Neptune was discovered 170 years ago with the help of observations of Uranus' orbit. A great demonstration of the scientific principle for its time. Recently, Earth and cis-Lunar based radio interferometry together with the Cassini orbiter have measured Saturn's orbit, 10 AU from the Sun, with very great precision. This has already helped determine some characteristics of a possible planet Nein 200-700 AU from here.
What would be the best way to gravitationally measure the yet unobserved mass concentrations in the Kuiper Belt and beyond, by observing orbits of objects among the known planets?
It takes alot to move Saturn to a measurable degree. What about a space probe with the sole purpose of being easy to locate at high precision, maybe in a highly inclined orbit? Or a family of them in different directions. Would its deplacement from the very weak gravity well of a planet Nine be detectable? If it is the history of Saturn that contributes crucially to the precision, what about an asteroid like Ceres which is known since 200+ years? Is there any special property of Saturn that makes it especially useful? Would a second object as well measured as Saturn be extra helpful by some kind of triangulation of distant gravitational influences?