text: Field Artillery Operations
  OBSERVED FIRE AND UNOBSERVED FIRE
  4-89. Fires are either observed or unobserved. Observed fire is the point of projectile impact or burst that
  can be seen by an observer, and can be adjusted on the basis of observation.
  4-90. Unobserved fire are projectile points of impact or burst not observed. The observer can attempt to
  adjust the round based on the sound of the impact.