Secure radio communications is always an issue. It is easy to become lax, and let the enemy know what we are doing.

Aviation was especially prone to problems. We often thought we were moving too fast for radio intercepts to have much use to the VC. And were right, usually.

We kept our radio frequencies and call signs the same, never changing them, or important air direction radio nets. Squadrons used their own call signs. The Danang Dasc call sign was Danang Dasc.

That all changed in the fall of 1970. Suddenly we had to change the call signs every day, using alpha numeric designators. We resented this, as it was confusing, and made it time consuming to know who you were talking to.

I found out why a number of years ago, when we saw a film about a captured VC / NVA radio intercept station. They had gleaned a substantial amount of information from careless radio operators. The radio operators would tell their friends about impending B-52 strikes with carefull made up codes, something lik "Big Bertha will be in your pos tommorow" The VC figured this out right away, and warned their forces.