Amplitude-companded single-sideband modulation

Amplitude-companded single-sideband (ACSB) is a narrowband modulation method using a single-sideband with a pilot tone, allowing an expander in the receiver to restore the amplitude that was severely compressed by the transmitter. The pilot tone serves as a frequency reference for the receiver, eliminating the signal distortion that would occur with single-sideband suppressed carrier modulation when the receiver is off frequency.

Source: Wikipedia — Amplitude-companded single-sideband modulation (CC BY-SA 4.0)