Motoaki Shimizu

The new tracking control system for Free-Space Optical Communications

The new tracking control system has been developed for Free-Space Optical Communications. It consists of the agile two-axis gimbals, the high performance fine-pointing-mechanism, the optical coarse acquisition sensor, and the optical fine tracking sensor. In the conventional tracking control systems, the two-axis gimbals is combined with the narrow range actuator for the fine pointing control, and it have the role of the coarse pointing control based on the GPS position and the body attitude. The each motion of the tracking mechanisms, therefore, is mutually disturbed by the individual control algorithms, and the obtained tracking accuracy is restricted by each motion. To improve the control performance of this two-stage type, the synchronized tracking-control system is proposed. The features of the system are the cooperative/predictive control using the each control/detective signals, and the improvement of the performance for disturbances, with the hollow-structured and rapid two-axis mirror gimbals. This control scheme is applied for the developed tracking control system, and makes it possible to achieve Free-Space Optical Communications. The paper shows the system configuration, the control algorithms and strategy for Free-Space Optical Communications, and the effect of the proposed control system by the several test results.1

Inertial stabilization system for small airborne SAR

An inertial stabilization system for small airborne synthetic aperture radar (SAR), which consists of an active damping mechanism and a two-axis gimbal, has been developed. Both high-frequency vibration isolation by the active damping and high stability of antenna direction under low-frequency angular disturbance have been successfully demonstrated. Also, a small airborne SAR prototype for disaster monitoring has been produced and prepared for flight tests.

Data transmission performed from a moving vehicle

A popular data transmission method for large-capacity communications is propagating a laser beam in free space. The high directivity of the coherent light reduces the propagation loss and provides strong irradiance at the receiving plane. The size of the optical antennas, or telescopes, that are used can therefore be much smaller than those used for propagating radio frequencies. In addition, the laser propagation technique provides a high data rate without frequency coordination, making installation of equipment simpler than for other methods. An optical link between the transmitter and the receiver must be maintained during data communications. However, using a sharp beam sometimes causes difficulties. This is especially the case when the relative positions of the communicating terminals change dramatically. Precise control over the direction of beam emission from the transmitter is required, and light arriving at the receiver must be accurately tracked and acquired.1–3 The optical communication equipment we have designed has two functions for beam tracking so that we can achieve such angular accuracy. The first is a coarse pointing function that controls the direction of the optical antenna so that it remains in contact with the accompanying terminal. A second, fine pointing, function is contained within an inner optical setup and is used to remove the residual angular errors. We conducted a demonstration of our laser communication technique using a moving car and a fixed ground station (as illustrated in Figure 1). We obtained a data rate of 40Gb/s at a wavelength of 1.5 m, over a distance of several hundred meters. We tested two different coarse pointing functions in our experiments. All of the optics in our moving terminal system, including the telescope and the fine pointing function, are installed within a ball-like container that also includes the coarse tracking function (see Figure 2). This terminal has been improved from Figure 1. Setup for the demonstration. The optical terminal on the car transmits data to the optical terminal on the ground with a data rate of 40Gb/s at a wavelength of 1.5 m.

Cooperative Control Algorithm of the Fine/Coarse Tracking System for 40Gbps Free-Space Optical Communication

The cooperative control algorithm of the fine/coarse tracking system has been developed to achieve 40Gbps free-space optical communication. The tracking control system consists of the agile two-axis gimbals as the coarse tracking system and the high performance fine-pointing-mechanism(fpm) as the fine tracking system with the optical coarse acquisition sensor and the optical fine tracking sensor. It applies for the aerial platform and the onground platform, and free-space optical communication has been carried out between them. In the conventional tracking control systems, the two-axis gimbals is combined with the narrow range actuator for the fine pointing control, and it has the role of the coarse pointing control based on the GPS position and the attitude of the platform. To improve the control performance of “this two-stage type”, especially during the motion of the