> a means by which remote transmitters on Earth were turned on and off<p>This sentence was real confusing. The transmitters were turned on and off by the people on Earth, but they were remote transmitters _on other parts of Earth_, not turned on and off remotely by the astronauts. The astronauts weren't turning Earth's transmitters on or off. Right?
Fun fact: when the Quindar tones were no longer needed, NASA initially removed them. However, it caused confusion in mission control since everyone was used to them, so NASA added them back.
This is one of the many things Ron Howard’s <i>Apollo 13</i> film depicted correctly, as opposed to how the tones were simulated on quite a few other TV shows and films about that era of space flight. For example, some of the more egregious depictions had the tones coming from <i>astronaut</i> transmissions, not the <i>ground</i> transmissions. Knowing this would have been a simple matter of research, but a lot of folks apparently didn’t do it.