It was the end of an uneventful day at the Earth Station, and nearly time for the day shift and engineers to go home, leaving just the watch-keepers in charge. I was acting as "resident engineer," second to assist the customer's engineers during the first few months of operation. Christmas was approaching, and the "short rains" season had arrived.
We sat in the break room, drinking tea and admiring the elegant white antenna outlined against the dark sky about a hundred meters from the main building, when a violent storm swept across the East African Rift Valley.
Suddenly, there was a dazzling flash of lightning followed almost instantaneously by a mind-numbing clap of thunder. The lights dimmed and recovered, but as our ears stopped ringing from the thunder, we became aware of a clamor of alarms from the control room. Clearly, there had been a direct hit on the antenna, and it was going to be interesting to see if the lightning protection systems had worked. Something exciting to put into my weekly report back to the UK.
The focal point of the control room was the illuminated mimic diagram, which, as we left to take our tea break, had been mostly comforting shades of green and white. Now, it was a mass of angry flashing red lights as just about every part of the station had tripped out and was demanding attention. Since the tracking system automatically applied the brakes when tripped to standby, the antenna would still be pointing at the satellite, and it would remain in range for some minutes.
Swift restoration of the receivers and transmitters was the first priority, and it should have been just the work of a couple of minutes to get everything working again using the remote control buttons on the console. We began to fear the worst when both receivers showed OK indications but the downlink and beacon level meters remained stubbornly down at noise levels -- no sign of signals. We were switching between the duplicated paramps and downconverters to try to find at least some signals when the transmitter, which had come back online without problems, showed some warnings then tripped out again. We tried the other transmitter and that too tripped after a few seconds.
It was now clear that we had a really serious problem, so we resigned ourselves to not going back to the city until the problem was solved. Leaving the deputy station manager in charge in the control room, we went to the antenna tower, only partly shielded from the torrential rain by the covered walkway, and checked out our own particular systems after getting the control room to enable local control. After half an hour we got together to swap notes.
The paramps and receiver guys were convinced that the receive down-chain was working perfectly but just wasn't getting any signals from the feed horn. The transmitter team reported that both transmitters would work fine into the water-cooled dummy load, but instantly trip on "reverse power" when connected to the outgoing waveguide. I was convinced that the servo and tracking system was working perfectly, but without any signal from the satellite, it couldn't track anything, so I left it in standby mode.
The "reverse power trip" is provided to protect the transmitters from damage if too much energy is reflected back into the output. This would only happen if the waveguide switches had been incorrectly set to divert the output to a dead end or open flange or some other grossly mismatched or damaged device. Visual inspection of the waveguide switches proved that this wasn't the case, yet as soon as the transmitter was connected to the transmit waveguide leading to the horn, it tripped out. We were mulling over the possibilities when someone spotted a warning light on the waveguide pressurization and radome rain-blower cabinet.
This was tucked away in a corner of the cabin at the back of the antenna, but the alarms were only shown locally and not integrated into the main station alarms. The cabinet had a compressor and desiccator system to provide clean dry air at low pressure to all the waveguides and the inside of the horn. The front of the horn was covered with a thin but strong orange Mylar radome, a bit like a drum skin, and sealed with a robust clamping ring some two feet in diameter. Arranged around the periphery were some air-blast nozzles that, in theory, could be used to sweep rain from the radome surface, but had turned out to be rather ineffective so were rarely used.
The alarm showed that pressurization had been lost in the horn itself, and the main transmit waveguide, but was OK in the rest of the system. We reluctantly concluded that the Mylar radome had been punctured by the lightning strike, but we still didn't understand why that should disrupt both transmit and receive functions.
The rain was still pouring down in buckets and the antenna was up at 60 degrees elevation, so the radome and feed-horn were invisible from the ground except from some distance from the tower, and by now the tropical night had descended in its usual abrupt fashion. In clear weather and daylight we could have inspected it with binoculars, but in the dark and in the pouring rain, that just wasn't possible.