Optical Satellite Communication

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Communication links between space crafts is an important element of space infrastructure, particularly where such links allow a major reduction in the number of earth stations needed to service the system. An example of an inter orbit link for relaying data from LEO space craft to ground is shown in the figure below

Interorbit link for relating data from LEO spacecraft to ground

Fig. 1
Inter orbit link for relaying data from LEO space craft to ground.

The above figure represents a link between a low earth orbiting (LEO) space craft and a geostationary (GEO) space craft for the purpose of relaying data from the LEO space craft back to the ground in real time. The link from the GEO Satellite to ground is implemented using microwaves because of the need to communicate under all weather conditions. However, the interorbit link (IOL) can employ either microwave or optical technology. Optical technology offers a number of potential advantages over microwave.

I. The antenna can be much smaller. A typical microwave dish is around 1 to 2m across and requires deployment in the orbit, An optical antenna (le a telescope) occupies much less space craft real estate having a diameter in the range of 5 to 30 cm and is therefore easier to accommodate and deploy.

II. Optical beam widths are much less than for microwaves, leading to very high antenna gains on both transmit and receive. This enables low transmitter (ie laser) powers to be used leading to a low mass, low power terminal. It also makes the optical beam hard to introsept on fan leading to convert features for military applications, consequently there is a major effort under way in Europe, USA and Japan to design and flight quality optical terminals.

 

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