An Hoa was a 'forward combat base' out in enemy territory. The base housed a Marine infantry regiment primarily there to protect an industrial area, including an iron mine, surrounded by a farming community. A river ran nearby and across the river existed a major Viet Cong base known by Marines as 'Arizona Territory' in an allusion to movie westerns about Indian country.
Being 'forward' meant the roads were impassable during the rainy season thus requiring a short airstrip alongside the base to resupply by air. Which during the rainy season also then needed a GCA radar system.
Rows of concertina backed by periodic sandbagged defensive bunkers completely surrounded the base. The airstrip needed to be inside that defensive perimeter which placed our control tower and GCA system between the airstrip and the defensive perimeter. Indeed, if the enemy broke through the defensive perimeter wire during an attack, they would be right in our operations area.
Which is what happened to the last MATCU that was there: Wikipedia: After midnight on 23 February, 1969, PAVN mortars hit An Hoa Combat Base triggering explosions in the ammunition dumps, followed by a sapper attack which was repulsed with gunship and artillery fire. So with the rainy season approaching we were sent to An Hoa to re-establish the GCA capability.
I arrived by helicopter with the operators and radio techs to find an abandoned complex of sandbag ramparts around repair huts and underground living quarters literally adjoining the perimeter defenses. The back door to the underground living quarters had stairs up to the surface about 20 yards from a perimeter defensive bunker where a barbed wire fence connected it to the next defensive bunkers.
The staff NCOs had separate individual living quarters near the repair area. Our living quarters consisted of two shipping containers placed side by side on the ground and then covered with dirt. One of the containers served as the dining area while the other, which had pools of water inside, held the sleeping arrangements. It appeared obvious that no one had been there for several months.
The replacement GCA radar system hung disassembled in its shipping racks next to its sandbag emplacement. The sections were supposed to be bolted into those shipping racks but whoever sent this to us didn't know much about the system. Not only were the disassembled sections tied by wire to the shipping frames, but each section should have had the connecting bolts screwed into the corresponding place of each segment.
Instead, a bag of bolts hung from the shipping frame. There was no way to know which bolt went into which section. Fortunately I had disassembled and reassembled this kind of system several times and knew how it fit together and which bolts likely went where. I began working to reassemble the radar while helping others reestablish the operations area.
I was senior sergeant in electronics but Skee was an equally senior sergeant in charge of the generators and communications wires. So we worked together to get our area operational. The wires used to communicate by phone from our operations area to the base operations across the runway had to be reinstalled. So I helped dig the ditch and lay the wires in between working on the radar. I also took the time to dig a drainage ditch so the water in our underground bunker would drain away. The radio techs set up the control tower and Skee got the generators working. There were other miscellaneous jobs that needed to be done, but I finally had the radar assembled and operating.
A staff NCO who had previously been a GCA technician took over aligning the radar and sent me to work on the abandoned TACAN system left by the previous operations. A TACAN broadcasts a radio signal from a rotating antenna so aircraft could tune to the signal frequency assigned to An Hoa. The radio produces a pulse whenever the antenna faces east. Since aircraft knew when it was pointed at them by the signal strength, and the pulse let them know when the signal was pointed east, they knew in what direction to fly to reach our location.
Only this TACAN didn't work at all. Multiple circuits throughout the system needed repair. I went about trying to troubleshoot the unit but in the process I made a stupid mistake. All electronic circuits start with a high voltage at the beginning and then as electricity flows in the circuit the voltage drops so that measuring the voltage toward the end of the circuit results in low voltage readings.
However, if a circuit is broken and electricity cannot flow, then the voltage measured anywhere before the break equals the highest voltage. Consequently I should have set the voltage meter to a high voltage each time and then clicked down until I got a reading.
Since I was repeatedly measuring small voltages in circuits I impatiently neglected resetting the voltage meter to a high voltage and ended up running across a broken circuit that blew out the voltage meter. I couldn't work on electronics until a new voltage meter was sent to me at this forward combat base.
Meanwhile the staff NCO who took over aligning the GCA radar wanted to show me how he had the radar set up. Radar not only shows reflections from aircraft as white blobs on the display, but it also picks up reflections from things on the ground. This "ground clutter" appears as patches of white areas on the display. The staff NCO pointed out how the glidepath lines were set to bring aircraft right down on a somewhat linear stretch of clear space amid the ground clutter, which he averred was the runway.
I didn't see the markers in the ground clutter on the display that usually designated the runway. As I looked at the ground clutter, however, I did notice some peculiarities.
"That's not the runway," I told the staff NCO.
"Yes it is," he pointed out the linear clear space in the ground clutter.
"No, that's the river. The runway should be over here," I pointed to another somewhat linear clear space in the ground clutter. "I'll get a truck and drive up and down the runway and you look for moving ground clutter." Which I did and they managed to find the runway on the display, along with the markers that designated the end of the runway.
The way he had aligned the radar would have guided some aircraft through a rainstorm right into the river. So, if I did nothing else in this war, I stopped some pilot from getting wet.