Right. Since each "mysterious" X was equipped with a "US Army Corps of Engineers" brass plate, it was kind of obvious they were calibration targets for something military. The article says "focusing", but they were probably used to get more precise orbital elements for satellites. This was pre-GPS, and calibration across geography on a planetary scale was tough.
> The article says "focusing", but they were probably used to get more precise orbital elements for satellites. This was pre-GPS, and calibration across geography on a planetary scale was tough.
My guess is that they served a function for calibrating ground control points rather than orbital elements. Even today, NORAD publishes orbital elements (TLEs) for spacecraft deduced by radar (and also optical means). I think other radio ranging techniques were also used before GPS. Trying to point at Earth-fixed point with only an inertial attitude (without a position/velocity) seems really tough.
Today, lots of CubeSats don't have GPS, so they eagerly await getting TLEs from NORAD. It seems like this Twitter is always the first with new data:
