Actually, given the primitive nature of the katusha, how could they be certain that it is actually following a true ballistic trajectory that would be traceable back to a point of origin with enough precision to avoid collateral damage?
If a katyusha is a primitive rocket, it can't help but follow a ballistic path.
If you get 3 paintings, you can redraw the parabola. If you get more than 3, you can use a more complex trajectory that would compensate for spin.
Baseball players, if they are good batters, try to pick up the ball off the pitchers hand, then sample 2 or 3 more locations. If they can get 3 more, for a total of 4, they can compensate for the ball's spin. If they only get 2 more, for a total of 3, they swing as for a fastball.
Modern radars get hundreds of paintings, but for ballistic targets, they use them to correct error and ambiguity, rather than to get a more complex model.
Upon triangulating the origin of fire, surround said town with artillery and send volleys until town is rubble. Search and mop-up, then onto next point of origin. It's what the Syrians did to defeat islamofascists. And it worked. Oh, and the media was not allowed in - had to rely on second hand information.