This paper describes the design and performance of a precision CW laser tracker. When tracking low acceleration targets such as satellites and airplanes, this tracker has an accuracy of approximately 25 microradians rms. The accuracy under these conditions is set by the static friction, the background noise present in the equivalent noise bandwidth of the tracker and the scintillation of the atmosphere. When tracking high acceleration targets such as rockets, the tracker has a tracking error which is essentially proportional to the relative angular acceleration. If the rocket acceleration and the tracker-target geometry are such as to cause a relative angular acceleration of 0.6 radians/second , the tracking angular error peaks to 0.3 mr at the instant of launch settling to 0.1 mr within 0.1 second thereafter and remaining at the level during the rest of the propulsion period after which it reduces further to 25 microradians during the coasting phase.
Thus, the tracking accuracy against low acceleration targets is comparable to the accuracy of a star tracker. However, the laser tracker has the added capability of measuring range to the target. This accuracy particularly at low altitudes exceeds that which can be provided by a high performance radar.
Interest in high precision tracking, of course, results from the instrumentation tracking requirements that arise at the missile ranges and other test stations. High precision tracking is also a necessary part of long-range optical communications which can be efficiently accomplished only by using very narrow beams. The advantage of optical tracking over radar tracking is that it is not affected by undesired reflections from surrounding objects, and the accuracy is somewhat less affected by variations in the index of refraction of the atmosphere. Laser tracking, as contrasted to passive optical tracking, has the advantage of discriminating against other optical sources and also has the capability of simultaneously measuring range
