The USA have for a long time been trying to track missiles after the main engine has burned out. The recent rise of manoeuvrable hypersonic gliders and cruise missiles has made this need more pressing, since they fly very low compared to ballistic missiles, and consequently they are mostly below the radar horizon.
China and Russia are developing such weapons, and since according to US generals “there are not enough islands in the Pacific to place enough radars to get a good coverage”, the solution for tracking them will have to come from space.
As demonstrated in the article Detecting hypersonics, if you do not have a sensor able to detect and track the missiles over an Earth background, then you need an extremely large number of satellites: at least 16 in GEO, and much more if using lower orbits. The Pentagon has arrived to a similar conclusion, and financed the development of the SBTSS below-the-horizon sensor. They do note, however, than extracting the faint signal from the background clutter caused by Earth’s surface is one of the toughest challenges of the architecture.
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