What geometric term does the flight path of a plane on radar
best represent

Nadir line

Explanation:

This line represents the lowest point on the ground and in terms of the light path, this line is seen to move along the aircraft when taking the measurements from above the radar. Zenith is just above the horizon of the nadir line in signing the plumb line. Geocentric zenith is an intersection with the celestial plane through the observer's position of the geometric center of the earth. This is considered important when taking photography from the lower angle of the plane. The azimuthal line runes parallel to the direction of the flight path. Most of this technology is used for the framing and scanning of the systems throughout the optical system directly beneath the platforms. Keeping track of the various ranges like the near, far, ground and slant range.