Can someone please explain the two modes USB/AM and how they are used, also are the HF radio's turned off on the ground? Thank you

"Can someone please explain the two modes USB/AM and how they are used, also are the HF radio's turned off on the ground?"There's probably no simple answer to this one (and my basic theory is a little hazy), but...HF Transmissions can be of various types. AM, Upper Sideband (USB), Lower Sideband (LSB), etc... For each type, you have to ensure that your aircraft receiver is compatible with the various types of ground transmitter (and vice versa).AM (Amplitude Modulation) HF (High Frequency) signals are composed of a carrier wave plus upper and lower sidebands (Sidebands are a side-effect of adding intelligence, such as music, voice, etc, to a basic High Frequency radio "carrier" signal.The boffins/scientists discovered that sending the carrier wave/signal and both upper and lower sidebands is wasteful, energy-wise (all the important data is carried in the sidebands). They figured out a way to use most of a transmitter's energy to transmit a single sideband by itself (if I remember correctly.... or perhaps a single sideband plus a small vestige of carrier signal?). However, the aircraft HF receiver has to add the missing stuff from the ground radio transmission in order to convert the sideband signal into something the pilots can hear.At some point, it was decreed that USB signals were to be the standard for aviation (earlier, aircraft radio receivers used to have a variety of modes, including upper and lower sideband, but it started to get confusing). You may still find panels with LSB on them, but usually the panel has modified wiring so that USB is selected, even in the LSB position. I don't know if AM is used for much these days by the pilots, except if you want to listen to the latest news or the cricket/baseball scores on national/international radio stations :-)HF is quite powerful and there are restrictions on its use during aircraft refuelling. It's effectiveness, however, varies with the earth's changing atmospherics. Sometimes you can't get a station 20 miles away, other times your signals will carry half way around the globe. UHF and VHF on the other hand are generally "line of sight". i.e. you can't talk to anyone if they are over the visible horizon (The higher you fly, the farther your signals will carry). Hope this makes sense.Cheers.Ian.