FR (Fixed Reference) and DR (Dynamic Reference) optics are two different types of optical systems used in surveying and geodesy.

FR optics have a stationary reference point or system that is fixed in location and position. It uses this fixed reference to measure angles and distances to other points. FR optics are commonly used in traditional surveying instruments like theodolites and total stations.

DR optics, on the other hand, use a dynamic reference point or system that can move or adapt to changes in position or orientation. This allows for more flexibility in measuring angles and distances, especially in dynamic or changing environments. DR optics are commonly used in GNSS (Global Navigation Satellite System) receivers and other modern surveying instruments.

In summary, the main difference between FR and DR optics is the nature of the reference point used for measurements - fixed in the case of FR optics and dynamic in the case of DR optics.