| Popis: |
Laser communications between satellites imply stringent demands concerning the accuracy of the Pointing, Acquisition and Tracking (PAT) Subsystem which is part of the transceiver package. In the scope of ESTEC and DFVLR contracts a breadboard model of a PAT subsystem is being developed. This subsystem is designed for a CO 2 -laser transceiver package. The PAT subsystem is composed of four opto-mechanical subassemblies namely the Coarse Pointing Assembly (CPA), the Fine Pointing Assembly (FPA), the Optical Nutator Assembly (ONA) and the Point-Ahead Assembly (PAA). The actual pointing mechanism is composed of the CPA and the FPA. The CPA provides the PAT with a hemispherical pointing range but allows only low dynamics and a limited pointing accuracy. Hence, the FPA's part is to cope with high-speed, small-angle precise beam steering. Furthermore, the FPA is used to generate the search pattern in the acquisition mode. The PAA deviates the received and the transmitted beam to compensate the effects caused by the propagation time of light between the satellites. By means of the conical scan tracking technique the ONA allows the generation of the tracking signal for the FPA from the same detector which is used for data reception. This paper describes the control loops and steering algorithms which are necessary for proper interactions of the subassemblies. In particular the acquisition strategy and the transition from the acquisition mode to the tracking mode is described. The subassemblies and the control electronics are modeled. Problems concerning the CPA singularity are discussed. The performances of the control loops and the interactions of the subassemblies are demonstrated by means of computer simulations verified by some hardware tests. |
