China has set about building over-the-horizon radars to track American aircraft carriers as they make their long journey across the Pacific Ocean:
The frequency of radio waves used by most radars, in the form of microwaves, travel in straight lines. This generally limits the detection range of radar systems to objects on their horizon (generally referred to as “line of sight” since the aircraft must be at least theoretically visible to a person at the location and elevation of the radar transmitter) due to the curvature of the Earth. For example, a radar mounted on top of a 10 m (33 ft) mast has a range to the horizon of about 13 kilometres (8.1 mi), taking into account atmospheric refraction effects. If the target is above the surface, this range will be increased accordingly, so a target 10 m (33 ft) high can be detected by the same radar at 26 km (16 mi). Siting the antenna on a high mountain can increase the range somewhat; but, in general, it is impractical to build radar systems with line-of-sight ranges beyond a few hundred kilometres.
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The most common type of OTH radar uses skywave or “skip” propagation, in which shortwave radio waves are refracted off an ionized layer in the atmosphere, the ionosphere. Given certain conditions in the atmosphere, radio signals transmitted at an angle into the sky will be refracted towards the ground by the ionosphere, allowing them to return to earth beyond the horizon. A small amount of this signal will be scattered off desired targets back towards the sky, refracted off the ionosphere again, and return to the receiving antenna by the same path. Only one range of frequencies regularly exhibits this behaviour: the high frequency (HF) or shortwave part of the spectrum from 3–30 MHz. The best frequency to use depends on the current conditions of the atmosphere and the sunspot cycle. For these reasons, systems using skywaves typically employ real-time monitoring of the reception of backscattered signals to continuously adjust the frequency of the transmitted signal.
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OTH systems are thus very expensive to build, and essentially immobile.
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A second type of OTH radar uses much lower frequencies, in the longwave bands. Radio waves at these frequencies can diffract around obstacles and follow the curving contour of the earth, traveling beyond the horizon. Echos reflected off the target return to the transmitter location by the same path. These ground waves have the longest range over the sea. Like the ionospheric high-frequency systems, the received signal from these ground wave systems is very low, and demands extremely sensitive electronics. Because these signals travel close to the surface, and lower frequencies produce lower resolutions, low-frequency systems are generally used for tracking ships, rather than aircraft.
I believe the tracking of surface ships has been done by satellites for the last few decades. OTH radar is a thing, but it was reduced in importance once the satellite systems were put in place.
All the discussions of Chinese tracking of US carrier strike groups indicate satellites are the primary method.