This from the link:
30th Dec 2001, 14:02
"In normal Amplitude Modulation (AM) signal generation a carrier and two side bands are produced. The side bands are designated Upper Side Band (USB) and Lower Side Band (LSB). It is actually not necessary to transmit all these signals to provide the receiver with enough information to reconstruct the original modulation, audio in this case. The carrier may be removed or attenuated, and so can one of the two sidebands. The resulting signals will require less transmitted power and will occupy less bandwidth.
USB is the norm for aeronautical HF communications, AM is used when a carrier is required such as while being checked for maintenance and as far as I am aware LSB is unused by acft."
A 1 Kw AM carrier requires 500 watts of audio to produce 100% modulation. The transmitted signal is made of the carrier plus the USB and the LSB which are spaced from the carrier by the audio frequency. Both side bands contain the same information. Commercial equipment (maritime, airline, etc.) save much energy by transmitting just 1 side-band. Receivers select a BFO frequency (beat frequency oscillator) as a reference carrier for the side-band.
The HF band is too low for Frequency Modulation (FM). An FM carrier contains all the audio (or digital) information and requires no extra modulator power for 100% modulation, plus it has other advantages.