Dave, you ask:
Is building movement due to wind a problem? Roof-to-roof on multi-story buildings at distances of a mile or so, with lasers? Or is it an insignificant factor?
Yes, this is what we were referring to as building sway over in the GBLX thread. A very small movement at the sending (transmitting) end of the air link can result in a wide swing at the receiving end, which could potentially result in missing the receiver altogether. When the two buildings are swaying out of synch, the problem randomly increases proportionally. Keep in mind that it is the center of the receive side pattern (delivery footprint) that signal concentration is the greatest.
At 200 feet, envision a circular footprint patern against a wall of only a couple of feet in diameter, with the center six inches or so carrying most of the transmitted energy intensity. The core six inches constitutes a "sweet spot." As in another analogy (I think it was in Dave Horne's upstream post) if you move the transmitter ever so slightly, it's akin to misaligning a rifle's aim, resulting in missing the target entirely. In practice, aligning one of these links actually makes use of internal rifle scopes, cross-hairs and all.
Given that the beam is very narrow to start with (and it's angle is chosen by the operator to be only between 3 and 5 degrees in most situations) it could be a show stopper unless measures are taken to offset the sway. One way to mitigate the effects of building sway is to open up the angle to, say, 7 to 10 degrees, thus increasing the footprint at the receiving end, but this also has other side effects such as reducing the potential distances that might be covered, or reducing the bit rate over the same distances. There's always a tradeoff between distance and bit rate in these scenarios. HTH.
Regards, Frank Coluccio |
