Optical Communications, 2nd Edition

10.3 SPATIAL TRACKING

After pointing and spatial acquisition have been achieved, there remains the task of maintaining the transmitted beam on the detector area in spite of beam wander or relative transmitter–receiver motion. This operation of keeping the receiver aperture properly oriented relative to the arriving optical field requires spatial tracking. This tracking is achieved by generating instantaneous pointing error voltages that are used to continually realign the optical hardware.

After successful acquisition of the incoming light beam, the beacon field should be focused at the center of the acquisition array, which is coaxially aligned to the position error sensor of the tracking subsystem. The acquisition threshold removes the array processing and enables the tracking operation using the centered, focused beam. The tracking subsystem then generates the error signals as the focused beam moves off-center, due to either line-of-sight beam motion or receiver platform jitter.

The tracking subsystem usually uses two separate (azimuth and elevation) closed loops, as shown in Figure 10.12. The tracking error is determined instantaneously for both azimuth and elevation coordinates by means of the position error sensor, which generates the error signals. The error signals are then used to control the alignment axis of the receiver lens. This is accomplished by some type of control loop dynamics, generally with separate servo loops for individual control of the azimuth and elevation ...