The Research Of Beacon Positioning In Laser Communications

As consequences of the high frequencies and the high directivity, compared withconventional radio frequency systems, there are several advantages offered by freespace optic systems, such as smaller size of the antenna, potential increase inmodulation bandwidth, lower inter-channel crosstalk, and stronger confidentiality.Applications, such as space links, could benefit from free space optic connections.Because of the high directivity of the transmitted beam, the communicationterminals have to point each other extremely precisely before communicating. Oneterminal receives and images the beacon emitted from another terminal, and then byusing the fast steering mirror, the optical system adjust the pointing direction of thetransmitted beam, thus correct the positioning error. Imaging error is the immediatecause of positioning error. The imaging error includes static imaging error anddynamic imaging error. The static imaging error mainly refers to the aberrationcaused by manufacturing error and alignment error of the optical system of thetelescope in the communication terminal. The dynamic imaging error means theimage jitter caused by atmospheric turbulence.According to the error distribution analysis, under the existing technicalconditions, diminishing the alignment error which mainly refers to the relative shifterror and the relative tilt error between the primary and the secondary mirror of thetelescope is an efficient means to improve positioning accuracy. Despite a freshlyaligned telescope being in good condition, factors as mechanism wear andtemperature changing would gradually bring unpredictable error to the systeminevitably. With a dearth of measure instruments and the limited conditions in mostapplications, errors couldn’t be estimated and corrected. Therefore, this thesisdeveloped a new method estimating and correcting the error based on themorphology of image directly. By introducing a new measure function to measurethe imaging quality, this method connected the value of this function and the value ofalignment error. By taking RC telescope system as an example, through simulationsand experiments, a mathematical model which can express the correspondencebetween the alignment error and the measure function was represented. Experimentsshowed that the alignment error could be corrected automatically based on thismodel, so that the static imaging errors could be diminished effectively by employing the new method.Atmospheric turbulence causes distortions of the wave phase and introducesdynamic imaging error. Moreover, the turbulence changes the shape and direction ofthe beam and introduces beam spreading and beam wander, which makes trackingand pointing more difficult. Base on the theory of laser beam propagation throughrandom media, functions for estimating dynamic imaging error were worked up.Several measurable parameters influencing the dynamic imaging error were studiedseparately，and functions for estimating the receiving efficiency were deducedafterwards. Base on those estimation functions, dynamic imaging error could bediminished by altering the parameters of the transmitter, thereby improvingpositioning accuracy and increasing receiving efficiency.