Optical Navigation SSI image taken for optical naviation; image typically consists of the limb and terminator of one main body (Jupiter or a satellite) and stars. The image is used to determine the position of the imaged body against a star background which enables the navigators to better estimate the relative position of the spacecraft and the imaged body.

OpNav stands for Optical Navigation, and means that images are obtained of Eros against the star background. These are measured (just like plates of asteroids and stars are measured to update asteroidal occultation predictions) to help determine where NEAR is relative to Eros, that is, to help navigate the spacecraft (since these observations are used in the overall orbit determination, based also on measurement of NEAR's radio signals, which is then used to plan thrusting maneuvers to target NEAR to Eros).

Office of the Chief of Naval Operations