A number of serious consortiums develop satellite communication networks. The objective of these communication projects is to service personal communication users almost everywhere on Earth. The intersatellite links in those projects use microwave radiation as the carrier. Free-space optical communication between satellites networked together can make possible high-speed communication between different places on Earth. Some advantages of an optical communication system over a microwave communication system in free space are: (1) smaller size and weight, (2) less transmitter power, (3) larger bandwidth, and (4) higher immunity to interference. The pointing from one satellite to another is a complicated problem due to the large distance between the satellite, the narrow beam divergence angle, and vibration of the pointing system. Such vibration of the transmitted beam in the receiver plane decreases the average received signal, which increases the bit error rate. We review: (1) the present status of satellite networks, (2) developing efforts of optical satellite communication around the world, (3) performance results of vibration effects on different kinds of optical communication satellite networks, and (4) seven approaches to overcome the problems caused by transmitter pointing vibration.

Laser satellite communication network-vibration effect and possible solutions

The basic free-space optical communication system includes at least two satellites. To communicate between them, the transmitter satellite must track the beacon of the receiver satellite and point the information optical beam in its direction. Optical tracking and pointing systems for free space suffer during tracking from high-amplitude vibration because of background radiation from interstellar objects such as the Sun, Moon, Earth, and stars in the tracking field of view or the mechanical impact from satellite internal and external sources. The vibrations of beam pointing increase the bit error rate and jam communication between the two satellites. One way to overcome this problem is to increase the satellite receiver beacon power. However, this solution requires increased power consumption and weight, both of which are disadvantageous in satellite development. Considering these facts, we derive a mathematical model of a communication system that adapts optimally the transmitter beam width and the transmitted power to the tracking system performance. Based on this model, we investigate the performance of a communication system with discrete element optical phased array transmitter telescope gain. An example for a practical communication system between a Low Earth Orbit Satellite and a Geostationary Earth Orbit Satellite is presented. From the results of this research it can be seen that a four-element adaptive transmitter telescope is sufficient to compensate for vibration amplitude doubling. The benefits of the proposed model are less required transmitter power and improved communication system performance.

Beam width and transmitter power adaptive to tracking system performance for free-space optical communication

Angular microvibrations of platform jitter on the optical inter-orbit communications engineering test satellite are measured in space during ground-to-satellite laser communication links. The microaccelerations are measured by the onboard accelerometers at a sampling rate of 2048 Hz. The angular microvibrations are estimated from the measured microaccelerations using the tracking characteristics of the laser communications terminal and the conversion factor on the basis of microvibration data obtained from ground-based tests. The power spectral density (PSD) of the satellite microvibrations is analyzed by using the fast Fourier transform analysis and the data length is examined according to the frequency resolution of the PSD. The in-orbit measurements of the PSDs are compared with those obtained from the ground test. The angular microvibrational base motion is estimated and a PSD up to 1024 Hz is additionally provided as a database of the real measurement results with previously obtained in-orbit measurements. The measured results will contribute to the angular jitter estimation and the design of a tracking control loop for space laser communication systems in the future.

In-orbit measurements of spacecraft microvibrations for satellite laser communication links

Morio ToyoshimaNational Space Development Agency ofJapan2-2-1 SengenTsukuba, Ibaraki 305-8505JapanE-mail: toyoshima.morio@nasda.go.jpKenichi ArakiCommunications Research Laboratory4-2-1 Nukui-KitamachiKoganei, Tokyo 184-8795JapanE-mail: araki@crl.go.jpAbstract. Angular microvibrations of platform jitter on a three-axisattitude-stabilized satellite are measured in space using an onboard la-ser communication terminal. Stable optical tracking control allows themeasurement of relative angular variations from the reference line ofsight of the optical link. The tracking accuracy in this measurement isless than 1 mrad rms. The angular variation and the drift rate on thesatellite are measured at a sampling rate of 500 Hz, and these data aredownlinked to a ground station via a satellite-to-ground optical commu-nication link. The power spectral density of the satellite microvibration iscalculated by a Fourier transform analysis. The frequency spectrum ofthe gimbal mirror’s angular variation is also obtained when the coarsetracking control is operational. The rms value agrees with the residualcoarse tracking error estimated before launch of the satellite. These re-sults will be useful for the future design of a tracking control loop foroptical communication systems.

In-orbit measurements of short term attitude and vibrational environment on the Engineering Test Satellite VI using laser communication equipment

Free-space optical communication between satellites networked together can make possible high-speed communication between different places on earth. The use of optical radiation as a carrier between the satellites creates very narrow beam divergence angles. Due to the narrow-beam divergence angle and the large distance between the satellites, the pointing from one satellite to another is complicated. The complication is due to vibration of the pointing system caused by two stochastic fundamental mechanisms: (1) tracking noise created by the electro-optic tracker and (2) vibrations created by internal satellite mechanical mechanisms. We derive mathematical models of signal, noise, approximate SNR, and approximate bit error rates of optical communication satellite networks as functions of the system parameters, the number of satellites, and the vibration amplitude. The optical intersatellite network model considered includes transmitter satellite, repeater satellites, and receiver satellite all networked together. These models are the basis for pointing system design of appropriate complexity and performance to make the network as simple and inexpensive as possible. An example of practical communication between Anchorage in Alaska to Johannesburg in South Africa by a free-space optical communication network composed of nine low-earth-orbit satellites is given. From the analysis it is clear that even low vibration amplitude of the satellite pointing systems decreases dramatically the network performance.

Performance limitations of free-space optical communication satellite networks due to vibrations-analog case

On ESA's large communication satellite OLYMPUS a set of three orthogonally arranged microaccelerometers is installed. The aim of this experiment is to characterize and observe the behaviour of the different mechanisms in space and to get measurements of the vibration levels which are of relevance for the design of optical communication payloads. During the commissioning phase of the spacecraft in October 1989 a lot of data recordings were made at ESTEC. The translation of the measured acceleration spectra into a base motion spectra shows a higher amount of vibration in the frequency region above 100 Hz, when compared with previously reported data from Landsat spacecraft. The main disturbing vibration sources, solar array drive and thruster firing, can not be characterized precisely enough before the launch. This confirms that measurements in space are mandatory before flying an optical payload. Therefore ESA plans to install this kind of equipment on several other spacecrafts which will be launched before launch of ESA's optical communication payload SILEX. In this paper the equipment on board OLYMPUS will be described followed by presentation of typical measured data. A specification for the micro-vibration environment with which an optical payload has to cope is then derived.

L. van Holtz

Papers

In-orbit measurements of microaccelerations of ESA's communication satellite Olympus