I was directly involved in a number of very serious US Navy situations during the Viet Nam era, but usually not at all involved with things happening in Viet Nam. HF comms were not always reliable, sometimes not even under the best of conditions, and like in foreign ports when SSB was becoming the primary HF short range voice communications mode. These were in the days before satellite communications. Satellites certainly have brought about reliable military and civilian communications, but IMO they are also very vulnerable to disruption, but for very different reasons. We had many very serious situations where ships could not communicate with higher authorities, or even with other ships within several hundred miles. HF communications was not always reliable. Many ship communications crews did not fully understand MF/HF propagation. It was in the time after WW2 and before the Navy Electronics Laboratory Center (NELC) defined Mininum Usable Frequency and Maxiumum Usable Frequency (MIN/MUF). Solar storms had their effects. We didn't have GPS and were reliant on clear skies to know precisely where we were -- if we could also get very accurate time signals from world wide time and frequency standards like WWV/CHU, etc. Otherwise our positions were determined by "dead reckoning" which wasn't a whole lot different than the ancient Vikings using ropes with knots for depth and direction, sometimes by pulling buckets on ropes to know speed. To add to it, we were using encrypted teletype for most of our communications. The stations we were attempting to communicate with had to have the same punch cards as we did. If we lost communication and synchronization, the code card had to be replaced with the next in the series. The previous card was sliced by razors in the machine, making it unusable as soon as the replacement door was opened for replacement. If the stations/ships got out of synchronization it became very difficult to figure out which card within the series that others were trying to use. Many times we were also under very short duration transmit periods, and often were under transmit power restrictions. This was to reduce the chance of being located by direction finders. Sometimes we ran without any radio transmissions and without any externally visible lighting. We didn't have anything like WSPR or some of the other one-way or two-way communications modes. Even Morse (CW) was only minimally effective in many circumstances. In times of somewhat less danger we ran with transmitters constantly sending random data so that we didn't lose synchronization with other communication platforms and sites. Overall, it was often difficult to reliably communicate with other important sites several hundreds, or sometimes a few thousand miles away.