Fixed, automated transmitters which transmit signals (typically CW) continuously to allow operators to check propagation. These transmitters are located around the globe. On HF they are mostly found on bands such as 10 meters above 28.1 MHz. They typically use low power and omni-directional antennas. It can be good operating strategy to tune to these stations during contests to learn when propagation might support QSOs even if activity and thus stations in the contest are not heard.
An Amateur Radio Blog by DX Engineering 
