I spent a year in the Marines in Vietnam working on ground control approach (GCA) radar systems in a Marine Air Traffic Control Unit (MATCU) at Phu Bai, An Hoa, Chu Lai, and Marble Mountain. I arrived at Phu Bai in December of 1968, well after all the nasty fighting in the Tet offensive in February. Phu Bai airport serves Hue, the old imperial capital of Vietnam, and a visit there showed they were still recovering from the damage inflicted during Tet.
I left in November of 1969 to be an instructor at the Marine Corps Electronics School at Twentynine Palms in southern California. I initially headed up an instructional team that taught radar fundamentals. We took a class of new Marines through the program after they completed Basic Electronics School.
Sometime later the (poorly named) fire-control radar course needed an instructor, so I was sent there. The fire-control radar didn't actually control the firing of guns or missiles, it directed the flight of friendly aircraft to bomb close support targets on the ground. I had originally been trained as a fire-control radar technician when I first started out in the Marines back in 1964.
The fire-control system had a stand-alone parabolic antenna cabled to an operator's hut. The antenna could lock onto a friendly aircraft with a narrow radar beam and by deriving the direction and distance and angle of elevation, the radar computers could know exactly where the aircraft was over the ground. Thus with the map coordinates of the radar itself, the derived coordinates of the acquired aircraft, and any target coordinates sent to us, we could tell an aircraft pilot exactly how to attack that target.
The computer was analogue, rather than digital. We used knobs to adjust voltages rather than enter data by keyboard. Our readouts looked a lot like automobile odometers. Numbers on dials showed the aircraft speed, direction, and other information. The computer calculated how long it took a bomb to fall from the aircraft to the ground. The dials then displayed the direction and time to drop which was radioed to the pilot.
The pilot might never even see the target. The aircraft could be flying above the clouds, or at night, but the fire-control radar knew where the aircraft was and where the target was, so it could calculate the direction the pilot needed to fly. It even corrected for the wind and for the rotation of the earth, so that when the bomb was released at altitude it would hit where the target would be at the end of the fall.
Naturally, this complex electronic system required that whenever I prepared the radar system for a class instruction, I had to maintain, calibrate, and test to ensure that it worked properly. The final test each time involved searching for a local aircraft and locking the system onto it. Then we could ensure that the system could maintain a "lock" and follow an aircraft while our readouts provided us the information needed to control the aircraft.
During maintenance, we didn't contact the aircraft, as we would in actual practice. We just grabbed whatever aircraft happened to be in the vicinity, locked on and observed that everything worked properly.
One time, however, things didn't quite work properly. I had just finished the alignment and calibration of the radar and started searching for an aircraft to lock onto. I found a passing aircraft at high altitude and everything seemed to work fine. The radar acquired the electronic return and began tracking the aircraft like normal.
But as I looked at the readouts something was wrong. First of all, the distance to the aircraft kept changing radically. Second, the aircraft moved in strange directions. Sometimes the aircraft seemed to move backwards. The altitude of the aircraft varied radically as well. No aircraft that I knew could behave this way.
This seemed like some extraterrestrial flying saucer that defied physical laws. I stepped outside the radar hut to see the antenna pointed high in the sky at what looked like an ordinary passenger airliner flying west. It flew straight and normal, not altering its flight path at all. I couldn't see any other aircraft in the area.
I went back into the radar hut and began checking the equipment again. I released the lock on the strange aircraft and tested all of the calibrations. Everything seemed normal. After a while I used the radar to find another aircraft to track, and this time nothing unusual happened. The equipment worked perfectly. I remained completely baffled about what had happened with the previous aircraft.
That night the evening TV news reported President Nixon had arrived on the west coast to visit his home in San Clemente. That meant Air Force One would have likely flown right over Twentynine Palms. The President's plane is equipped with electronic counter-measures to prevent it from being shot down. The fire-control radar I used would have seemed to those on Air Force One exactly as if a missile firing system had a "lock" on the President's plane. I wondered if I momentarily caused a panic on the President's plane.
It could have been worse. Back in 1965, we learned that a Marine fire-control radar unit near the DMZ in Vietnam calibrated its system and then locked on a nearby aircraft that turned out to be a U.S. attack plane returning from an airstrike in North Vietnam. It fired a beam-rider missile and blew up the radar. Fortunately for me, Air Force One didn't have beam-rider missiles.