I don't know for certain, but I think the main assumption for passive ranging is that the object isn't moving, as something on the ground would be. That's the only way such a measurement could be made. When I modeled GoFast (less precisely than I did Gimbal), there was a pivot point on the sightline where the object could be at rest. At 12,000 feet, I measured 5 knots. No such point exists in the Gimbal case; the slowest speed I could find is 75 knots around 16 NM. And in Gimbal only one angle is slewing significantly; in GoFast both are.
So, radar, maybe. But Lehto called the range number a guess, and radar wouldn't be a guess. A passive trig calculation, based on both azimuth and elevation slew rate, would be.