5. Establishing the physical link: scoring, acquisition and tracking

Unlike RF transmissions, for which open-loop pointing is largely acceptable, closed-loop pointing is often required for optical transmissions. Only a feedback loop based on an external source (laser signal) can achieve the required pointing levels. The most elegant solution to this problem is for each terminal to track a signal from the other.

When this state of respective pursuit is reached, the physical link is said to be established.

The physical link is established by a synchronized sequence between the two terminals. It consists of several main stages:

• the link management entity requests the establishment of the link to the two terminals concerned, specifying their respective positions and the precise time at which acquisition is to begin;

• the two terminals...