. Air Force veteran. It represents no official position, and information is neither complete nor fully accurate at this time. For questions on specifics, please contact the author for advisement. Information was derived from personal experience, conversations and correspondence from friends and associates. Data was collected, collated and written to provide a general insight into activities from 1966 to 1975 in Southeast Asia during the time of the Vietnam conflict. All official, unofficial, and personal pages on the world-wide web were researched for any pertinant information, and credit is given to all parties where known. General links are provided wherever possible to enhance the reader's understanding of allied subjects not covered here. The author actively encourages all vistor comments, corrections, and complaints for the enhancement of both accuracy and presentation style of these materials.

Author: C. Jeppeson ,
Invert Radio Maintenance,
Sgt., USAF (30454); NKP, 1973
Last Updated: 09/11/00